Organisation for Economic Co-operation and Development

ENV/CBC/MONO(2024)23

Unclassified English - Or. English

2 December 2024
ENVIRONMENT DIRECTORATE
CHEMICALS AND BIOTECHNOLOGY COMMITTEE

Revised Consensus Document on the Biology of Wheat (Triticum aestivum L.)

Series on the Harmonisation of Regulatory Oversight in Biotechnology

No. 76

JT03556802
OFDE

This document, as well as any data and map included herein, are without prejudice to the status of or sovereignty over any territory, to the
delimitation of international frontiers and boundaries and to the name of any territory, city or area.
2  ENV/CBC/MONO(2024)23

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23 3

OECD Environment, Health and Safety Publications

Series on the Harmonisation of Regulatory Oversight in Biotechnology

No. 76

Revised Consensus Document on the Biology of Wheat (Triticum aestivum L.)

Environment Directorate

ORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENT

Paris 2024

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
4  ENV/CBC/MONO(2024)23

Also published in the Series on Harmonisation of Regulatory Oversight in Biotechnology:

No. 1, Commercialisation of Agricultural Products Derived through Modern Biotechnology: Survey Results (1995)

No. 2, Analysis of Information Elements Used in the Assessment of Certain Products of Modern Biotechnology
(1995)

No. 3, Report of the OECD Workshop on the Commercialisation of Agricultural Products Derived through Modern
Biotechnology (1995)

No. 4, Industrial Products of Modern Biotechnology Intended for Release to the Environment: The Proceedings of
the Fribourg Workshop (1996)

No. 5, Consensus Document on General Information concerning the Biosafety of Crop Plants Made Virus Resistant
through Coat Protein Gene-Mediated Protection (1996)

No. 6, Consensus Document on Information Used in the Assessment of Environmental Applications Involving
Pseudomonas (1997)

[No. 7, Consensus Document on the Biology of Brassica napus L. (Oilseed Rape) (1997) – REPLACED with
Consensus Document on Brassica crops (Brassica spp). No. 54 (2012)]

No. 8, Consensus Document on the Biology of Solanum tuberosum subsp. tuberosum (Potato) (1997)

[No. 9, Consensus Document on the Biology of Triticum aestivum (Bread Wheat) (1999) – REPLACED with Revised
Consensus Document on the Biology of Wheat (Triticum aestivum L.). No. 76 (2024)]

No. 10, Consensus Document on General Information Concerning the Genes and Their Enzymes that Confer
Tolerance to Glyphosate Herbicide (1999)

No. 11, Consensus Document on General Information Concerning the Genes and Their Enzymes that Confer
Tolerance to Phosphinothricin Herbicide (1999)

No. 12, Consensus Document on the Biology of Picea abies (L.) Karst (Norway Spruce) (1999)

No. 13, Consensus Document on the Biology of Picea glauca (Moench) Voss (White Spruce) (1999)

[No. 14, Consensus Document on the Biology of Oryza sativa (Rice) (1999) – REPLACED with Revised Consensus
Document on the Biology of Rice (Oryza sativa L.) No. 70 (2021)]

No. 15, Consensus Document on the Biology of Glycine max (L.) Merr. (Soybean) (2000)

No. 16, Consensus Document on the Biology of Populus L. (Poplars) (2000)

No. 17, Report of the OECD Workshop on Unique Identification Systems for Transgenic Plants, Charmey,
Switzerland, 2-4 Oct. 2000 (2001)

No. 18, Consensus Document on the Biology of Beta vulgaris L. (Sugar Beet) (2001)

No. 19, Report of the Workshop on the Environmental Considerations of Genetically Modified Trees, Norway,
September 1999 (2001)

No. 20, Consensus Document on Information Used in the Assessment of Environmental Applications Involving
Baculoviruses (2002)

No. 21, Consensus Document on the Biology of Picea sitchensis (Bong.) Carr. (Sitka Spruce) (2002)

No. 22, Consensus Document on the Biology of Pinus strobus L. (Eastern White Pine) (2002)

No. 23, Revised 2006: OECD Guidance for the Designation of a Unique Identifier for Transgenic Plants (2006)

No. 24, Consensus Document on the Biology of Prunus spp. (Stone Fruits) (2002)

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23 5

No. 25, Module II: Herbicide Biochemistry, Herbicide Metabolism and the Residues in Glufosinate-Ammonium
(Phosphinothricin)-Tolerant Transgenic Plants (2002)

No. 26, Output on the Questionnaire on National Approaches to Monitoring/Detection/Identification of Transgenic

Products (2003)

No. 27, Consensus Document on the Biology of Zea mays subsp. mays (Maize) (2003)

No. 28, Consensus Document on the Biology of European White Birch (Betula pendula Roth) (2003)

No. 29, Guidance Document on the Use of Taxonomy in Risk Assessment of Micro-organisms: Bacteria (2003)

No. 30, Guidance Document on Methods for Detection of Micro-organisms Introduced into the Environment: Bacteria
(2004)

No. 31, Consensus Document on the Biology of Helianthus annuus L. (Sunflower) (2004)

No. 32, An Introduction to the Biosafety Consensus Documents of OECD’s Working Group for Harmonisation in
Biotechnology (2005)

No. 33, Consensus Document on the Biology of Papaya (Carica papaya) (2005)

No. 34, Consensus Document on the Biology of Pleurotus spp. (Oyster Mushroom) (2005)

[No. 35, Points to Consider for Consensus Documents on the Biology of Cultivated Plants (2006) – REPLACED with
Revised Points to Consider document No. 67 (2020)]

No. 36, Consensus Document on the Biology of Capsicum annuum Complex (Chili, Hot and Sweet peppers) (2006)

No. 37, Consensus Document on Information Used in the Assessment of Environmental Application involving
Acidithiobacillus (2006)

No. 38, Consensus Document on the Biology of Western White Pine (Pinus monticola Dougl. ex D. Don) (2008)

No. 39, Abstracts of the OECD Expert Workshop on the Biology of Atlantic Salmon (2006)

No. 40, Consensus Document on the Biology of Pinus banksiana (Jack Pine) (2006)

No. 41, Consensus Document on the Biology of the Native North American Larches: Subalpine Larch (Larix lyallii),
Western Larch (Larix occidentalis), and Tamarack (Larix laricina) (2007)

No. 42, Consensus Document on the Safety Information on Transgenic Plants Expressing Bacillus thuringiensis –
Derived Insect Control Protein (2007)

No. 43, Consensus Document on the Biology of Douglas-Fir (Pseudotsuga menziesii (Mirb.) Franco) (2008)

No. 44, Consensus Document on the Biology of Lodgepole Pine (Pinus contorta Dougl. ex. Loud.) (2008)

No. 45, Consensus Document on the Biology of Cotton (Gossypium spp.) (2008)

No. 46, Consensus Document on Information Used in the Assessment of Environmental Applications Involving
Acinetobacter (2008)

No. 47, Guide for Preparation of Biology Consensus Documents (2008)

No. 48, Consensus Document on the Biology of Bananas and Plantains (Musa spp.) (2009)

No. 49, Consensus Document on the Biology of Picea mariana [Mill.] B.S.P. (Black spruce) (2010)

No. 50, Guidance Document on Horizontal Gene Transfer between Bacteria (2010)

No. 51, Consensus Document on Molecular Characterisation of Plants Derived from Modern Biotechnology (2010)

No. 52, Guidance Document on the Use of Information on Pathogenicity Factors in Assessing the Potential Adverse
Health Effects of Micro Organisms: Bacteria (2011)

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
6  ENV/CBC/MONO(2024)23

No. 53, Consensus Document on the Biology of Cucurbita L. (Squashes, Pumpkins, Zucchinis and Gourds) (2012)

No. 54, Consensus Document on the Biology of the Brassica Crops (Brassica spp.) (2012)

No. 55, Low Level Presence of Transgenic Plants in Seed and Grain Commodities: Environmental Risk/Safety
Assessment, and Availability and Use of Information (2013)

No. 56, Consensus Document on the Biology of Sugarcane (Saccharum spp.) (2013)

No. 57, Consensus Document on the Biology of Cassava (Manihot esculenta Crantz) (2014)

No. 58, Consensus Document on the Biology of Eucalyptus spp. (2014)

No. 59, Consensus Document on the Biology of Common bean (Phaseolus vulgaris L.) (2015)

No. 60, Consensus Document on the Biology of Cowpea (Vigna unguiculata (L.) Walp.) (2015)

No. 61, Report of the OECD Workshop on Environmental Risk Assessment of Products derived from New Plant
Breeding Techniques (2016)

No. 62, Consensus Document on the Biology of Sorghum (Sorghum bicolor (L.) Moench) (2016)

No. 63, Consensus Document on the Biology of Tomato (Solanum lycopersicum L.) (2016)

No. 64, Consensus Document on the Biology of Atlantic salmon (Salmo salar) (2017)

No. 65, Consensus Document on the Biology of Mosquito Aedes aegypti (2018)

No. 66, Consensus Document on the Biology of Apple (Malus domestica Borkh.) (2019)

No. 67, Revised Points to Consider for Consensus Documents on the Biology of Cultivated Plants (2020)

No. 68, Consensus Document on the Biology of Safflower (Carthamus tinctorius L.) (2020)

No. 69, Developments in Delegations on Biosafety Issues, April 2020 – March 2021 (2021)

No. 70, Revised Consensus Document on the Biology of Rice (Oryza sativa L.) (2021)

No. 71, Developments in Delegations on Biosafety Issues, April 2021 – May 2022 (2022)

No. 72, Developments in Delegations on Biosafety Issues, June 2022 – April 2023 (2023)

No. 73, Consensus Document on Environmental Considerations for Risk/safety Assessment for the Release of
Transgenic Plants (2023)

No. 74, Developments in Delegations on Biosafety Issues, May 2023 – February 2024 (2024)

No. 75, COLLATION OF THE ANSWERS FOR QUESTIONNAIRE Enhanced Information Exchange on New
Breeding Techniques: 2024 Results (2024)

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23 7

About the OECD

The Organisation for Economic Co-operation and Development (OECD) is an intergovernmental

organisation in which representatives of 38 countries in North and South America, Europe and the Asia
and Pacific region, as well as the European Union, meet to co-ordinate and harmonise policies, discuss
issues of mutual concern, and work together to respond to international problems. Most of the OECD’s
work is carried out by more than 200 specialised committees and working groups composed of member
country delegates. Observers from several Partner countries and from interested international
organisations attend many of the OECD’s workshops and other meetings. Committees and working groups
are served by the OECD Secretariat, located in Paris, France, which is organised into directorates and
divisions.

The Environment, Health and Safety Division publishes free-of-charge documents in twelve different
series: Testing and Assessment; Good Laboratory Practice and Compliance Monitoring; Pesticides;
Biocides; Risk Management; Harmonisation of Regulatory Oversight in Biotechnology; Safety of
Novel Foods and Feeds; Chemical Accidents; Pollutant Release and Transfer Registers; Emission
Scenario Documents; Safety of Manufactured Nanomaterials; and Adverse Outcome Pathways.
More information about the Environment, Health and Safety Programme and EHS publications is available
on the OECD’s World Wide Web site (https://www.oecd.org/en/topics/chemical-safety-and-biosafety.html).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
8  ENV/CBC/MONO(2024)23

This publication is available electronically, at no charge.

For this and many other Biosafety publications, consult the OECD’s
World Wide Web site
(https://www.oecd.org/en/topics/biosafety-novel-food-and-feed-safety.html)

or contact:

OECD Environment Directorate,

Environment, Health and Safety Division
2 rue André-Pascal
75775 Paris Cedex 16
France

E-mail: ehscont@oecd.org

© OECD 2024
Applications for permission to reproduce or translate all or part of this material should be made to: Head of Publications
Service, RIGHTS@oecd.org, OECD, 2 rue André-Pascal, 75775 Paris Cedex 16, France
OECD Environment, Health and Safety Publication

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23 9

FOREWORD

The consensus documents prepared by the OECD Working Party on the

Harmonisation of Regulatory Oversight in Biotechnology (WP-HROB) contain
information for use during the regulatory assessment of the environmental safety
(or ‘biosafety’) of a particular product. In the area of plants, these are being
published on information on the biology of certain species of crops and trees,
selected traits that may be introduced into plant species, and biosafety issues
arising from certain general types of modifications made to plants.
This document addresses the biology of wheat (Triticum aestivum L.).
Australia and the United States served as the co-leads in the preparation of this
document, and the draft has been revised based on the input from other member
countries and stakeholders.
The WP-HROB endorsed this document, which is published under the
responsibility of the Chemicals and Biotechnology Committee of the OECD.
When reading these documents, it is recommended to consult the following OECD
documents with a broader scope which describe core principles for biotechnology
risk assessments.
OECD (2023), OECD Consensus Document on Environmental Considerations for
the Release of Transgenic Plants, https://doi.org/10.1787/62ed0e04-en.
OECD (2022), “Revised points to consider on plant biology consensus
documents”, in Safety Assessment of Transgenic Organisms in the Environment,
Volume 9 https://doi.org/10.1787/e49bd2e8-en

Acknowledgements: The WP-HROB is grateful for the valuable contributions made

to the drafting of this document by Deshui Zhang and Subray Hegde, United States
Department of Agriculture, and the staff from the Office of the Gene Technology
Regulator, Department of Health and Aged Care, Australia.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
10  ENV/CBC/MONO(2024)23

Table of contents

SECTION 1. Species or taxonomic group 12

1.1. Classification and nomenclature 12
1.2. Morphological characteristics and uses 16
1.3. Geographic distribution, natural and managed ecosystems and habitats, cultivation and
management practices, and centres of origin and diversity 19

SECTION 2. Reproductive biology 28

2.1. Generation time and duration under natural circumstances, and where grown or managed 28
2.2. Reproduction 28

SECTION 3. Genetics and breeding 36

3.1. Basic genetic information 36
3.2. Genetic diversity or variability 37
3.3. Methods of classical breeding 38
3.4. Next generation breeding 41
3.5. Intraspecific gene flow 42

SECTION 4. Hybridisation and introgression 43

4.1. Wheat gene pool 43
4.2. Natural facility of interspecific crossing 44
4.3. Experimental crosses 45

SECTION 5. General interactions with other organisms (ecology) 54

5.1. Interactions in agricultural ecosystems 54

References 59
Annex A. Most common diseases and pests in Triticum aestivum 74
Annex B. Biotechnology applications for wheat improvement 78
References in Annexes 80

FIGURES
Figure 1.1. Origin of Triticum aestivum 16
Figure 1.2. Morphology of the Triticum aestivum plant 17
Figure 1.3. Map of wheat production across the world 20

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  11

Figure 1.4. The Fertile Crescent of the Near East 27

Figure 2.1. Features of the wheat spikelet 29
Figure 3.1. Cartoon karyogram of the bread wheat genome 37
Figure 3.2. Schematic representation of the breeding process in bread wheat 38
Figure 4.1. Genetic manipulations of interspecific and/or intergeneric hybrids for chromosomal interchanges
and alien gene introgression 48

TABLES
Table 1.1. The genera of the tribe Triticeae 13
Table 1.2. The Triticum species and subspecies 15
Table 1.3. Wheat quality characteristics for various food types 18
Table 1.4. World wheat market in seasons 2020-2021 and 2021-2022 21
Table 4.1. List of intergeneric species that have been crossed with wheat 47
Table 5.1. Common weeds in wheat crops 55
Table A.1. Viruses, mycoplasms 74
Table A.2. Bacteria 75
Table A.3. Fungi 75
Table A.4. Animals 76

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
12  ENV/CBC/MONO(2024)23

SECTION 1. Species or taxonomic group

1.1. Classification and nomenclature
Triticum aestivum L., bread wheat, is one of the top two cereal crops grown in the world
for human consumption, along with rice (FAO, 2014). It is an annual grass that belongs to
the family Poaceae, subfamily Pooideae and tribe Triticeae (Clayton et al., 2015).
Recommended texts for a comprehensive overview of wheat breeding are The World
Wheat Book: A History of Wheat Breeding, Volumes 1 and 2 (Angus et al., 2011; Bonjean
and Angus, 2001).
Common names for T. aestivum are wheat, cultivated wheat and bread wheat. It is of note
that the word ‘wheat’ is also used in the literature to refer to a number of species that are
related to T. aestivum. Some of these species exist only in the wild but others have wild
and domesticated forms that are currently cultivated or were cultivated in the past. In this
document, either the scientific name or the term ‘bread wheat’ will be used to refer to T.
aestivum.
1.1.1. The tribe Triticeae Dumort
The tribe Triticeae is a group of plants that is vital to human subsistence and was key in
the advent of agriculture in the Fertile Crescent of the Near East (Kilian et al., 2009). It
includes the cereal crops wheat (Triticum sp.), barley (Hordeum sp.), rye (Secale sp.) and
triticale (Triticosecale sp.) that account for one third of the cereal production of the world
(Feuillet, Langridge and Waugh, 2008). The tribe Triticeae contains 32 genera and over
500 species, of which approximately 100 are annual and 400 are perennial grasses
(Feldman and Levy, 2015; Liu et al., 2016; Wang and Lu, 2014).
Hybridisation has played an important role in Triticeae speciation and evolution (Liu et al.,
2016). The Triticeae polyploid1 species originated by an unknown mechanism, though is
likely to be a result of multiple rounds of homoploid and polyploid hybrid speciation
(Marcussen et al., 2014).
The basic chromosome number 2 in Triticeae is seven (x = 7; Heslop-Harrison, 1992).
Triticeae species have ploidy levels ranging from 2x to 12x (Liu et al., 2016). Cultivated
species are diploid 3 (2n = 2x = 14; barley and rye), tetraploid 4 (2n = 4x = 28; durum or
pasta wheat), or hexaploid 5 (2n = 6x = 42; bread wheat, spelt and triticale 6 ) (Feuillet,
Langridge and Waugh, 2008). Every diploid species has its own genome and the polyploid
species contain the genomes of their diploid progenitors. The genomes are represented

The statistical data for Israel are supplied by and under the responsibility of the relevant Israeli
authorities. The use of such data by the OECD is without prejudice to the status of the Golan Heights,
East Jerusalem and Israeli settlements in the West Bank under the terms of international law.
1
Polyploid: organism containing more than two homologous sets of chromosomes.
2 Basic chromosome number: the number of chromosomes in each set of chromosomes, which is

constant for any one species of plant or animal. It is represented by the letter x. The number of
chromosomes in each somatic cell is 2n.
3 Diploid: organism containing two sets of chromosomes (2x in somatic cells).

4 Tetraploid: organism containing four sets of chromosomes (4x in somatic cells).

5 Hexaploid: organism containing six sets of chromosomes (6x in somatic cells).
6 Tetraploid to octaploid variants also exist, but cultivated versions are primarily hexaploid (Bernard

and Bernard, 1987)

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  13

by the symbols A to W, Ta, St, and Ns (Liu et al., 2016). Taxonomic treatment of these
species is challenging and often controversial. To reflect the new and robust taxonomic
information at either a morphological or molecular level, or both, frequent updates have
been made to taxonomic treatments of Triticeae species, leading to the creation of multiple
taxonomic names at either the generic or species level for the same taxon (Barkworth and
von Bothmer, 2009; Bernhardt, 2015). Based on the most commonly used taxonomy
(Barkworth and von Bothmer, 2009; Bernhardt, 2015; Feldman and Levy, 2015; Liu et al.,
2016; Wang et al., 1994), the genera of the tribe Triticeae are summarised in Table 1.1..

Table 1.1. The genera of the tribe Triticeae

Genus1, 2, 3, 4 Ploidy Genomic composition1, 2, 4, 5 Growth habit2, 3, 4
level3, 4
Aegilops L. 2x-6x B, C, D, M, N, X, U, BU, CU, CD, DM, DN, Annual
MU, DDM, BDM, DMU
Agropyron Gaertn. 2x-6x P Perennial
Amblyopyrum (Jaub. & Spach.) Eig 2x T Annual
Anthosachne Steud. 6x StHW Perennial
Australopyrum (Tzvelev) Á. Löve 2x W Perennial
Crithopsis Jaub. & Spach. 2x K Annual
Dasypyrum (Coss. & Dur.) T. Dur. 2x-4x V Perennial/Annual
Douglasdeweya C.Yen, J.L. Yang, & B.R. Baum 4x StP Perennial
Elymus L. 2x-12x St plus at least one of H, W, Y Perennial/Annual
Elytrigia Desv. 2x-10x St, E, H, N Perennial
Eremiun Seberg & Linde-Laursen 6x Ns Perennial
Eremopyrum (Ledeb.) Jaub.&Spach 2x-4x F, Xe Annual
Festucopsis C.E. Hubb 2x L Perennial
Henrardia C.E. Hubb 2x O Annual
Heteranthelium Jaub.&Spach 2x Q Annual
Hordelymus (Jessen) Harz 4x/10x Ns Perennial
Hordeum L. 2x-6x H, I, Xa, Xu, HXa Perennial/Annual
Hystrix Moench 4x StH or Ns Perennial
Kengyilia C. Yen & J.L. Yang 6x StPY Perennial
Leymus Hochst 4x-8x Ns Perennial
Pascopyrum Á. Löve 8x St, H, N Perennial
Peridictyon Seberg, Fred. & Baden 2x Xp Perennial
Psathyrostachys Nevski 2x Ns Perennial
Pseudoroegneria (Nevski) Á. Löve 2x-4x St Perennial
Roegneria K. Koch 4x/6x St, Y Perennial
Secale L. 2x R Perennial/Annual
Stenostachys Turcz. 4x HW Perennial
Taeniatherum Nevski 2x Ta Annual
Thinopyrum Á. Löve 4x-12x E sometimes with P, St, or L Perennial
Triticum L. 2x-6x A, B/G, D Annual

Sources: 1 Bernhardt (2015); 2 Barkworth and von Bothmer (2009); 3 Feldman and Levy (2015); 4 Liu et al. (2016); and
5 Wang et al. (1994).

1.1.2. The Triticum-Aegilops complex

The genera Triticum and Aegilops, sometimes referred to as the wheat group (Feldman
and Levy, 2012), contain the diploid species that through hybridisation gave rise to modern
cultivated wheat (see Section 1.1.4.). Some Triticum and Aegilops diploid species are

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
14  ENV/CBC/MONO(2024)23

closely related, differing only in small morphological features, and are relatively easy to
cross with each other resulting in polyploid species, to the extent that it has even been
proposed that these two genera should be merged (Bowden, 1959; Yen, Yang and Yen,
2005).
Aegilops species, commonly known as goatgrass, are annual grasses that occur around
the Mediterranean Sea and Central Asia (van Slageren, 1994). The species are diploid,
tetraploid or hexaploid and contain the genomes D, U, C, M, N and S. Hybridisation
between Aegilops and Triticum occurs between diploid and polyploid species both naturally
and by human-assisted crossing (van Slageren, 1994).
1.1.3. The genus Triticum L.
The genus Triticum L. includes bread wheat and its close relatives (Table 1.2.) (Matsuoka,
2011). This genus consists of six species of annual grasses that are diploid (section
Monococcon), tetraploid (section Dicoccoidea), or hexaploid (section Triticum). The
genomes found in Triticum are A, B, D and G. The Triticum species exist in cultivated and
wild forms, except for T. urartu that exists only in a wild form and T. aestivum and
T. zhukovskyi that exist only as cultivated forms (Matsuoka, 2011).
The most important wheat species grown today are the hexaploid bread wheat
(T. aestivum) and the tetraploid durum or pasta wheat (T. turgidum subsp. durum) but
during the history of wheat domestication other wheats were cultivated.
● Diploid wheat

T. monococcum subsp. monococcum or einkorn wheat (genome AmAm) arose from the
domestication of wild einkorn (Table 1.2.) and was probably the first wheat species widely
cultivated, starting around 10,000 years ago in South Eastern Türkiye (Heun et al., 1997;
Feuillet, Langridge and Waugh, 2008). Einkorn wheat cultivation has been largely
abandoned and replaced by tetraploid and hexaploid wheats (Kilian et al., 2009). Notable
locations where it is still grown are provided in Table 1.2., where it is grown as food, feed,
or a source of genetic variation for breeding (Nesbitt and Samuel, 1996; Ozkan et al., 2007;
Perrino, 1996).
T. urartu (genome AA) was never domesticated but played a critical role in wheat evolution
as the donor of the A genome found in polyploid wheats (Kilian et al., 2009).
● Tetraploid wheat

The tetraploid wheat species originated with the hybridisation of diploid species of Triticum
and Aegilops. One of the two lineages of tetraploid wheat is T. turgidum (genome BBAA),
known as emmer wheat. Domestication of emmer wheat gave rise to a range of subspecies
that were cultivated across the globe for thousands of years (Table 1.2.). One of them,
durum wheat (T. turgidum subsp. durum), is widely cultivated today. Durum wheat is
consumed as macaroni and semolina products (Matsuoka, 2011).
● Hexaploid wheat

The hexaploid wheat species emerged through natural hybridisation between tetraploid
cultivars and diploid Triticum and Aegilops species (Huang et al., 2002; Kilian et al., 2009).
T. aestivum (genome BBAADD) accounts for 90% of the world wheat production today and
is composed of several subspecies (Table 1.2.).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  15

Table 1.2. The Triticum species and subspecies

Section Species and Seed Genome Common Geographic distribution3
subspecies1 form2 composition names
Monococcon T. monococcum L.
(2n = 2x = 14) subsp. aegilopoides Hulled A bA b Wild einkorn Balkans, N. Greece, W. Türkiye
(syn. T. boeticum)
subsp. monococcum Hulled AmAm Cultivated einkorn Türkiye, Italy and Spain, Transcaucasia
T. urartu Tumanian ex Hulled AA Fertile Crescent
Gandilyan
Dicoccoidea T. turgidum L. BBAA
(2n = 4x = 28) subsp. dicoccoides Hulled Wild emmer S.E. Türkiye, Israel, S. Syria, N. Iraq, W. Iran
subsp. dicoccon Hulled Cultivated emmer India, Ethiopia, Yemen, Iran, E. Türkiye,
Transcaucasia4, the Volga Basin, ex-
Yugoslavia, Central Europe, Italy, Spain
subsp. durum Naked Durum or Mediterranean climate areas
macaroni wheat
subsp. polonicum Naked Polish wheat S. Europe, Türkiye, Iraq, Iran, Armenia, N.W.
India
subsp. turanicum Naked Khorassan wheat Europe, United States, Iran, Middle East
subsp. turgidum Naked Rivet wheat Portugal, United Kingdom, Spain
subsp. carthlicum Naked Persian wheat Caucasia5, Iraq, Iran
subsp. paleocolchicum Hulled Georgian wheat Georgia
T. timopheevii (Zhuk.) GGAA
Zhuk.
subsp. armeniacum Hulled Wild timophevii Transcaucasia
subsp. timopheevii Hulled Cultivated Transcaucasia, Armenia, N. Iraq, Iran
timopheevii
Triticum T. aestivum L. BBAADD Common wheat
(2n = 6x = 42) subsp. aestivum Naked Bread wheat Temperate regions of the world
subsp. compactum Naked Club wheat Mountains of Afghanistan, Alps
subsp. sphaerococcum Naked Indian dwarf Afghanistan, Bukhara, N.W. India
wheat
subsp. macha Hulled Macha wheat Georgia/Transcaucasia
subsp. spelta Hulled Spelt Central Europe, Middle East, United States,
Canada
T. zhukovskyi Menabde Hulled GGAAAmAm Transcaucasia, Armenia, N. Iraq, Iran
& Ericz

Notes: 1. Based on Van Slageren (1994). For other taxonomic classifications see Wheat Genetic Resource Centre (Kansas State University).
2.Hulled wheat has a hull (dry outer covering) that adheres to the grain. Hull-less wheat or “naked” wheat has an outer hull that is
loosely attached to the kernel so it generally falls off during harvesting (see Section 1.2.1.).
3.Based on Matsuoka (2011) and Zaharieva et al. (2010). N, north; S, south; W, west; E, east.
4.Transcaucasia: geographical region in the vicinity of the southern Caucasus Mountains on the border of Eastern Europe and Western
Asia. Transcaucasia roughly corresponds to modern Georgia, Armenia, and Azerbaijan.
5. Caucasia: region situated at the border of Eastern Europe and Western Asia between the Black Sea and the Caspian Sea. It extends
across territories of the countries Georgia, Armenia, Azerbaijan and Russian Federation.
Source: Adapted from Matsuoka (2011).

1.1.4. The origin of Triticum aestivum

Bread wheat originated from two hybridisation events involving three different diploid
progenitors in the Triticum and Aegilops genera (Figure 1.1.). T. urartu was the donor of
the A genome of bread wheat (Chapman, Miller and Riley, 1976; Dvořák, 1976; Kimber
and Sears, 1987; May and Appels, 1987) while Ae. tauschii was the donor of the D genome
(Huang et al., 2002; Kihara, 1944; McFadden and Sears, 1946). Recommended texts for

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
16  ENV/CBC/MONO(2024)23

a comprehensive overview of wheat breeding are The World Wheat Book: A History of
Wheat Breeding, Volumes 1 and 2 (Bonjean and Angus, 2001; Bonjean, Angus and van
Ginkel, 2011). The identity of the B genome donor is still unknown and there is some
controversy about this issue (Huang et al., 2002). Much evidence suggests that either Ae.
speltoides or its undiscovered ancestor from the section Sitopsis were the donor of what
became the B genome in tetraploid and hexaploid wheats (Kilian et al., 2006; Petersen et
al., 2006; Sallares and Brown, 2004; Sarkar and Stebbins, 1956).
The first step in the creation of bread wheat involved the hybridisation of T. urartu (AA) with
the B genome donor to generate the tetraploid emmer wheat (T. turgidum subsp.
dicoccoides; genome BBAA) around 0.5 million years ago. Analysis of chloroplast and
mitochondrial genomes showed that this hybridisation occurred with the B genome
progenitor as the maternal parent (Tsunewaki, 1988). The second step occurred around
8,000 years ago and involved hybridisation between the tetraploid emmer wheat (genome
BBAA) and Ae. tauschii (genome DD) to form T. aestivum (genome BBAADD). Emmer
wheat was the maternal parent in this hybridisation event and provided the B genome
cytoplasm to T. aestivum (Tsunewaki, 1988). Since wild populations of T. aestivum have
never been found, it is thought that cultivated emmer wheat, not the wild version, hybridised
with Ae. Tauschii.

Figure 1.1. Origin of Triticum aestivum

Note: Diagram representing the two hybridisation events that gave rise to bread wheat and the evolutionary history of
durum wheat, the second most cultivated wheat. Letters in brackets represent the genome composition of the species.
Source: Illustration courtesy Maria Alonso, Office of the Gene Technology Regulator (OGTR), Australia.

1.2. Morphological characteristics and uses

1.2.1. Morphological characteristics of bread wheat
The mature bread wheat plant is an annual grass that consists of a central stem from which
leaves emerge at opposite sides (Figure 1.2.A). The stem is erect, hollow or pithy,
glabrous7, and up to 1.2 m tall. It is made up of repeating segments which contain a node,
a hollow internode, a leaf and a bud and terminates in the ear of the wheat plant (Kirby,
2002). Lateral branches called tillers, which emerge from buds near the base of the stem,
can produce an ear at their terminal too (Figure 1.2.A) (Kirby, 2002; Setter and Carlton,
2000b). The root system is composed of seminal roots produced by the young plant during
germination, and adventitious roots that arise later from the basal nodes of the plant to
become the permanent root system. The root system can grow 1-2 m deep, but most roots
are concentrated in the top 30 cm of soil (Kirby, 2002).

7 Glabrous: free from plant hairs (trichomes). Smooth.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  17

As in all grasses, the leaves consist of the sheath and the blade (Figure 1.2.A), with the
leaf sheath wrapping around the stem (Setter and Carlton, 2000b). The flag leaf blades are
20-38 cm long, about 1.3 cm wide and have an ascending, arching or floppy disposition.
At the base of the leaf blade, where it joins the sheath, are a membranous ligule and a pair
of small hairy projections known as auricles (Figure 1.2.B) (Kirby, 2002). The aspect of
ligules and auricles is used to determine the species of cereal seedlings (Agriculture
Victoria, 2012b). The final leaf before the ear is called the flag leaf.

Figure 1.2. Morphology of the Triticum aestivum plant

Notes: A, Diagram of a wheat plant. B, Detail of the leaf blade and sheath junction. C, Detail of a spike (ear). D, Different
parts of a spikelet. E, Detail of the floral organs. F, Diagram of a cross section of a wheat seed.
Source: Illustration courtesy Maria Alonso, OGTR, Australia.

The ear of bread wheat is an erect floral spike about 5-10 cm long. The spike is made of
two rows of spikelets (Figure 1.2.C and D). The spikelets contain the florets (flowers) and
are arranged on opposite sides of a central rachis (central stalk of the spike; Setter and
Carlton, 2000b). Further details on floral biology are located in Section 2.2.1..
The caryopsis or grain of bread wheat is 7.5-8.5 mm long, 3.5-3.75 mm wide, and ovoid-
ellipsoid in shape (Figure 1.2.F). The grain is made up of the bran coat, the endosperm
and the embryo (Figure 1.2.F) (Setter and Carlton, 2000b).. The endosperm makes up
83% of the wheat grain and stores the starch and protein (aleurone) important both for the
developing plant and flour production (Setter and Carlton, 2000b).
1.2.2. Uses of bread wheat
The primary use of bread wheat is to produce flour, the main ingredient for baking bread.
Bread wheat flour is also used to produce other baked goods, confectionery products,
noodles and wheat gluten or seitan (a powdered form of purified wheat gluten, used as
an alternative to soy based products in vegetarian cooking) (Pomeranz, 1987).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
18  ENV/CBC/MONO(2024)23

Bread wheat for human food is classified into grades according to a number of quality
attributes that dictate its suitability for various end-uses (Table 1.3.). The main quality
attributes are grain hardness, protein content and flour strength type (Peña, 2002).
Grain hardness is determined by the way components are packed in the endosperm cells
and refers to the grain’s resistance to being fractured (Peña, 2002). Hard wheats require
longer milling times and more milling energy and produce a larger amount of damaged
starch. Hard wheats are preferred for bread making because the damaged starch
increases the water absorption of the dough. In contrast, the cookie and cake industries
use soft wheat flour since this type of baking requires free water in the form of vapour to
expand the doughs and batters.

Table 1.3. Wheat quality characteristics for various food types

Product type Grain hardness Protein content (%) Flour strength type
Leavened breads
Pan-type, buns Hard >13 Strong-extensible
Hearth-French Hard-medium 11-14 Medium-extensible
Steamed Hard-soft 11-13 Medium-weak
Unleavened breads
Arabic Hard-medium 12-14 Medium-extensible
Chapati, tortilla Medium 11-13 Medium-extensible
Crackers Medium-soft 11-13 Medium
Noodles
Yellow alkaline Medium 11-13 Medium-strong
White Medium-soft 10-12 Medium
Cookies, cakes, pastries Soft-Very Soft 8-10 Weak, Weak-extensible

Source: Peña (2002)

Grain protein content in bread wheat is 8-17%, depending on the genetic make-up and on
external factors associated with the crop. Most of the wheat grain protein is gluten. The
gluten found in wheat flour forms a viscoelastic mass when in contact with water and
determines most of the viscoelastic properties of wheat flour doughs. Gluten viscoelasticity
is commonly known as flour or dough strength and it is the main factor dictating the end-
use of a wheat variety in bread and pasta making. Flour strength is dependent on both
protein content and protein composition of the wheat grain (Peña, 2002).
Other than its primary use as a human food source, bread wheat has a number of
alternative uses. These include, but are not limited to, use in animal feed, starch
production, bioethanol production, brewing of wheat beer, the production of wheat-based
cat and pet litter, wheat-based raw materials for cosmetics and to make wheat straw
composites. Wheat straw can also be used as a fuel for heat and electricity (OGTR, 2017;
Shevkani et al., 2017).
The use of wheat grain in the animal feed market has increased in recent years
concomitantly with the worldwide demand for meat. The amount of wheat used as feed
varies each year depending on its price competitiveness with respect to coarse grains like
corn and sorghum. Although an average of 19% of wheat was used for feed worldwide in
the 2014/2015, 2015/2016 and 2016/2017 seasons, this proportion can reach 42-49% in
developed countries in Europe and Australia, which potentially surpasses the amount of
wheat destined for food (Heuzé et al., 2015; Tasmanian Institute of Agriculture, 2014).
The main consumers of feed wheat are the pork and poultry industries, the beef feedlot

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  19

industry and the dairy industry. Wheat grain can be fed whole or processed in many
different ways like dry rolling, steam rolling, flaking or grinding followed by pelleting (Heuzé
et al., 2015). Feed wheat is often surplus to human requirements or is low quality wheat
unsuitable for human consumption. However, wheat is increasingly grown specifically for
feed purposes, which has led to the introduction of specialty feed wheat lines. Wheat can
also be used as winter pasture and forage source (Heuzé, Trann and Baumont, 2015).
Wheat forage may be grazed and/or cut for hay and silage. Dual purpose wheat varieties
can also be used that provide good quality forage during late fall and winter and
harvestable grains the next summer (Heuzé, Trann and Baumont, 2015).
The forecast of industrial utilisation of wheat in 2021/2022 is 24.4 million tonnes (IGC,
2022), the majority of which is for use in the starch industry followed by bioethanol
production (FAO, 2021). Wheat-based ethanol production represents a large fraction of
production in the EU and Canada that accounted for 4.36% and 1.40% of global ethanol
production in 2008, respectively (Saunders, Izydorczyk and Levin, 2011). Production of
ethanol from grain wheat involves hydrolysis of extracted starch to glucose or maltose,
which is then fermented to produce ethanol and carbon dioxide (Nigam, 2001; Sparks
Companies Inc., 2002). The co-product of this process is used in animal feed, which
reduces the cost of ethanol production. However, wheat-based ethanol production is less
efficient than corn or sugar beet-based production (European Biomass Industry
Association, 2017). In countries where wheat is a major agricultural crop, wheat-based
ethanol would benefit from the development of high starch, low protein varieties of wheat
(similar to feed wheat varieties) with characteristics that are ideally suited for bioethanol
production.

1.3. Geographic distribution, natural and managed ecosystems and habitats,

cultivation and management practices, and centres of origin and diversity
1.3.1. Geographic distribution
Wheat (T. aestivum and T. durum combined) 8 is the most abundant crop world-wide,
occupying 22% of the total cultivated area in the world (Leff, Ramankutty and Foley, 2004),
which accounts for more than 216,000,000 hectares (Monfreda et al., 2008). The most
intensive wheat cultivation occurs in the temperate latitudes of both hemispheres (Curtis
et al., 2002; Heyne, 1987). Bread wheat production is concentrated between the parallels
of latitude 30-60° in the North Temperate Zone and 27-40° in the South Temperate Zone
(Briggle and Curtis, 1987; Curtis et al., 2002; Kimber and Sears, 1987; Nuttonson, 1955;
Korber-Grohne 1988, Geisler 1991). The northern limit of bread wheat cultivation in Europe
lies in southern Scotland (United Kingdom) (60° latitude) and occasionally beyond (central
Scandinavia up to 64° latitude) and in North America, wheat is grown to about 55° latitude.
Cultivation in the Northern Hemisphere extends as far south as in the mountain regions of
Mexico, and at the Equator on the high lands of Ecuador and Colombia (Nuttonson, 1955;
Percival, 1921). Wheat grows well from sea level up to heights of about 4500 m above sea
level (Briggle and Curtis, 1987; Kimber and Sears, 1987). Wheat is most prevalent in the
Great Plains of the United States, the Canadian Prairie Provinces, the Indus and the upper
Ganges Valleys, along the Kazakhstan and Russian border, and in southern Australia
(Figure 1.3.) (Heyne, 1987; Leff, Ramankutty and Foley, 2004). Wheat is also found
throughout Europe, in southern South America, in parts of eastern Africa, and in eastern
People’s Republic of China (Leff, Ramankutty and Foley, 2004).

8 90-95% of wheat worldwide production corresponds to T. aestivum and the rest to T. turgidum
durum.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
20  ENV/CBC/MONO(2024)23

Figure 1.3. Map of wheat production across the world

Source: AndrewMT, CC BY-SA 3.0, via Wikimedia Commons.

The global production of wheat (T. aestivum and T. durum combined) forecast for 2021-
2022 is estimated at 785.8 million tonnes (FAO, 2021). Food consumption accounts for
most of the global wheat utilisation followed by its use as animal feed (Table 1.4.). Human
wheat consumption, per person, was 67.3 kg per annum worldwide (2020-2021) with
an average of 62.4 kg per capita in developing countries and 93 kg per capita in developed
countries.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  21

Table 1.4. World wheat market in seasons 2020-2021 and 2021-2022

World balance 2020-2021 Estimate* 2021-2022 Forecast*
Production 774.8 785.8
Trade1 186.2 187.2
Total utilisation 759.5 778.6
Food 524.7 530.9
Feed 144.7 155.5
Other uses 90.1 92.2
Ending stocks2 291.0 298.7

Note: * million tonnes.

1 Exports based on a common July/June marketing season.
2 Wheat accumulated in world inventories.

Source: FAO (2021).

The top ten wheat producers in 2021 (listed in decreasing order) were the European Union
(hereafter “EU”), People’s Republic of China (hereafter “China”), India, Russian Federation
(hereafter “Russia”), United States, Canada, Ukraine, Australia, Pakistan and Republic of
Türkiye (hereafter “Türkiye”). Of these, the major exporters forecast for 2021/2022 are
Russian Federation, European Union, the United States, Canada and Australia, and the
total tonnage of wheat exported worldwide is estimated to reach 187.2 million tonnes.
The largest importers of wheat are Egypt, China, Indonesia, Türkiye, Algeria, Brazil,
Bangladesh, Philippines, Nigeria and Japan (FAO, 2021).
1.3.2. Ecosystems and habitats where the species occur natively, and where it has
naturalised
Bread wheat is a crop plant species with low competitive ability, and it has no natural
habitat outside cultivation (Tutin, 1980). There are a number of reports of bread wheat
becoming naturalised in areas where it is not a native species, including California
(Calflora, 2019), the Canadian Prairies and North American central Great Plains (Harker
et al., 2005 and references therein). Although bread wheat plants do not have high
potential for weediness (Keeler, 1989), they may sometimes be found in ‘disturbed’ areas
where there is little or no competition from other ‘weed’ species (e.g. waste places, fallow
fields, along roadsides). However, their survival at such sites is limited to short periods
(Illinois wildflowers, 2019) and there are no indications that bread wheat plants can become
established as a self-sustaining population on a long-term basis (Newman, 1990).
1.3.3. Agronomic, silvicultural, and other intensively managed ecosystems where
the species is grown or occurs on its own, including management practices
Bread wheat is grown across a wide range of environments around the world. It is a cool
season crop requiring a minimum temperature for growth of 3-4°C, with optimal growth
occurring around 25°C and tolerance of temperatures to a maximum of about 32°C. Seed
germination may occur between 4-37°C, with the optimal temperature ranging from 12-
25°C (Acevedo, Silva and Silva, 2002).
Winter and spring wheats differ in the length of their life cycle and temperature
requirements (see Section 2.1.). Spring wheat is planted in locations with severe winters
and flowers in the same year yielding grain in approximately 90 days. The cold tolerance
for seedlings of spring wheat is -5°C and usually require temperatures between 7-18°C for
5-15 days for floral induction (Evans, Wardlaw and Fischer, 1975; Acevedo, Silva and
Silva, 2002). Winter wheat is grown in locations with less severe winters and will only head

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
22  ENV/CBC/MONO(2024)23

after it has received a cold treatment (vernalization). Winter wheat has a maximum cold
tolerance of about -25°C and requires temperatures between 0-7°C for 30- 60 days (Evans,
Wardlaw and Fischer, 1975; Acevedo, Silva and Silva, 2002). Winter wheat is therefore
planted in the fall and harvested in the spring of the following year. Flowering begins above
14°C (Acevedo, Silva and Silva, 2002).
Wheat grows best in well drained soils. Wheat will grow in areas receiving 250-1750 mm
annual precipitation, but most wheat production occurs in areas receiving 375-875 mm
annually (Briggle and Curtis, 1987; Kimber and Sears, 1987). Wheat can be grown in
dryland (rain-fed) or under irrigation. More than 95% of the developed world’s wheat is
grown in dryland (Sayre, 2002). Large wheat production areas in the United States,
Canada and Australia are produced under low rainfall conditions, whereas most wheat
production in Western Europe is produced under favourable rainfall conditions (Sayre,
2002). Winter wheat is grown in Argentina, Australia, Brazil, China, the EU, India, Pakistan,
Ukraine and the United States. Spring wheat grown under irrigated conditions is more
common in Canada and Russia and is also grown in northwest India, Southern-Central
China and the United States (Sayre, 2002; Becker-Reshef et al. 2023).
General agronomic practices for dryland bread wheat
Winter-dominant rainfall areas contribute a major part of the world’s dryland wheat
production. In these areas 70-100% of the annual rainfall occurs during the growing
season, from late fall when wheat is sown to early summer when it is harvested (Anderson
and Impiglia, 2002).
No-till or low-till practices in wheat dryland faming are common as they help to conserve
moisture and improve soil structure, reduce erosion, increase yields and in some cases
decrease disease (Jarvis et al., 2000). Another major trend is stubble retention where the
growing surface is covered by previous crop residues. This improves water retention and
increases nitrogen availability (Anderson and Impiglia, 2002). Dryland wheat is sown at a
rate of 30-50 kg/ha in most regions to achieve a plant population of at least 700,000
plants/ha (70 plants/m2). In irrigated Mediterranean conditions, highest yields can be
achieved at 400-500 plants/m2 (Lloveras et al., 2004). Plant populations below this density
may result in a reduction in yield and increased weed competition (DAF, 2012). Advances
in equipment for minimum and no-till systems has incorporated sowing implements that
create furrows where seeds are deposited and subsequently covered with soil. The furrow
harvests water into the seed row and ensures good seed soil contact (Agriculture Victoria,
2012a). Optimum sowing depth for wheat is around 50-70 mm and seeds are placed in
rows from 15-50 cm apart (Anderson and Impiglia, 2002).
Continuous wheat growing is becoming less common and wheat cultivation is more
frequently part of cereal-fallow, cereal-grain-legume, cereal-pasture and cereal-oilseed-
legume rotations. Continuous wheat is still common in some places, particularly the United
Kingdom and other parts of Europe, though it may come with lower yield stability than by
using crop rotations, but this potentially evens out in the long-term (Macholdt et al., 2020;
Macdonald, 2018; St-Martin et al., 2017; Steinmann and Dobers, 2016). Fixed rotations
are common in the more traditional areas around the Mediterranean basin, but in other
areas more flexible cropping sequences are likely to be found, driven by fluctuations in the
prices of wheat and other products. Crop rotation is practiced to control diseases, weeds
and insects, to improve soil fertility (mainly from the inclusion of legumes that fix nitrogen),
to spread the risk of crop failure and to stabilise farmer income (Anderson and Impiglia,
2002).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  23

The three main nutrients required for successful production of a wheat crop are nitrogen
(N), phosphorus (P) and potassium (K). Depending on soil type and historical use of the
land, these nutrients may be deficient in the soil. It is estimated that every two tonnes per
hectare of wheat grain takes 42 kg of N, 9 kg of P, 10 kg of K and 2.5 kg of sulphur out of
the soil (Laffan, 1999). Protein production in the wheat grain is reliant on substantial
nitrogen levels in the soil, therefore higher amounts of N are required if high grain yield and
protein levels are expected (GRDC, 2015). Nitrogen fertiliser is commonly added to
the field before sowing but it can be added again in fractionated doses prior to flowering,
which can be beneficial in Mediterranean growth climates (Laffan, 1999). The trace
element deficiencies (zinc, manganese and iron) are most prevalent in winter-rainfall areas
and those associated with alkaline soils. These deficiencies can be corrected with small
additions to the base fertiliser. The presence of boron and aluminium in the soil can cause
toxicity and this is best addressed by using tolerant cultivars (Anderson and Impiglia,
2002).
Weed management can be the most significant cost in wheat production (Bowran, 2000).
Seeds of some weeds, when harvested and mixed with the wheat grain, can reduce flour
quality (Wolff, 1987). Other costs associated with weeds are yield loss from competition
between the crop and the weeds and the cost of applying appropriate control measures
(Anderson and Impiglia, 2002). As wheat is most sensitive to weeds during the early stages
of its life cycle, the reduction of the weed seed bank in the seasons before cropping and
early management of weeds will reduce the weed-associated risks of crop losses
(Anderson and Impiglia, 2002). Integrated weed management strategies including
agronomic approaches (rotations, row spacing, seed densities, stubble management etc.),
biological approaches (for example choice of herbicide resistant cultivars) and chemical
approaches to control weeds are likely to be the most effective (Bowran, 2000; GRDC,
2014, 2015).
Leaf and root diseases can affect dryland wheat production but grain yields have seldom
been high enough to warrant chemical spray for disease control on a routine basis
(Anderson and Impiglia, 2002). The exception is through Europe, where the highest yields
have historically been obtained, accompanied by high use of fungicides (Jenkins and
Lescar, 1980; Jørgensen et al., 2008). Breeding for resistance and agronomic control
methods are more widely used. Root diseases that are transmitted to the wheat crop from
alternative grass hosts have been successfully controlled by the use of break crops such
as pulses and non-legume broadleaf crops.
Generally, insects do not pose a major threat to wheat in dryland areas. The chances of
insect damage are greater in better seasons and in higher yielding crops. Monitoring of
pest numbers and establishing economic thresholds can reduce the unnecessary use of
pesticides and allow the selection of chemicals that are most effective on the pest species.
Breeding for resistance to pests is also an important component of integrated pest
management systems (Anderson and Impiglia, 2002).
For more information on wheat pests and diseases see Section 5 and Annex A.
General agronomic practices for irrigated bread wheat
In contrast to dryland wheat, irrigated wheat production systems depend almost entirely on
conventional intensive tillage practices especially where irrigation is by flood irrigation or
by furrows (Sayre, 2002). In addition, crop residue yields can be quite high in irrigated
systems making it difficult to retain stubble or implement zero-tillage planting systems.
Irrigated wheat is generally sown on raised beds separated by 70-90 cm wide furrows. The
furrows are used for water delivery and to allow better access to the plants which improves

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
24  ENV/CBC/MONO(2024)23

mechanical weed control, irrigation water management and fertiliser use efficiency (Sayre,
2002). By using the same beds for successive crops, the need for tillage is reduced and
crop residues can be kept and spread in the furrows. Irrigated wheat is sown at a rate of
100-200 kg/ha to achieve a population of 100-150 plants/m2 of bed area (GRDC, 2012;
Rawson and Gómez Macpherson, 2000).
In South and East Asia, irrigated wheat is mostly grown in a continuous rice-wheat rotation
(Chhokar et al., 2007; Sayre, 2002). When not grown in rotation with rice, wheat is
cultivated in rotation with various upland crops including cotton, soybean, sugar cane,
maize and sorghum. In Mexico, Chile, Egypt and Zimbabwe, wheat is grown in annual
rotations with maize, soybean or cotton (Sayre, 2002).
Fertiliser use and management is crucial in irrigated production systems since yield
potential is high, leading to an extensive removal of essential nutrients from the soil.
Nitrogen tends to be the nutrient that is applied at highest rates and costs the most for
most farmers growing irrigated wheat (Sayre, 2002). To increase use efficiency, nitrogen
is best applied at different developmental stages coinciding with the periods of highest
nitrogen demand (Lacy and Giblin, 2006; Ortiz-Monasterio, 2002; Sayre, 2002).
Phosphorus may be applied at sowing near the seed when required (Lacy and Giblin,
2006).
Weed control must be undertaken before and after sowing to avoid yield loss (Chhokar et
al., 2007). Early removal of weeds with pre-emergent herbicides consistently produces
greater yields than if weeds are left until the crop has tillered.
Due to the high plant density in irrigated wheat cultivation, plants are prone to lodging 9 and
foliar diseases (Lacy and Giblin, 2006). The profitability of fungicide application increases
in high-yielding irrigated wheat. Disease control through a combination of disease-resistant
varieties and fungicide seed and leaf treatments is recommended (Sayre, 2002). It is also
advisable to sow wheat after a break crop or long fallow to improve soil health and reduce
root disease (Rawson and Gómez Macpherson, 2000). Lodging can also be addressed
through sowing lodging resistant varieties.
Harvesting and processing bread wheat
Harvest generally occurs in late spring and early summer. Grain must be harvested in a
timely manner to minimise pre-harvest losses due to shattering, pre-harvest sprouting, bird
damage or weathering. Ideally wheat is harvested when the moisture content is 10-20%
(Setter and Carlton, 2000a). Alternately, grain may be harvested at a moisture content
higher than is safe for storage and then dried in windrows, sheaves, stooks or shocks
(Payne, 2002). During threshing, cracking and breaking the grain should be avoided since
damaged grain incurs greater damage from storage moulds and insects, and reduces
marketability.
After a crop has been harvested and threshed, the grain, if necessary, is cleaned, i.e. by
removal of inert matter, seed of weeds, other crops and other varieties, and seeds that are
diseased, damaged and deteriorated. Wheat seed cleaning is performed using mainly
screens, indented cylinders and air (for review see Payne, 2002; van Gastel, Bishaw and
Gregg, 2002). Wheat seed can be treated with fungicides or insecticides for protection
during storage.

9 Lodging is when a plant has been flattened in the field or damaged so that it cannot stand upright, e.g. as
a result of weather conditions or because the stem is not strong enough to support the plant.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  25

Bread wheat seed production

Since wheat is a self-pollinating crop and the grain can be used as seed, farmers tend to
replant their own seed (van Gastel, Bishaw and Gregg, 2002). Wheat farmers typically
purchase a relatively small amount of a new variety of improved seed and then save their
own seed for planting for quite a long period of time. Depending on the growing region
there can be royalty requirements or restrictions on seed saving, particularly where the
seeds are protected by intellectual property laws.
When a variety is officially released, the small amount of breeder seed received from the
breeder (agricultural research centre or private company) is multiplied through a number
of generations before it becomes available to the farmers in larger quantities as certified
seed. Each generation is produced under strict supervision and must meet seed quality
standards. There are three categories of seed under the OECD Decision Revising the
Schemes for the Varietal Certification or the Control of Seed Moving in International Trade
(‘OECD Seed Schemes’) [OECD/LEGAL/0308]; Pre-Basic Seed, Basic Seed, and Certified
Seed (OECD, 2022). These categories, all derived from the parental material or breeder
seed, are summarised by van Gastel, Bishaw and Gregg (2002):
“Breeder seed is the initial source of seed and is usually produced by the breeder.
It is the source for the production of pre-basic or basic seed. Pre-basic seed is
the progeny of the breeder seed and is usually produced under the supervision of
a breeder or his designated agency. This generation is commonly used for crops
that have low multiplication ratios and where large quantities of certified seed are
required. Basic seed is the progeny of breeder or pre-basic seed and is usually
produced under the supervision of a breeder or his designated agency and under
the control of a seed quality control agency. Certified seed is the progeny of basic
seed and is produced on contract with selected seed growers under the supervision
of the seed enterprise, public or private. Certified seed can be used to produce
further generations of certified seed or can be planted by farmers for grain
production.” (van Gastel, Bishaw and Gregg, 2002)
The OECD Seed Schemes Rules and Regulations for cereals prescribe, for all self-
fertilising species which include wheat, that the basic or certified seed crops must be
isolated from other cereal crops by a definite barrier or a space sufficient to prevent seed
mixture during harvest. Additional isolation distance and specifications are prescribed for
seed crops to produce certified seed of a hybrid variety. Controls on previous uses of the
field (previous cropping) include that the field must not have been used to grow wheat for
the two previous years (OECD, 2022).
Volunteer bread wheat
As with all crops cultivated and harvested at the field scale, some wheat seeds may escape
and remain in the soil until the following season when they germinate giving rise to
volunteer plants either before or following seeding of the succeeding crop. Seed loss in
wheat crops is the result of natural plant shedding, weather events like hail storms or
harvest operations. Wheat seed losses at harvest have been documented as 0.8-6%
(Anderson and Soper, 2003). A 2% grain loss at harvest with a crop yield of 3,000 kg/ha
and a 1000-seed weight of 25 g will leave approximately 240 seeds/m 2 in the field, noting
that current yields can be higher (see Section 2.2.3.) resulting in greater potential for
volunteers (Anderson and Soper, 2003). This is similar to a different study in which seed
loss after harvest was 219 or 60 seeds/m2 depending on the harvester used (Komatsuzaki
and Endo, 1996). Studies examined persistence of volunteer wheat under a number of
different farming systems in Canada and found that most volunteer spring wheat emerged

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
26  ENV/CBC/MONO(2024)23

one year after the dispersal of seeds and none persisted after three years (De Corby et al.,
2007; Harker et al., 2005). Not all dispersed seeds will germinate the following years as
most of them degrade in the soil. The average density of wheat volunteers in the first year
post-dispersal was 3.3 plants/m2 when 190 seeds/m2 were dispersed (Harker et al., 2005)
and 21.5 plants/m2 after dispersing 500 seeds/m2 (De Corby et al., 2007). An Australian
study found a wheat volunteer density of 0.7-5.6 plants/m2 post-harvest depending on
farming practices (Wicks et al., 2000). Overall, the timing of volunteer emergence and
seedling density is variable across growing seasons and locations (Anderson and Nielsen,
1996; Anderson and Soper, 2003; De Corby et al., 2007; Harker et al., 2005). Factors
accounting for this variability may be weather conditions, crop management practices and
the wheat varieties that differ in their rate of seed shattering and the degree of dormancy
of their seeds. In general, volunteer wheat seed emergence is low and as long as volunteer
wheat is controlled and not allowed to set seed that would increase the seed bank, it does
not represent a major problem to farmers (Harker et al., 2005).
When uncontrolled, volunteer wheat can compete for nutrients and water with succeeding
crops and cause severe yield losses (Friesen et al., 1990; Lemerle et al., 2016; Marshall
et al., 1989; O’Donovan, Kirkland and Sharma, 1989). Volunteer wheat can also harbour
insect pests and diseases that can negatively affect the upcoming wheat crop and
neighbouring fields. Diseases that build up on volunteer wheat include cereal rusts, root
lesion nematodes, Rhizoctonia bare patch and crown rot (Coutts et al., 2017). Pests
commonly found in volunteer wheat include white grub, wireworm, army cutworm, Hessian
fly, aphids and mites (Bell et al., 2016). In addition to feeding damage, the latter two pests
can transmit viruses that cause wheat diseases. In particular, the wheat curl mite is
the vector of Wheat streak mosaic virus, Triticum mosaic virus and Wheat mosaic virus, all
of which can be devastating for wheat crops (Coutts et al., 2008; Klein et al., 2016; Thomas,
Hein and Lyon, 2004).
It is recommended to control volunteer wheat at least three weeks prior to
the establishment of the next wheat crop in order to reduce the risk of disease transmission
(Thomas, Hein and Lyon, 2004). Volunteer wheat can be controlled by tillage and/or
herbicides. While tillage is often the most cost-effective method to control volunteer wheat,
tillage prior to planting may result in moisture loss due to evaporation from the seed
germination zone that is vital for timely crop establishment especially on dryland fields.
Herbicides provide a good option for controlling volunteer wheat as well as other weeds
(Bell et al., 2016).
1.3.4. Centres of origin and diversity
All Triticum species are native to the Fertile Crescent of the Near East, where Western
agriculture originated after the last ice age, 12,000-9,500 years ago. The region of the
Fertile Crescent extends through the modern-day countries of Israel, Jordan, Türkiye,
Syria, Iran and Iraq (Figure 1.4.). Within the Fertile Crescent, agriculture originated in a
core area in South Eastern Türkiye where the closest wild relatives of einkorn, emmer,
barley, rye, chickpea and lentil still grow (Kilian et al., 2009). Wild cereals were cultivated
for centuries in this region but they were gradually replaced due to selection for
domesticated cultivars. This domestication process lasted up to one millennium (Kilian
et al., 2009).
Tetraploid emmer wheat was domesticated in South Eastern Türkiye. From this area,
emmer expanded across Asia, Europe and Africa. North East expansion met
the distribution of Ae. tauschii and allowed the emergence of hexaploid T. aestivum around
8,000 years ago. This hybridisation event took place in the corridor from Armenia to

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  27

the South Western coast of the Caspian Sea (Feuillet, Langridge and Waugh, 2008; Kilian
et al., 2009; Matsuoka, 2011).

Figure 1.4. The Fertile Crescent of the Near East

Note: Red line delimits the region known as the Fertile Crescent while the blue line surrounds the core area in
the Karacadag range (KK) where agriculture originated
Source: Modified by Brandon McMahon based on Kilian et al. (2010).

Wheat domestication
Crops differ from their wild ancestors in several plant features, collectively referred to as
the domestication syndrome. The most important wheat traits selected for during
domestication were the hulled seeds and brittle rachis. Additional selections resulted in the
increase in seed size, loss of seed dormancy, reduction of plant height, and changes to
photoperiod and vernalisation response (Kilian et al., 2009).
Wild wheats and early-cultivated wheat varieties were characterised by hulled seeds that
required drying to be liberated from the chaff. In addition, the spikelets of the wild ears fell
apart at ripening through fragmentation of the spike stem (rachis) resulting in seed
shattering and dispersal. Most modern varieties of wheat, including bread wheat, have
non-hulled or naked seeds. The modified leaves that form the hull (glumes and palea; see
Section 1.2.1.) are thinner in these varieties and fall off at harvesting. Modern varieties of
wheat also have spikes with a tough non-brittle rachis that keeps the mature spike together
until it is harvested. With the introduction of these two traits in cultivated wheat, the
harvesting of the grains became efficient (Kilian et al., 2009).
For early wheat varieties to flower, the plants had to experience a period of cold in the
winter (a process called vernalisation), followed by exposure to long days in spring (they
were responsive to day-length or photoperiod). These responses were adapted to
the environmental conditions in the Fertile Crescent. With the spread of agriculture to
different environments, better suited wheat types were selected that responded differently
to these environmental cues. Today spring and winter-type varieties of wheat are available
(see Section 2.1.).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
28  ENV/CBC/MONO(2024)23

SECTION 2. Reproductive biology

2.1. Generation time and duration under natural circumstances, and where grown
or managed
Wheat (Triticum aestivum L.) is an annual crop, and its life cycle (from seed to seed)
comprises successive but distinct growth-development phases, including emergence,
tillering, booting, heading, flowering and ripening (Nuttonson, 1955). Considerable
difference exists in the duration of life cycle of different types and varieties of wheat.
Furthermore, the duration of each development phase in the wheat life cycle often varies
considerably between varieties with different ecological-geographical origins (Nuttonson,
1955).
Wheat varieties are broadly categorised into two distinct types, i.e. winter wheat and spring
wheat, based on their growth habits (Curtis et al., 2002; Nuttonson, 1955). The life cycle
of winter wheat and spring wheat generally lasts about 180-300 days and 100-170 days,
respectively, depending on the genotype, geographic location, and environment conditions
(Asseng et al., 2012). The main difference between the two types of wheat is that winter
wheat requires a vernalisation process (a period of exposure to low temperature for
transition from vegetative to reproductive stage), whereas the spring wheat does not
require such a vernalisation process (Nuttonson, 1955; Smith, 1995). A third, less clearly
defined, type is facultative wheat, noted for a low vernalisation requirement that allows
them to be sown earlier in spring (Braun and Sãulescu, 2002). After vernalisation is
completed, varieties that are sensitive to photoperiod require a certain day-length to flower.
Sensitivity to photoperiod differs amongst modern wheat varieties. Most cultivated wheats
today flower faster as the day-length increases, but they do not require a particular length
of day to induce flowering (Acevedo, Silva and Silva, 2002). Winter wheat is generally sown
in the fall in the Northern Hemisphere, is in a dormant state over winter, flowers in spring,
and is harvested around the summer of the next year, while spring wheat is normally sown
in the spring and harvested in later summer or early fall (Curtis et al., 2002; Nuttonson,
1955). When winter wheat is sown in the spring, it usually remains prostrate throughout
the growing season but will not develop culms or spikes (Nuttonson, 1955). Winter wheat
is normally grown in regions where fall seedlings can survive winter, including the central
and southern plains of the United States, western Europe, the Balkans, southern Russia,
and China (Allan, 1980). Spring wheat is normally grown in colder regions that are
unfavourable for winter wheat, including the northern plains of the United States,
the Canadian Prairies, Argentina, and northern and central Russia, so plants grow, flower
and set seed before the short summer ends. However, spring wheat is also grown as a fall-
sown crop in regions with mild winters, such as Mexico, Brazil, India, Australia, and
southwestern United States, where plants flower during the warm and wet winter, and are
harvested before hot and dry summer conditions set in (Allan, 1980; Nuttonson, 1955).

2.2. Reproduction
2.2.1. Floral biology
The wheat inflorescence is a determinate and composite spike, consisting of two rows of
spikelets arranged on opposite sides of a central rachis (Allan, 1980; Lersten, 1987). The
rachis is tough, such that it does not disarticulate on maturity and prevents seed shattering.
Each spikelet is about 10-15 mm in length, has a short spikelet axis (rachilla) at the bottom,

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  29

and comprises multiple (usually two to nine) florets, which are encompassed by two small
bract leaf-like glumes (Figure 1.2.D, Figure 2.1.) (De Vries, 1971; Lersten, 1987; Waines
and Hegde, 2003). In some wheat varieties, the glumes can have awns up to 30 mm long,
while the lemmas can have awns up to 80 mm long. As described in the literature (Lersten,
1987; Murai, 2013; Waines and Hegde, 2003; Willenborg and Van Acker, 2008), each floret
is enveloped by two leaf-like structures, i.e. a lemma (awn or awnless) and a palea, and
comprises the sexual organs that include one pistil, three stamens (each consists of an
anther and a filament) and two lodiculae (Figure 1.2.E, Figure 2.1.) (Setter and Carlton,
2000b). The pistil, the centrally located female part of a floret, consists of the ovary, which
contains one ovule and two filamentous styles, each terminating with a feathery stigma.
The stamen, the male part of a floret, is composed of a filament and an anther containing
pollen grains. The two lodiculae are attached to the ovary and swell during anthesis, forcing
the lemma and palea apart to facilitate pollination of the stigma from the dehisced anther.

Figure 2.1. Features of the wheat spikelet

3rd floret Awn

Awn
Palea
44thth Floret
floret
Stigma
Anther 2nd floret

Lemma

1st glume 2nd glume

1st floret

Note: The wheat inflorescence (spike or head) is composed of spikelets, which include multiple florets encompassed by two small
bracts (glumes). Each floret is enveloped by two leaf-like structures, lemma and palea, and inside sits the reproductive organs,
including three stamens and a pistil consisting of the ovary that contains the ovule and two filamentous styles.
Source: Photograph by K-State Research and Extension, Kansas State University. Captions adapted from Waines and Hegde
(2003).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
30  ENV/CBC/MONO(2024)23

2.2.2. Pollination mode, pollen dispersal, pollen viability

The general flowering process in wheat has been investigated for over a century and is
well documented (De Vries, 1971; Leighty and Sando, 1924). Wheat starts to flower
several days after the spike emerges, flowering generally begins near the upper part of
the spike and continues in both an upward and downward direction (De Vries, 1971;
Poehlman, 1959b). Flowering occurs throughout the day, and a spikelet requires three to
four days while a plant needs up to eight days to finish blooming, which is influenced by
both meteorological conditions and genotypes (De Vries, 1971; Leighty and Sando, 1924).
Wheat is predominantly chasmogamous (open) when flowering (De Vries, 1971), but some
wheat varieties always keep their flowers closed (cleistogamy) (Ueno and Itoh, 1997).
During the flowering process, the two lodiculae within a floret swell, become very turgid,
and push the lemma and palea open. While or after the floret opens, the filaments of the
stamens rapidly elongate, making anthers extrude from the glumes (De Vries, 1971). The
stigmas remain within the glumes in most wheat varieties, but can expose themselves
outside the glumes in some varieties or when pollination is delayed. The frequency of
chasmogamous flowers varies among different varieties and different meteorological
conditions (De Vries, 1971). The percentage of extruding anthers ranges from 12-99%
depending on the environmental conditions and genotypes (De Vries, 1971). Low moisture
and high stress conditions often promote chasmogamous flowering and increase
outcrossing (Waines and Hegde, 2003). The duration of floret opening varies from 11-66
minutes, but it can vary considerably due to differences in genotype and weather conditions
(Leighty and Sando, 1924). When conditions are unfavourable for the opening of
the glumes, the anthers may not protrude from the glumes, and thus shed all of their pollen
inside the flower (Leighty and Sando, 1924; Poehlman, 1959b).
Despite the fact that most of the wheat florets are flowering chasmogamously (De Vries,
1971; Ueno and Itoh, 1997), wheat is predominantly self-pollinating because wheat anther
normally dehisces to disperse pollen while the stamens are still within the floret leading to
self-pollination (Lersten, 1987; Willenborg and Van Acker, 2008). It is reported that less
than 10% of pollen grains fall on the stigma of the same floret, 9-12% remain on the anther
(De Vries, 1971) and as much as 30-80% shed outside of the floret into the air from the
extruded anthers (Hegde and Waines, 2004; Leighty and Sando, 1924). Once attached to
a stigma surface, the pollen tube starts to grow within about 1-2 hours, and fertilisation
occurs 30-40 hours later (Chandra and Bhatnagar, 1974; De Vries, 1971). At some time
point shortly after the anthers shed pollen, the lodiculae lose their turgidity and collapse,
leading to the closing of the floret (De Vries, 1971).
Wheat normally has a very low natural cross-pollination rate of less than 1% (Garber and
Quesenberry, 1923; Harrington, 1932; Poehlman, 1959a). Cross-pollination in wheat is
mainly facilitated by wind dispersal of pollen because wheat flowers are small,
inconspicuous, nectarless, scentless, and unattractive to insect pollinators. The cross-
pollination rate varies with cultivars, distance from the pollen source, and environmental
conditions (Beri and Anand, 1971; Dong et al., 2016; Hucl, 1996; Joppa, McNeal and Berg,
1968; Khan, Heyne and Arp, 1973; Loureiro et al., 2012; Martin, 1990; Matus-Cádiz, Hucl
and Dupuis, 2007). It is estimated that wheat pollen generally travels only one metre in still
air due to its relatively heavy weight, which limits mobility (Waines and Hegde, 2003;
Willenborg and Van Acker, 2008). Under field conditions, the number of wheat pollen
grains decrease significantly as the distance increases from the pollen source (Dong et al.,
2016; Hucl and Matus-Cádiz, 2001; Jacot et al., 2004; Khan, Heyne and Arp, 1973;
Loureiro et al., 2007b). In studies, most of the pollen grains were found to be concentrated
within 5 m of the pollen source and only a few pollen grains were present at >30 m (Dong

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  31

et al., 2016; Hucl and Matus-Cádiz, 2001; Jacot et al., 2004). However, a pollen slide study
conducted by Pioneer Hi-Bred International (Kansas) showed that viable wheat pollen
could be found as far as 1,000 m away from a very large pollen source (Hegde and Waines,
2004; Virmani and Edwards, 1983). Hucl and Matus-Cádiz (2001) reported no outcrossing
beyond 27 m in their study. Wheat pollen movement is also affected by a number of other
factors such as percent of anther extrusion, pollen production per anther, and the number
of anthers per unit area (Beri and Anand, 1971; Joppa, McNeal and Berg, 1968; Willenborg
and Van Acker, 2008). In addition, high humidity tends to decrease cross-pollination rate
(0.1%) while warm and dry weather leads to higher cross-pollination rates (3.7-9.7%)
(Mandy, 1970). Thus, genotype and environmental factors such as wind, temperature, and
humidity play a strong effect on pollen dispersal.
Both the quantity and viability of wheat pollen are relatively low. As reviewed by De Vries
(1971), the number of pollen grains per anther ranged from 581-3,867 in different wheat
varieties under various field conditions. The average number of pollen grains per
inflorescence of wheat is calculated to be about 10% of rye and 2.5% of maize. Wheat
pollen is only viable for a short time period and under field conditions, wheat pollen viability
is lost within 15-20 minutes (De Vries, 1971). Optimal viability of wheat pollen is
approximate 30 minutes under field conditions at warm temperatures (20°C) and moderate
relative humidity (60%) (De Vries, 1971). After flowering, the stigma receptivity can
generally last for 7-8 days, but the first 2-4 days are most suitable for pollination (De Vries,
1971).
2.2.3. Seed production and natural dispersal
The number of wheat seeds per area is the product of spikes per area and seeds per spike
(Acevedo, Silva and Silva, 2002; Poehlman, 1959b). Spikes per area is dictated by
seedling rate, tillering and tiller survival. Seeds per spike is impacted by a number of factors
such as genotype, emergence time, tiller population density, weed competition, fertility,
and biotic and abiotic stresses (Acevedo, Silva and Silva, 2002). Modern wheat varieties
have been bred with high seed production potential. In 1951, the worldwide average wheat
seed yield was close to 1 tonne/ha, this then increased to 2 tonnes/ha by the early 1980s
(Curtis et al., 2002), and has now climbed to approximately 3.5 tonnes/ha, ranging from 1
tonne/ha to 11 tonnes/ha (OECD, 2021). The significant increases in wheat seed yield are
mainly attributed to genetic improvements and better cultural methods (see Section 1.3.3.),
particularly the breeding development of the high-yielding, disease-resistant semi-dwarf
wheat cultivars (Heyne, 1987; Poehlman, 1959b). It is noted that the development of new
wheat cultivars alone cannot deliver such a significant yield increase. It is the simultaneous
improvements in varieties and in cropping practices, including the application of
agrochemicals (e.g. fertilisers, pesticides, and herbicides), controlled irrigation and new
cultivation methods, as a whole that contributes to the wheat yield increase (Pingali, 2012).
For example, it is estimated that the average wheat yield in the United States would be
reduced by 16% without nitrogen fertiliser (Stewart et al., 2005). Like fertilisers, crop-
protection chemicals (pesticides and herbicides) also have contributed to the significant
wheat yield increase (Oerke and Dehne, 1997).
Wheat is planted primarily for seed production, and it has been domesticated to reduce the
loss of spike fragility to maximise harvest-ability of the produce (Feldman and Levy, 2015;
Fuller and Allaby, 2009), thus, natural dehiscence of the seed from the spike at maturation
occurs only to a small extent (Anderson and Soper, 2003). However, there exists
substantial genotypic variations in natural dehiscence among different wheat cultivars
(Anderson and Soper, 2003; Clarke, 1985). Natural dehiscence also varies with season
and time (Clarke, 1985; Willenborg and Van Acker, 2008). For example, Clarke (1985)

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
32  ENV/CBC/MONO(2024)23

reported that the cumulative natural shattering loss of the five tested wheat cultivars ranged
from 4-27 g/m2 over a period of two weeks.
Wheat seeds shattered from the parent plants may experience different dispersal
movement and fates under the biotic and abiotic influences (Bakker et al., 1996;
Nathan and Muller-Landau, 2000). Most shattered wheat seeds are anticipated to fall
beneath the plant with only a short distance of movement. Some shattered wheat seeds
enter the seed bank with different fates, e.g. dead, alive, active or dormant, etc. (see
Section 2.2.4.). Wheat seeds on the soil surface or even in the seed bank may be further
dispersed by animal or physical forces (wind and water). Wheat seeds can be dispersed
through the faeces of birds as the seeds were shown to survive passage through emus
(Dromaius novae-hollandiae) (Davies, 1978) and other birds (Twigg et al., 2009). Similarly,
wheat seeds also can be dispersed via the faeces of mammals such as the white-tailed
deer (Odocoileus virginianus Zimm.) (Myers et al., 2004), red deer (Cervus elaphus L.) and
fallow deer (Dama dama L.) (Malo and Suárez, 1995). In addition, pest animals such as
kangaroos (Macropus spp.), rabbits (Oryctolagus cuniculus), mice (Mus musculus) and
rats (Rattus spp.) are potential dispersers of viable wheat seeds (OGTR, 2017). Wheat
seeds, especially for seeds with awns, can also be dispersed by animals through adhering
on their fur (Sorensen, 1986; Yoshioka et al., 2017). However, the long-distance dispersal
of seeds by migratory animals is typically rare, but also difficult to measure and predict
(Nathan et al., 2008).
Wheat seeds on the soil surface can also be dispersed through abiotic forces such as wind,
surface water movement, and soil erosion (Bakker et al., 1996; Nathan and Muller-Landau,
2000). The distance of wind-dispersed seeds depends upon seed mass and
meteorological conditions, and only seeds with a mass less than about 0.05 mg have
the potential to be dispersed over long distance by wind (Bakker et al., 1996). Wheat seeds
have an average mass of about 30 mg and are not expected to be dispersed over a long
distance by wind except in a storm. When the intensity and amount of rain cause overland
flows, the rain-wash-water can also contribute to the horizontal or surface movement of
seeds. There is a lack of specific reports on wheat seed dispersal though abiotic forces.
Also, like the seed dispersal by migratory animals, the distance of both wind- and water-
mediated wheat seed dispersal is difficult to predict.
2.2.4. Seed viability, longevity and dormancy, germination, seedling viability and
establishment
The viability and longevity of wheat seeds are extremely variable depending upon a variety
of factors, including genotypes, seed production factors, and environmental conditions
(Anderson and Soper, 2003). Wheat seeds in soil may experience several fates, including
immediate germination, a short dormant period followed by germination, predation,
microbial decomposition, or seed death (Anderson and Soper, 2003; Willenborg and Van
Acker, 2008). Because wheat seeds do not have a hard seed coat and are relatively large,
they are prone to rapid decomposition, particularly in moist soils, and are generally not very
persistent (De Corby et al., 2007; Wilson and Hottes, 1927). Classical burial studies
showed that wheat seeds were short-lived in soil, normally persisting for less than one year
(Anderson and Soper, 2003). De Corby et al. (2007) investigated the emerging time and
recruitment of volunteer spring wheat after broadcasting and incorporating seeds into the
soil in fall at 500 seeds/m2, and showed that wheat seeds emerged early and the wheat
seed recruitment level was low. The total cumulative emergence of wheat ranged from 0.9-
13.1% (or 5-66 seedlings/m2), with an overall mean of 4.3%. The majority of the remaining
wheat seeds decomposed in the soil (De Corby et al., 2007). However, other field studies
showed that volunteer wheat seedlings were still emerging 16 months and occasionally

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  33

even two years after harvest (Anderson and Soper, 2003). Harker et al. (2005) observed
wheat persistence up to three years after seed dispersal at eight sites across Western
Canada. Similarly, volunteer spring wheat is reported to persist for even up to five years in
the seed bank (Beckie and Owen, 2007). The difference in seed longevity between the
burial studies and field studies may be attributed to the after-ripening period of seeds
before being destined in seedbank, the seed densities in the soil, and the soil moisture
levels (Anderson and Soper, 2003; Percival, 1921).
Dormancy is an effective survival mechanism for wild plant species to prevent or delay
germination and maintain longevity under unsuitable or even suitable conditions (Fuller
and Allaby, 2009). While wheat has undergone domestication against dormancy,
it normally still possesses a sustained level of primary dormancy, which is desirable to
prevent pre-harvest sprouting of wheat seeds on the plant and help maintain seed quality
(Nyachiro et al., 2002). Wheat seed dormancy levels vary among different genotypes and
environmental factors (Komatsuzaki and Endo, 1996; Reddy, Metzger and Ching, 1985).
Temperature is shown to be one of the most influential environmental factors affecting
the induction of seed dormancy during seed development and expression of dormancy
during seed germination. Low temperatures (10-15°C) during the grain-filling period is
shown to induce and prolong wheat seed dormancy (Nyachiro et al., 2002; Reddy, Metzger
and Ching, 1985). Conversely, low temperature (15°C) during seed germination is effective
for breaking seed dormancy (Reddy, Metzger and Ching, 1985). In addition, high nitrogen
application rates and high seed nitrogen contents are shown to decrease seed dormancy
and increase rain-induced pre-harvest sprouting in genotypes with moderate or low levels
of resistance, although results were inconsistent among different environmental conditions
(Morris and Paulsen, 1985). There is no reported induction of secondary dormancy in
buried wheat seed (De Corby et al., 2007).
The germination and emergence of wheat seeds are impacted by environmental and
varietal factors. For germination to be initiated, the wheat seed must be viable, be free of
any primary dormancy condition, and be subjected to the appropriate environmental
conditions, including a proper temperature range, a suitable degree of moisture, and
a supply of oxygen (Percival, 1921). Temperature plays a significant role on wheat seed
germination (Nyachiro et al., 2002). Wheat seeds can germinate over a wide range of
temperatures from 4-37°C, with the optimal temperature ranging from 12-25°C (Acevedo,
Silva and Silva, 2002). Low temperatures of 10-15°C resulted in higher germination
percentages than higher temperatures of 25-30°C (Nyachiro et al., 2002; Reddy, Metzger
and Ching, 1985; Wilson and Hottes, 1927). Temperature is also shown to have a large
effect on emergence time. In the range of 5-20°C, wheat seedling emergence time
increases when temperature decreases (Lafond and Fowler, 1989). Wheat seeds are
capable of germinating under widely differing moisture conditions, but around 50% seed
moisture content of its dry weight appears to be the optimal (Wilson and Hottes, 1927).
Low soil water potential is shown to increase the time of emergence, and wheat seeds are
capable of germinating and emerging at much lower soil water potentials when
the temperature decreases (Lafond and Fowler, 1989). Wheat seedling emergence is also
adversely impacted by other environmental factors, such as low oxygen diffusion rate
(Hanks and Thorp, 1956), strong soil surface crust (Anzooman et al., 2018; Hanks and
Thorp, 1956) and flooding treatments (Ueno, Fujita and Yamazaki, 1999). In addition to
environmental factors, both wheat seed germination and emergence also vary significantly
between different genotypes (Anzooman et al., 2018; Ueno, Fujita and Yamazaki, 1999).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
34  ENV/CBC/MONO(2024)23

2.2.5. Asexual propagation (apomixis, vegetative reproduction)

Apomixis is a type of asexual seed development from the maternal tissues of the ovule in
flowering plants without the meiosis and fertilisation process (Bicknell and Koltunow, 2004;
Koltunow and Grossniklaus, 2003). Apomixis can provide obvious advantages in crop
enhancement, including the fixation of heterosis through the formation of true-breeding
hybrids, simplification of hybrid seed production without requiring male sterile lines and
isolation, and increased opportunity for developing superior gene combinations (Asker,
1979; Baum, Lagudah and Appels, 1992; Hanna and Bashaw, 1987). Apomixis has been
described in more than 400 flowering plant taxa, and it has been commonly observed in
polyploid wild species of grasses (Bicknell and Koltunow, 2004; Hanna and Bashaw, 1987).
However, apomixis is very rare in crop species with the exceptions in tropical forage
grasses and subtropical fruit trees, such as mango, mangosteen and citrus (Bicknell and
Koltunow, 2004). There has been no report of apomixis in wheat.
In an attempt to use apomixis for crop genetic improvement, numerous attempts have been
made to transfer the apomixis trait from wild related species into cultivated crops through
wide hybridisations (Savidan, 2001). In Triticeae tribe, Elymus rectisetus (Nees in Lehm.)
A. Löve & Connor (2n = 6x = 42, SSYYWW), which is endemic to Australia and New
Zealand, is the only known apomict (Liu, Wang and Carman, 1994). The mechanisms for
the high level of or obligate apomixis in E. rectisetus have been investigated by Crane and
Carman (1987) and Liu, Wang and Carman (1994). To transfer apomixis from E. rectisetus
to wheat, apomictic E. rectisetus has been crossed with wheat (Liu, Wang and Carman,
1994; Peel et al., 1997). Although certain aspects of apomixis in E. rectisetus are observed
in the hybrids, absolute confirmation of true apomixis in wheat remains elusive (Peel et al.,
1997).
A special wheat strain, Salmon, is shown to be able to parthenogenetically produce
progenies from egg cells when its nucleus is transferred into heterologous cytoplasms of
some related species such as Aegilops (Kumlehn et al., 2001; Matzk, 1996). Wheat strain
Salmon was derived from a cross between two octoploid Triticales (2n = 8x = 56,
BBAADDRR), but it possesses the same hexaploid genome formula as that of normal
wheat (2n = 42, BBAADD) except for two structural chromosomal changes, an 1BL-1RS
translocation (the short arm of wheat chromosome 1B is replaced by the short arm of rye
chromosome 1R) and a deletion on wheat chromosome 2B (Tsunewaki, 1964). Such a
Salmon system of wheat shows a high incidence (up to 90%) of polyhaploid
parthenogenesis from the unfertilised but reduced egg cell (Matzk, 1996). Intergeneric
hybrids between barley (Hordeum vulgare) and wheat were also shown to induce apomixis.
Mujeeb-Kazi (1981) successfully made the intergeneric crosses between H. vulgare cv.
Manker and T. aestivum cvs. Bonza and Chinese Spring, resulting in the production of
F1 hybrids (2n = 4x = 28, HBAD). However, these F1 hybrids were all male sterile, and were
then backcrossed to wheat parent cultivars to produce backcross-1 (BC1) seeds.
The BC1 plants were found to contain 28 or 27 chromosomes with the similar genome
composition as F1 hybrids (2n = 4x = 28, HABD) rather than the expected BC1 genome
composition (2n = 7x = 49, HBBAADD). These BC1 plants were proved to be derived from
haploid parthenogenesis (Mujeeb-Kazi, 1981). Apomixis has also been discovered in
wheat rye F1 hybrids (Silkova, Shchapova and Kravtsova, 2003).
Wheat normally does not naturally reproduce through a vegetative approach, although
vegetative reproduction can be artificially induced through in vitro tissue culture. Currently,
the majority of the methods used for in vitro wheat clonal propagation involve plant
regeneration from dedifferentiated tissues or cell suspensions (Bhaskaran and Smith,
1990; Vasil, 1987). Plant regeneration, however, is a transient and sporadic event, and

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  35

only a few genotypes respond favourably to callus induction and plant regeneration
(Tanzarella and Greco, 1990). Furthermore, in vitro plant regeneration often induces
genomic instability and variation (Jain, 2001; Larkin et al., 1984; Larkin and Scowcroft,
1981).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
36  ENV/CBC/MONO(2024)23

SECTION 3. Genetics and breeding

3.1. Basic genetic information
3.1.1. Cytogenetics
Bread wheat is a polyploid plant that originated with the hybridisation of three diploid
grasses, each of them contributing their genomes to the newly formed species (Figure
1.1.). As a result, the genome of bread wheat is formed by three subgenomes: A, B and D
(see Section 1.1.4.). Each genome is composed of seven chromosome pairs that are
represented by the numbers 1 to 7. To differentiate the subgenome that they belong to,
the chromosomes are named as 1A, 1B, 1D to 7A, 7B and 7D, respectively (Figure 3.1.).
The 21 pairs of homologous chromosomes of bread wheat fall into seven homoeologous
groups. Each group contains one pair of chromosomes from the A, B, D subgenomes.
For instance, homoeologous group 1 contains the pairs 1A, 1B and 1D. Homoeologous
chromosomes share a high degree of gene synteny and DNA sequence homology, but
also differ by a number of non-coding highly repetitive DNA sequences (Feldman and Levy,
2012). A standard karyotype nomenclature has been described (Gill, Friebe and Endo,
1991) that allows ready distinction of the chromosomes. A brief review by Gill (2015)
discusses the history and developments of wheat chromosome analysis techniques.
During meiosis, homologous chromosomes are able to pair and recombine (intragenomic
pairing) while pairing of homoeologous chromosomes is prevented (intergenomic pairing).
This ensures the formation of 21 pairs of chromosomes during meiosis, and results in the
regular segregation of chromosomes in the gametes and their subsequent inheritance by
the next generation. This diploid like meiotic behaviour in wheat allopolyploids has been
critical for their establishment. It ensures high fertility, genetic stability, and diploid-like
inheritance of traits (Feldman and Levy, 2012). Two different mechanisms are involved.
Firstly, the difference in DNA sequence of homoeologous chromosomes makes it difficult
for them to pair at meiosis. Secondly, the gene Ph1 located on chromosome 5B (Riley and
Chapman, 1958; Sears, 1976) restricts pairing to homologous chromosomes (Hegde and
Waines, 2004).
3.1.2. Genome characteristics
The genome size of bread wheat is approximately 17 000 000 000 base pairs of DNA (17
Gigabases - Gb). It is approximately forty times bigger than the rice genome
(450 Megabases; Goff et al., 2002), six times bigger than the maize genome (2.3 Gb;
Schnable et al., 2009) and five times bigger than the human genome (3.2 Gb; Makałowski,
2001). As a hexaploid, the bread wheat genome contains six alleles of each gene in
contrast to diploid organisms whose genome contains two alleles per gene. Due to
the relatively recent origin of bread wheat, the gene copies found in the different genomes
(homoeologous genes) are functional and have more than 97% sequence similarity across
coding sequences (IWGSC, 2014; Uauy, 2017).
The advent of next-generation sequencing methods allowed the sequencing of
the genome of the reference hexaploid wheat line ‘Chinese Spring’ which was assembled
into the 21 constituent chromosomes (IWGSC, 2014). The genomes of five additional
varieties have been released; i.e. the bread wheat varieties Robigus, Paragon, Claire and
Cadenza and the durum wheat variety Kronos (Uauy, 2017). The complete or partial
genomes of the diploid relatives of bread wheat, including T. urartu, T. monococcum,

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  37

Ae. tauschii and Ae. speltoides, have also been sequenced (Brenchley et al., 2012; Jia et
al., 2013; Ling et al., 2013).

Figure 3.1. Cartoon karyogram of the bread wheat genome

Note: The cartoon depicts the organisation of the bread wheat genome into 21 pairs of chromosomes, which are derived
from three subgenomes.
Source: Illustration courtesy Maria Alonso, OGTR, Australia.

Each bread wheat subgenome has a size of approximately 5.5 Gb with more than 80%
comprising highly repetitive transposable elements (IWGSC, 2014). Coding sequences
represent less than 2% of the genome. It has been estimated that the wheat genome
contains 106,000 functional protein-coding genes, with gene number estimates ranging
between 32,000 and 38,000 for each subgenome. This is consistent with the number of
genes found in the genomes of the related diploid species (IWGSC, 2014). The genes are
also highly conserved, with more than 99% sequence identity between the subgenomes
and their respective diploid relatives (IWGSC, 2014). Analysis of gene expression in bread
wheat revealed that each subgenome exhibits a high degree of regulatory and
transcriptional autonomy and there is no evidence for a genome-wide transcriptional
dominance of one of the three subgenomes (IWGSC, 2014). This is in contrast to other
polyploid crops like cotton, Brassica rapa and maize where one of the genomes is more
transcriptionally active than the others (IWGSC, 2014).

3.2. Genetic diversity or variability

Bread wheat’s success as a crop is due to its broad adaptability to different environments.
Compared to its diploid and tetraploid relatives, bread wheat has diverse photoperiod and
vernalisation requirements; improved tolerance to abiotic stresses; and better resistance

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
38  ENV/CBC/MONO(2024)23

to several pests and diseases. It can also be used in producing a greater variety of food
products (Dubcovsky and Dvorak, 2007). These characteristics can be explained by the
genetic diversity found in bread wheat. The high rate of DNA changes and the buffering
effects of polyploidy can quickly lead to a greater diversity of characteristics (Dubcovsky
and Dvorak, 2007).
Repetitive transposable elements in the wheat genome have been found to have a high
replacement rate (Dubcovsky and Dvorak, 2007). Movement of repetitive transposable
elements can generate knock-out mutations by insertion of repetitive elements into genes
or by deletion of genes. While knock-out mutations could be lethal or have strong effects
in diploid species, they may have only subtle dosage effects in bread wheat, if they affect
only one of the three homoeologous genes in the wheat genome. The transposable
elements can also facilitate gene duplication, with duplicated genes ending up with new
functions. An average of 23.6% of the genes on each chromosome is duplicated on the
same chromosome, which is a higher percentage than in other cereals such as rice,
sorghum, barley, maize and foxtail millet (IWGSC, 2014). The sequencing of the bread
wheat genome has also revealed that its genes contain a higher proportion of mutations
leading to amino acid changes with a predicted large impact on protein functionality than
their closest diploid and tetraploid relatives (IWGSC, 2014). Therefore, the buffering effect
of polyploidy may result in the development of novel protein functions.

3.3. Methods of classical breeding

As with other crops, the ultimate goal of bread wheat breeding is to accumulate favourable
traits into one cultivar. In pedigree breeding this is a multi-step process. First, plants that
display variation for a given trait of interest must be selected and crossed to generate a
breeding population (Figure 3.2.). After crossing the parental lines, the first hybrid
generation (F1) is allowed to self-pollinate (Figure 3.2.). The traits of interest segregate in
the F2 population. The next step of the breeding process is to select the best performing
individuals in the F2 and subsequent generations and to let them self fertilise in order to
generate homogenous lines (homozygous genotypes) with fixed traits. These homogenous
lines are evaluated at multiple geographic locations to identify the ones best adapted to
different environments. Reviews of conventional methods for bread wheat breeding are
available (Allard, 1999; Baenziger and DePauw, 2009; Simmonds, 1986).

Figure 3.2. Schematic representation of the breeding process in bread wheat

Source: Illustration courtesy Maria Alonso, OGTR, Australia.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  39

An alternative breeding strategy is composite crossing, used in both organic and

conventional systems (Knapp et al., 2019). In this strategy, a population with several traits
is grown and replanted over several generations with minimal selection aside from natural
condition variations, with the resulting population expected to become better adapted to
local conditions over time (Suneson, 1956). Initial trait selection can be important for
resulting quality traits (Brumlop, Pfeiffer and Finckh, 2017). See also Section 3.3.2., Bulk
Selection.
3.3.1. Methods to create genetic and phenotypic variation
Early breeding programs resulted in an improvement of yield and increased disease
resistance in wheat, however these improvements were brought about at the cost of an
overall reduction in genetic diversity in the species. Diversity was lost as the improved
cultivars, emanating from breeding programmes, replaced wheat landraces across
the world (Warburton et al., 2006). Breeders have since focused on expanding the genetic
base of wheat by using methods that create genetic variation like the ones mentioned in
this section (Warburton et al., 2006).
When a new breeding program is initiated the selected parental varieties must be crossed.
Crossing generates variation through genetic recombination at meiosis. Since bread wheat
is mostly a self-pollinated species, the crossing of parental lines to generate hybrid wheats
usually occurs in the controlled environment of a glasshouse (Simmonds, 1989). The
different bread wheat varieties and their wild relatives constitute the genetic sources that
provide the starting material for breeding new crop cultivars. Their seeds are stored in gene
banks that have been established in many countries and that supply this genetic material
to international breeding programs. The International Maize and Wheat Improvement
Center (CIMMYT) manages the world’s most diverse maize and wheat collections10. Other
important collections are held at the Svalbard Seed Vault 11 and the Russian Institute of
Plant Genetic Resources (VIR12).
Many important bread wheat cultivars have genes from wild relatives (Gale and Miller,
1987; Schneider, Molnár and Molnár-Láng, 2008) (For more detail, see Section 4). In vitro
tissue culture techniques are used when wheat cultivars are crossed with wild relatives to
bring new genes into wheat (Baenziger and DePauw, 2009). These embryo rescue
techniques support the development of the hybrid embryo that otherwise would die. The
Ph1 gene has been shown to prevent intergenomic pairing at meiosis in hybrids of bread
wheat and related Triticeae species (Jauhar and Chibbar, 1999; Riley, Chapman and
Kimber, 1959). However, intergenomic pairing is advantageous for wheat breeding as
meiotic recombination can result in the transfer of beneficial genes into the wheat genome.
Wheat breeders have developed several methods to promote homoeologous pairing in
hybrids by reducing the activity of the Ph1 gene (Gale and Miller, 1987).
If a trait of interest does not occur in the existing genetic resources, there are methods to
generate genetic variation. Mutations, i.e. changes in DNA sequence, can be induced by
exposing wheat seeds to chemical mutagens or to ultraviolet or ionising radiation. These
mutagenic techniques are non-targeted, that is genes are mutated at random, and this may
generate a trait of interest. An example of novel trait introduction via mutation breeding is
resistance to the imidazolinone class of herbicides. Imidazolinone-resistant wheat was
originally developed through seed mutagenesis of ‘Fidel’ winter wheat followed by selection

10 http://www.cimmyt.org/tag/germplasm-bank/
11 https://www.seedvault.no
12 https://vir.nw.ru

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
40  ENV/CBC/MONO(2024)23

of herbicide resistant plants. Imidazolinone-tolerant wheat cultivars have been released in

Argentina, Australia, Canada, and the United States (reviewed in Baenziger and DePauw,
2009).
Hybridisation is another method used to introduce diversity and high-performing crops.
Due to self-pollination in wheat it is more difficult to achieve hybridisation than in other
crops, and by the year 2012 less than 1% of planted wheat was from hybrid seed (Longin
et al., 2012). A range of methods are employed to prepare hybrid seed, reviewed in AGRI-
FACTS (2002), though at present it is primarily through use of chemical hybridisation
agents (Longin et al., 2012). Strategies continue to be developed to produce hybrids, for
example by inducing cytoplasmic male sterility, or using biotechnology techniques
(Castillo, Atienza and Martin, 2014; Kempe, Rubtsova and Gils, 2014). For further detail,
see Section 4.
3.3.2. Selection methods
Selection is the process of determining the relative worth of individuals from a segregating
population and propagating chosen individuals from generation to generation until the traits
of interest are fixed. Methods for selecting while inbreeding to develop a cultivar are
pedigree selection, bulk selection, single-seed descent, doubled haploid breeding and
backcrossing (reviewed in Baenziger and DePauw, 2009).
Pedigree selection. In this method, the breeding of individual plants from a segregating
population are selected and propagated and the genealogy of each line is recorded.
The pedigree breeding method is the most labour-intensive method but provides
the greatest detail of genetic information. It is generally used to create new lines and
cultivars that combine the best traits from parent lines.
Bulk selection. In this method, plants are chosen which express individual advantages,
and a sample of the aggregate of the seed is propagated in the next cycle of inbreeding.
In this case, the breeder often relies extensively on natural selection or relatively simple
selection techniques within the bulk population for removing unwanted types or retaining
desirable types, as the population is harvested en masse with no individual progeny
testing.
Single-seed descent. Single-seed descent is a method to achieve homozygosity while
often practicing minimal selection. The method consists of selfing a random sample of
F2 derived plants in each generation and advancing only one seed per plant. When inbred
lines have been produced, selection can be based on data from replicated field trials for
agronomic performance, biotic and abiotic stress tolerance, and end-use quality testing.
This method is usually applied when crossing elite wheat varieties in which many of
the favourable alleles are already fixed.
Doubled haploid breeding. Doubled haploid technology is used to rapidly generate
homozygous lines. The end-point is the production of a random set of inbred lines for
subsequent assessment. The method involves production of plants from haploid tissue by
doubling the chromosomes. The resultant plant will be completely homozygous and
homogeneous. Two predominant methods are available to create doubled haploids
(i) anther culture and (ii) the wheat-by-maize system (Tadesse et al., 2013). In anther
culture, wheat pollen grains are placed on artificial medium to develop into embryos.
Production of wheat haploids through wheat × maize crossing involves using maize pollen
to fertilise bread wheat. This results in the production of a haploid embryo that is sustained
by embryo rescue techniques. Doubled haploid cultivars have been released in a number

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  41

of countries and some have become dominant cultivars (reviewed in Baenziger and
DePauw, 2009).
Backcrossing. Backcrossing is a method of recurrent crossing to introduce a desirable
trait in a specific genetic background. The parental source of the desirable trait is
designated the donor parent, and the parent in which the trait is introduced is the recurrent
parent. At the end of the breeding, the recurrent parent has the new trait. Backcrossing
has been used effectively as a short term breeding strategy to incorporate dominant genes
for the control of devastating pathogens, such as those causing stem rust, in otherwise
highly productive and adapted cultivars (reviewed in Baenziger and DePauw, 2009).

3.4. Next generation breeding

Deep sequencing and genomics technologies have brought new possibilities to the field of
wheat breeding. The availability of a reference genome makes it possible to identify
candidate genes responsible for agronomic traits of interest. Some of the applications of
new technologies in wheat breeding are outlined in this section.
3.4.1. Marker-assisted selection methods
Traditionally, selection has been based on the phenotype of the individuals. However,
contemporary breeding is more reliant on genotypic selection, which is based on molecular
markers. Molecular markers are DNA sequences that can be easily tracked and quantified
in a population and are linked to a particular trait of interest, such as disease resistance.
Detection of markers tightly linked to traits can then rapidly predict the phenotypes of a
large selection of segregating individuals at an early stage of development, often well
before phenotypic screening would be possible, and at reduced cost. The application of
single marker-trait associations to crop breeding is known as marker-assisted selection
(MAS) (Hayward et al., 2015). Until now, only a limited number of molecular markers were
available for wheat breeding. With the sequencing of multiple bread wheat varieties, more
molecular markers are being discovered. Having enough markers across the entire
genome makes it possible to have marker-assisted selection on a genomic scale (Genome
Wide Marker Assisted Selection or GWAS) (Hayward et al., 2015).
A novel method of wheat breeding called Genomic Selection (GS) has also been deployed
(Bassi et al., 2016). In this method, a small population of plants called the training
population is genotyped and phenotyped and a statistical model is used to identify which
molecular markers are linked to the traits of interest. Then a bigger set of individuals, called
the breeding population is only genotyped. The statistical model is then used to predict
the performance of the individuals for various traits based on the molecular markers they
contain. The individuals predicted to have the best characteristics are propagated in
the subsequent breeding cycles.
3.4.2. Mutational genomics
Due to bread wheat being a polyploid, the effects of knock-out mutations in a single gene
are often masked by the compensating effect of the two other functional homoeologous
genes. This phenomenon is referred to as gene redundancy. Knocking out all three
homoeologous genes may uncover hidden genetic variation which may be useful for crop
improvement (Uauy, 2017). Recently, a population of chemically induced mutants of bread
wheat was produced and the genes in the mutants were sequenced (Krasileva et al.,
2017). On average each mutant carried more than 5,000 mutations in different genes. Most
importantly, at least one knock-out mutation was identified for almost every gene in the
genome. Mutants carrying mutations in one of three homoeologous genes can now be

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
42  ENV/CBC/MONO(2024)23

selected from this population and crossed to generate a triple mutant. Thus, gene
redundancy can be overcome and the role of these genes elucidated. Introduction of traits
into wheat can be accomplished through the use of biotechnology (see Annex B).

3.5. Intraspecific gene flow

Bread wheat is a cultivated species with no known wild or weedy strains (see Section 1).
However, cultivated varieties can successfully cross-breed, either naturally or under
controlled conditions (for further detail, see Section 4). Gene transfer may occur when the
parent lines are grown in proximity to one another and flower simultaneously as they are
sexually compatible (Waines and Hegde, 2003). The progeny of such crosses will be fertile
(Matus-Cádiz et al., 2004).
Intraspecific pollen-mediated gene flow has been studied at field and commercial scales in
Canada (Matus-Cádiz, Hucl and Dupuis, 2007; Matus-Cádiz et al., 2004). The authors
showed that intraspecific gene flow in individual plants could be detected at less than
0.01% up to 300 m away when a 16 ha pollinator block was used, or 2.75 km for a 30 ha
pollinator block (Matus-Cádiz, Hucl and Dupuis, 2007; Matus-Cádiz et al., 2004). Gene
flow was dependent on environmental conditions, with higher gene flow observed in cooler,
more humid and wetter conditions (Matus-Cádiz et al., 2004). The authors suggest that the
0.01% trace rate observed in individual samples can be considered a worst-case scenario
if compared with gene flow rates averaged across samples (Matus-Cádiz, Hucl and
Dupuis, 2007). In the first growing season, one hybrid seed was confirmed out of three
million seeds (0.00003% gene flow rate); and in the second season, nine hybrid seeds
were confirmed out of ten million seeds (0.00009% gene flow rate; Matus-Cádiz, Hucl and
Dupuis, 2007). Isolation distances of up to 45 m were recommended for wheat to reduce
pollen-mediated gene flow to negligible levels (Hanson et al., 2005b; Hucl and
Matus-Cádiz, 2001).
The rate of intraspecific pollen-mediated gene flow in South-eastern Australia has been
shown to be lower than that observed in other countries (Gatford et al., 2006). Using a
series of small pollinator blocks, these authors measured a maximum gene flow rate of
0.055% at 8 m from the pollen source (Gatford et al., 2006). The pollinator blocks used in
this study were smaller than the ones used in Matus-Cádiz, Hucl and Dupuis (2007), which
can lead to lower outcrossing rates. However, this low level of gene flow could also be
explained by environmental and morphological factors. Low relative humidity and warmer
temperatures could have accelerated pollen desiccation. Hot, dry weather conditions have
been shown to lower pollen viability to less than 15 minutes (D’Souza, 1970). It has also
been suggested that as most Australian elite cultivars have a closed flower structure, floral
morphology of the recipient could play a role in the gene flow rates observed (Gatford et al.,
2006). Based on these results, the authors recommended a 12 m separation between GE
(genetically engineered, or ‘genetically modified’) and non-GE crops (Gatford et al., 2006).
Another study in Switzerland examined outcrossing between GE and non-GE wheat of the
same and different lines. This study found that gene flow rates between non-GE and GE
lines varied between parental lines, with distance, and with the location of crops in relation
to one another (direction). In one experiment, gene flow rates declined from 0.7% at 0.5 m
to 0.03% at 2.5 m (Rieben et al., 2011). A case-by-case approach was recommended in
determining the likelihood of gene flow between GE and non-GE crops due to the range of
factors that might influence gene flow rates (Rieben et al., 2011).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  43

SECTION 4. Hybridisation and

introgression
Wide or distant hybridisation between different species or genera provides a way to
combine diverged genomes into one nucleus. Such wide hybridisations break what is
known as the species and/or genera barrier for gene transfer, and make it possible to
exchange genetic information between species or genera (Feuillet, Langridge and Waugh,
2008; Jiang et al., 1993; Mujeeb-Kazi et al., 2013; Sharma and Gill, 1983). Wide
hybridisation and introgression are common in natural ecosystems, and have played
important roles in plant species evolution, especially for wheat (Abbott, 1992; Anderson,
1949; Feldman and Levy, 2015; Kimber and Sears, 1987; Matsuoka, 2011; Rieseberg and
Wendel, 1993). Wide hybridisation has also been increasingly pursued by breeders to
increase genetic variations and enhance wheat improvement (Ceoloni et al., 2015;
Feldman and Levy, 2015; Feuillet, Langridge and Waugh, 2008; Mujeeb-Kazi and Hettel,
1995; Mujeeb-Kazi and Rajaram, 2002; Sharma and Gill, 1983).

4.1. Wheat gene pool

The overview of the Triticeae species in Section 1.1., especially their genome
compositions, helps with better understanding of wide hybridisations in wheat. The species
in the tribe Triticeae are very diverse, including both annual and perennial life cycles, self-
or cross-pollinating types, and a wide range of ploidy levels from diploidy to dodecaploidy
with extremely diverse genomic compositions (Table 1.1.).
From a genetic perspective, the related species of wheat in the tribe Triticeae can be
classified into primary, secondary and tertiary gene pools (Feuillet, Langridge and Waugh,
2008; Harlan and de Wet, 1971; Jiang et al., 1993; Mujeeb-Kazi and Rajaram, 2002). The
primary gene pool species consist of hexaploid wheat landraces, cultivated tetraploid T.
turgidum, wild T. dicoccoides, the A genome donor T. urartu and D genome donor Ae.
tauschii (Jiang et al., 1993; Mujeeb-Kazi and Rajaram, 2002; following taxonomy of van
Slageren, 1994). Gene transfers from the primary pool species into wheat can occur
through homologous chromosome recombination in direct cross and backcross
hybridisations (Jiang et al., 1993).
The secondary gene pool species include closely related polyploid Triticum and Aegilops
species, which share one genome with the three genomes of wheat (Jiang et al., 1993).
The diploid Aegilops species in the Sitopsis section, however, are also placed in this gene
pool because of their reduced chromosome pairing and difficulties in achieving gene
transfer with wheat. Gene transfers from the secondary pool can occur through
homologous recombination between the homologous genomes or recombination among
the non-homologous genomes in direct cross and backcross hybridisations (Feuillet,
Langridge and Waugh, 2008; Jiang et al., 1993; Mujeeb-Kazi and Rajaram, 2002).
The tertiary gene pool species include the diploid and polyploid species that contain non-
homologous genomes compared to wheat genome (Jiang et al., 1993). Thus, wide
hybridisation between wheat and its tertiary gene pool species is difficult to make and the
gene transfers from this gene pool cannot occur by homologous recombination. However,
the genomes of the tertiary pool species are homoeologous (genetically related) to the
wheat genomes, and thus gene transfer from these species can be achieved through

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
44  ENV/CBC/MONO(2024)23

inducing homoeologous recombination (Feuillet, Langridge and Waugh, 2008; Jiang et al.,
1993; Mujeeb-Kazi and Rajaram, 2002).

4.2. Natural facility of interspecific crossing

4.2.1. Natural hybridisation
Relatively few wild relative species within the tribe Triticeae can hybridise with wheat under
natural conditions because wheat is primarily a self-pollinated crop and genetic barriers
exist between wheat and its wild relative species (Mujeeb-Kazi and Hettel, 1995; Sharma
and Gill, 1983). The most important obstacles to wide hybridisation include 1) cross
incompatibility between wheat and its wild species, 2) inviability of F 1 hybrid, and 3) sterility
of the F1 hybrid or its progeny (Sharma and Gill, 1983). However, wide hybridisation
followed by introgression has occurred during wheat evolution and is an important source
of genetic variation in natural populations (Feldman and Levy, 2015; Liu et al., 2016). The
origin and domestication of polyploidy Triticum species involved natural intergeneric and
interspecific hybridisation and introgression (Feldman and Levy, 2015; Marcussen et al.,
2014; Matsuoka, 2011).
Natural hybridisation between the polyploidy species of the genera Triticum and Aegilops,
particularly between those sharing one common genome, is a frequent phenomenon (van
Slageren, 1994; Zaharieva and Monneveux, 2006). Wheat and Aegilops species frequently
grow in sympatry in the Mediterranean area, Northern Europe and the United States, and
frequent spontaneous intergeneric hybridisation between them have been reported (Arrigo
et al., 2011; Hegde and Waines, 2004; van Slageren, 1994; Zaharieva and Monneveux,
2006). For example, jointed goatgrass (Ae. cylindrica), a winter annual weedy form of wild
wheat (allotetraploid, 2n = 4x = 28, CCDD), has been widely reported to infest wheat fields
and hybridises with wheat to form fertile hybrids under natural field environment conditions
(Donald and Ogg, 1991; Gaines et al., 2008; Hanson et al., 2005a; Morrison, Crémieux
and Mallory-Smith et al., 2002; Seefeldt et al., 1998; Stone and Peeper, 2004; Zaharieva
and Monneveux, 2006; Zemetra, Hansen and Mallory-Smith, 1998). Natural hybrids
between Ae. cylindrica and T. aestivum have been reported in the coasts of the Black Sea
of Bulgaria with male sterility frequency between 99.22% and 100% (Stoyanov, 2013).
Natural hybrids have been observed and reported between wheat and a number of species
in Aegilops genus, including Ae. biuncialis, Ae. columnaris, Ae. crassa, Ae. cylindrica, Ae.
geniculata, Ae. juvenalis, Ae. neglecta, Ae. speltoides, Ae. tauschii, Ae. triuncialis, Ae.
umbellata, and Ae. ventricosa, (Dorofeev, 1969; Loureiro et al., 2006, 2007a; van
Slageren, 1994; Zaharieva and Monneveux, 2006).
Many natural wheat × rye hybrids have been discovered (Dorofeev, 1969; Leighty, 1915;
Leighty and Sando, 1928; Meister, 1921) since the first wheat × rye hybrid was reported in
1873 (Wilson, 1873). The wheat × rye hybrids are normally male-sterile, and the seeds, if
any, set on F1 hybrid plants are generally due to spontaneous backcrossing with wheat
(Briggle, 1969; Muntrzing, 1974). However, intermediate and fertile wheat × rye hybrids
that were amphiploid involving wheat plus rye genomes were also reported (Silkova,
Shchapova and Kravtsova, 2003). The discovery of amphiploid wheat × rye hybrids
resulted in the development of the man-made crop Triticale (Briggle, 1969). The name of
Triticale is derived from the combination of the names of two genera involved, Triticum and
Secale, and it is actually a polyploid (principally cultivated as hexaploid, tetraploid to
octaploid variants exist) derived from doubling the chromosome number of the sterile F1
hybrid of a cross between hexaploid wheat (T. aestivum) or tetraploid wheat (T. turgidum)
and diploid rye (S. cereale) (Bernard and Bernard, 1987).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  45

4.2.2. Natural introgression

Many domesticated plants hybridise spontaneously with their wild relatives (Felber et al.,
2007; Jarvis and Hodgkin, 1999). Detailed analyses of naturally occurring wide
hybridisations have shown that the wide cross hybrids often backcrossed to one or both
parents repeatedly leading to the infiltration of germplasm of one species into another, and
such a process was named “introgressive hybridisation” (Anderson and Hubricht, 1938;
Hegde and Waines, 2004), and the consequences were described as the “introgression”
of one species into another (Anderson, 1949).
Natural introgression has been shown to have occurred frequently and played an important
role for the evolution of and genetic diversity of cultivated wheat. Wild tetraploid wheat (T.
turgidum, genomes BBAA) has been proved to introgress its genetic materials into
cultivated wheat, enriching the genetic diversity of the wheat A and B genomes (Dvorak et
al., 2006). Introgression from rye (S. cereale) to wheat was also reported to have occurred
spontaneously in a Portuguese wheat landrace, ‘Barbela’ (Ribeiro-Carvalho et al., 1997,
2001). Natural crosses and introgression between wheat and the Aegilops species,
especially the jointed goatgrass (Ae. cylindrical, CCDD), have been frequently observed
since the early nineteenth century, probably due to their close relationship (Arrigo et al.,
2011; Gandhi et al., 2006; Hegde and Waines, 2004; Jacot et al., 2004; Pajkovic et al.,
2014; Schneider, Molnár and Molnár-Láng, 2008). The D genome of wheat is considered
to have a greater likelihood of being transferred into jointed goatgrass than genes located
on A and B genomes of wheat (Hegde and Waines, 2004; Zaharieva and Monneveux,
2006). Alternatively, natural introgression of genes can also occur from wheat into its wild
relative species such as jointed goatgrass (Gandhi et al., 2006; Hegde and Waines, 2004).
It is noteworthy that the transfer of genes related to the fitness or competitiveness from
wheat to its wild relative species could pose a potential weediness risk (Hegde and Waines,
2004).

4.3. Experimental crosses

Wheat breeders are facing challenges to make further genetic improvements due to the
scarcity of wheat varieties and landraces with desired genetic variations. The ample
number of wild relatives of wheat in tribe Triticeae can provide tremendous genetic
variability, and breeders have been attempting to make artificial wide cross hybridisations
to tap new genes in wild species for wheat genetic improvement (Ceoloni et al., 2015;
Kimber and Feldman, 1987; Mujeeb-Kazi, 1995; Mujeeb-Kazi et al., 2013; Ogbonnaya
et al., 2013). With the advancement of hybridisation techniques and embryo culture,
numerous hybridisations have been successfully made, not only between wheat and its
related species within the genus Triticum (interspecific), but also between wheat and more
distant relatives in other genera of the Triticeae tribe (intergeneric), and wide hybridisation
has become a common approach used in genetic improvement of wheat (Ceoloni et al.,
2015; Jiang et al., 1993; Liu et al., 2016; Mujeeb-Kazi and Hettel, 1995; Mujeeb-Kazi and
Rajaram, 2002; Mujeeb-Kazi et al., 2013; Sharma and Gill, 1983).
4.3.1. Interspecific hybridisations
Interspecific crosses have been successfully made between wheat and other Triticum
species that have genomes similar to the wheat A, B, or D genome. These Triticum species
include diploid wheat (T. monococcum and T. urartu), tetraploid wheat (T. turgidum, T.
timopheevii, T. carthlicum, T. durum, T. dicoccum, T, dicoccoides, and T. araraticum) and
hexaploid wheat (T. spelta, T. zhukovskyi, T. compactum, T. sphaerococcum, and T.
macha) (Mujeeb-Kazi and Hettel, 1995; Mujeeb-Kazi et al., 2013; Sharma and Gill, 1983).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
46  ENV/CBC/MONO(2024)23

4.3.2. Intergeneric hybridisations

In contrast to the above-described interspecific crosses, the species used in intergeneric
crosses are very diverse genomically and normally difficult to cross with wheat; even when
successful, the resulting hybrids have little or no intergenomic chromosome association
(Mujeeb-Kazi, 1995). Significant genotypic variations in crossability of wheat varieties and
wild relatives exist (Jiang et al., 1993; Zeven, 1987). The crossability of common wheat
with its related genera is controlled by several crossability genes (Kr genes) or QTLs (Lein,
1943; Liu et al., 2014; Luo, 1989). Other factors, including the ploidy level, the species
choice for the female parent, and various wide cross techniques such as emasculation,
pre-pollination chemical treatments and embryo rescue culture also have a big effect on
the success of intergeneric hybridisation (Gupta, Mishra and Kumar, 2018; Jiang et al.,
1993; Mujeeb-Kazi, 1995; Sharma and Gill, 1983).
Wheat has been successfully crossed with many species of the allied genera within the
tribe Triticeae, such as Aegilops, Agropyron, Secale, Dasypyrum (synonym of Haynaldia),
Hordeum and Elymus (Baum, Lagudah and Appels, 1992; Ceoloni et al., 2015; Jiang et
al., 1993; Liu et al., 2016; Mujeeb-Kazi and Hettel, 1995; Sharma and Gill, 1983). In
particular, the genus Aegilops can be crossed readily with common wheat since it is
the most closely related genus to wheat in Triticeae tribe (Schneider, Molnár and
Molnár-Láng, 2008; Zhang et al., 2015). The majority of hybrid wheat lines, including
chromosome additions, substitution, and translocation lines have been produced between
common wheat and Aegilops (Schneider, Molnár and Molnár-Láng, 2008). It is worthy to
note that Aegilops had been subsumed into Triticum genus in the past due to their close
relationship, resulting in different names for Aegilops species at the generic level (Kimber
and Feldman, 1987; Mujeeb-Kazi and Hettel, 1995). The Thinopyrum genus
(synonymously Agropyron or Elytrigia) is the most widely used perennial to cross with
wheat (Ceoloni et al., 2015; Mujeeb-Kazi, 1995; Mujeeb-Kazi et al., 2013). Almost all of
the basic genomes in the Triticeae species have been combined, either singly or in
combination, with the genomes of bread wheat through wide hybridisations (Ceoloni et al.,
2015; Jiang et al., 1993; Liu et al., 2016; Mujeeb-Kazi, 1995). A summary list of genera
and species in tribe Triticeae that have been successfully crossed with wheat to form
intergeneric hybrids is shown in Table 4.1.. This summary table by no means represents a
complete list of all the successful wide hybridisations, and the number of new intergeneric
hybrids with more distantly related species is expected to be constantly increasing.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  47

Table 4.1. List of intergeneric species that have been crossed with wheat
Genera Species
Aegilops bicornis, biuncialis, caudata, columnaris, comosa, crassa, cylindrica, dichasians, geniculata, juvenalis,
kotschyi, longissima, mutica, ovata, peregrina, searsii, sharonensis, speltoides, squarrosa, tauschii,
triaristata, tripsaccoides, truncialis, umbellulata, uniaristata, variabilis, ventricosa
Agropyron caespitosum, ciliare, cristatum, desertorum, distichum, elongatum, intermedium, junceum, michnoi,
mongolicum, podperae, smithii, scirpeum, trachycaulum, villosum, yezoense
Dasypyrum villosum
Elymus altissimus, anthosachnoides, canadensis, caninus, caucasicus, ciliaris, cylindricus, dahuricus,
dolichatherus, fibrosus, giganteus, kamoji, nipponicus, parviglumis, pendulinus, rectisetus, repens,
scabrus, semicostatus, shandongensis, tibeticus, trachycaulus, tschimganicus, tsukushiensis
Elytrigia acutum, pungens, repens, varnense
Hordeum bogdanii, bulbosum, californicum, chilense, depressum, distichum, geniculatum, jubatum, marinum,
pubiflorum, pussillum, spontaneum, vulgare
Leymus angustus, cinereus, innovatus, mollis, multicaulis, racemosus, triticoides
Psathyrostachys fragilis, huashanica, juncea
Pseudoroegneria geniculata, scythica, stipifolia, strigosa
Secale africanum, ancestrale, cereale, montanum, vavilovii
Thinopyrum bessarabicum, curvifolium, distichum, elongatum, gentryi, intermedium, junceiforme, junceum,
ponticum, sartorii

Sources: Ceoloni et al., 2015; Jacot et al., 2004; Jiang et al., 1993; Liu et al., 2016; Molnár-Láng, Linc and Szakács, 2013;
Mujeeb-Kazi, 1995; Mujeeb-Kazi et al., 2013; Schneider, Molnár and Molnár-Láng, 2008; Sharma and Gill, 1983;
Smith, 1942; Wang, 2011.

Species outside the tribe Triticeae have also been tested for intergeneric crosses with
wheat, including maize (Zea mays) and sorghum (Sorghum bicolor) (Liu et al., 2014). The
cross between wheat (T. aestivum, 2n = 42) and maize (Z. mays, 2n = 20) led to
the production of hybrid zygotes with one complete haploid chromosome set from each
parent (Laurie and Bennett, 1986). Maize chromosomes, however, are subsequently
eliminated, resulting in the production of haploid wheat (Laurie and Bennett, 1986, 1988b).
Similarly, wheat had been crossed with sorghum (S. bicolor, 2n = 20) as well as pear millet
(Pennisetum glaucum, 2n = 14), resulting in fertilised hybrid zygotes with one complete
haploid chromosome set from each parent followed by the rapid elimination of the sorghum
or pear millet chromosomes (Laurie, 1989; Laurie and Bennett, 1988a).
4.3.3. Genetic manipulations of wide hybrids for alien gene introgressions
The success of wide hybridisations between wheat and its allied species in the tribe
Triticeae and beyond, as described above, is only the first necessary step for gene
introgression. In many wide hybridisations, there exists post-hybridisation barriers such as:
homologous chromosome not being present for paring during meiosis, sterility and genome
elimination/loss that can impede the gene transfer between wheat and its related wild
relatives (Ceoloni et al., 2015; Gupta, Mishra and Kumar, 2018; Jiang et al., 1993; Liu
et al., 2014; Mujeeb-Kazi and Rajaram, 2002). To overcome these barriers and facilitate
gene introgression, a variety of genetic strategies have been used to manipulate the hybrid
genomes for genetic improvement (Table 4.1.).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
48  ENV/CBC/MONO(2024)23

Figure 4.1. Genetic manipulations of interspecific and/or intergeneric hybrids for chromosomal
interchanges and alien gene introgression

Source: Based on Liu et al. (2014).

F1 hybrids and amphihaploids

Wide crosses between two distantly related species lead to the generation of the F1
hybrids, bringing distantly-related parental genomes together into the same nucleus
(Ceoloni et al., 2015; Jiang et al., 1993; Liu et al., 2016; Mujeeb-Kazi and Hettel, 1995;
Mujeeb-Kazi and Rajaram, 2002; Mujeeb-Kazi et al., 2013; Sharma and Gill, 1983).
F1 hybrid genomes normally are amphihaploids (containing the haploid set of
chromosomes from each of the parent species) and in most cases exhibit little or no
intergenomic chromosome association and exchange because the non-homologous
chromosomes from each parental genome cannot pair with each other during meiosis
(Baum, Lagudah and Appels, 1992; Jiang et al., 1993; Maan, 1987). Furthermore, such
F1 amphihaploid hybrids are normally sterile because the irregular meiotic division of single
set of non-homologous chromosomes results in the gametes with incomplete set of
chromosomes (Baum, Lagudah and Appels, 1992; Maan, 1987; Mujeeb-Kazi et al., 2013).
Both the sterility and absence of intergenomic association in F1 amphihaploid hybrids
hinder the introgression of genes between wheat and its wild relative species (Loureiro
et al., 2009; Mujeeb-Kazi et al., 2013).
Ploidy alterations
F1 amphihaploids can sometimes go through certain ploidy alterations either at the whole
ploidy or individual chromosome level, resulting in the conversion of amphihaploids into
amphiploids (containing at least one complete diploid set of chromosomes from each
parent species) or leading to the generation of aneuploidy progenies with the addition,
deletion or substitution of one or a few chromosomes (De Storme and Mason, 2014;
Feldman and Levy, 2012; Mujeeb-Kazi and Rajaram, 2002; Sharma and Gill, 1983). The
ploidy alterations via chromosome doubling or backcrossing can often restore the fertility

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  49

of hybrid progenies, facilitate the introgression of genes from wild related species into
wheat, and play an important role in the speciation and diversification of wheat (Mujeeb-
Kazi and Hettel, 1995; Zaharieva and Monneveux, 2006). In addition, some F 1
amphihaploids can eliminate the entire set of genomes from one parent followed by
genome doubling, resulting in the formation of double polyhaploid (Figure 4.1.) (Laurie,
1989; Laurie and Bennett, 1986, 1988a). These ploidy alterations can occur either
spontaneously or through cytogenetic manipulation (De Storme and Mason, 2014; Maan,
1987; Mujeeb-Kazi and Hettel, 1995).

Whole genome duplication and amphiploids

As described above, wheat itself was derived from two wide crosses, each resulting in the
formation of complete amphiploids and eventually the generation of the allohexaploid
wheat (Feuillet, Langridge and Waugh, 2008). Like many other crop species such as oats,
cotton and tobacco, wheat is presumed to have undergone a spontaneous whole genome
duplication through unreduced gametogenesis (with somatic chromosome number)
followed by subsequent fertilisation of unreduced female and male gametes, leading to the
generation of an amphiploid from natural wide hybridisations (De Storme and Mason, 2014;
Matsuoka, 2011). See also Section 1.1.1.. The whole genome duplication can also be
achieved through artificial treatment with antimicrotubule chemicals such as colchicine,
oryzalin, amiprophosmethyl (APM), trifluralin, and pronamide (Liu et al., 2014). Artificial
whole genome duplication processes have been employed in a variety of applications, e.g.
developing new alloploid crops such as the creation of Triticale (X Triticosecale Wittmack),
production of double haploids lines, and generation of amphidiploids as the bridge of alien
gene introgression into crops (Liu et al., 2016).
Amphiploid F1 hybrid plants between three species of the Aegilops genus and different T.
aestivum cultivars can show certain self-fertility, with averages of F1 hybrids bearing F2
seeds of 8.17%, 5.12% and 48.14% for Ae. biuncialis, Ae. geniculata and Ae. triuncialis,
respectively (Loureiro et al., 2009). All the F2 seeds studied were spontaneous amphiploids
(2n = 10x = 70) (Loureiro et al., 2009), evidence for possible spontaneous formation of
amphiploids between these three Aegilops species and hexaploid wheat.
A large number of synthetic amphiploids (complete or partial) have been developed with
many species and genera in tribe Triticeae (Liu et al., 2016). Triticale as the first man made
cereal crop was an amphiploid developed through the artificial cross between wheat and
rye (Briggle, 1969; Falk and Kasha, 1981; Li et al., 2015; Oettler, 2005). Synthetic
hexaploid wheat (2n = 6x = 42, BBAADD) has been produced by several approaches,
including: 1) crossing T. turgidum (2n = 4x = 28, BBAA) × Ae. tauschii (2n = 2x = 14, DD)
followed by artificial whole genome duplication; 2) crossing wheat (2n = 6x = 42, BBAADD)
× Ae. tauschii (2n = 2x = 14, DD); 3) crossing wheat (2n = 6x = 42, BBAADD) × T. turgidum
(2n = 4x = 28, BBAA) (Liu et al., 2016; Mujeeb-Kazi and Hettel, 1995; Ogbonnaya et al.,
2013). The A-genome synthetic hexaploids (2n = 6x = 42, BBAAAA) have also been
produced through crossing durum wheat (2n = 4x = 28, BBAA) with A genome diploid
species T. monococcum or T. urartu (2n = 2x = 14, AA) (Mujeeb-Kazi and Hettel, 1995).
Unlike the amphiploid crops such as wheat, Triticale, oat, and cotton that have already
gone through a long domestication and evolution process, the synthetic amphiploids are
unsuitable for being directly used as crops because they contain excessive alien genetic
materials with many undesirable genes, known as linkage drag, and are often associated
with genome shock and meiotic instability (Gaeta and Chris Pires, 2010). However, the
synthetic amphiploid wheat lines are easy to cross with wheat and provide a convenient
way to transfer genes from T. turgidum and Ae. tauschii into wheat (Ogbonnaya et al.,

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
50  ENV/CBC/MONO(2024)23

2013). Thus, wide hybridisation-derived amphiploids are mostly used as potent bridge
germplasm from which further genetic manipulations can lead to exploitable products such
as alien gene introgression or alien chromosome substitution, addition, and translocation
lines where undesired linkage drag is largely minimised (Ceoloni et al., 2015; Jiang et al.,
1993; Mujeeb-Kazi et al., 2013).

Aneuploidy

Aneuploidy lines contain increased or lost dosage of chromosomes compared to their wild
type counterparts. The hexaploid nature of wheat makes it tolerant to a certain level of
chromosome dosage changes and can lead to the generation of aneuploids at relatively
high frequency (De Storme and Mason, 2014; Feldman and Levy, 2005). The systematic
production of aneuploid lines in wheat was first reported in 1954, and these lines include
monosomic (2n = 20II + 1I = 41), nullisomic (2n = 20II = 40), trisomic
(2n = 20II + 1III = 43), tetrasomic (2n = 20II + 1IV = 44), and nulli-tetrasomic
(2n = 19II + 1IV = 42) (Joppa, 1987; Sears, 1954). In addition, more complicated forms of
aneuploids exist, such as a double monosomic line missing one chromosome from each
of two pairs of homologous chromosomes (2n = 19II + 1I + 1I = 40) or a double tetrasomic
line with an additional pair of two pairs of homologous chromosomes
(2n = 20II + 1IV + 1IV = 46) (Heyne, 1987; Joppa, 1987; McIntosh, 1987). There are also
aneuploidy lines called ditelosomic that lacks a pair of chromosome arms rather than
the whole chromosomes as well as deletion lines lacking a segment of a chromosome
(Endo and Gill, 1996). These aneuploids have been used extensively for the genetic and
genomic studies of wheat (Joppa, 1987; McIntosh, 1987; Qi et al., 2007).
Aneuploid F1 progenies have been reported in many interspecific and intergeneric crosses
such as the crosses of wheat with H. vulgare, Thinopyrum repens and Agropyron
desertorum (Mujeeb-Kazi and Hettel, 1995; Mujeeb-Kazi et al., 2013). Aneuploids can also
be generated from the selfed progenies or the backcross progenies of wide cross-derived
amphiploids (Mujeeb-Kazi and Hettel, 1995; Mujeeb-Kazi et al., 2013). From the wheat
genetic improvement point of view, the aneuploids are often more preferred over
the amphiploids (complete or partial) and this is especially true for the single chromosome
addition, substitution and translocation lines with a wheat genetic background. This is
because aneuploids carry less alien genetic material, with a reduced likelihood of linkage
drag and are much easier and more efficient to lead to translocations or subtle exchanges
through cytogenetic manipulation and breeding selection (Mujeeb-Kazi et al., 2013;
Qi et al., 2007). The rapid development of molecular cytogenetic, molecular marker, and
sequencing technologies further aid the development and identification of addition,
substitution and translocation lines (Qi et al., 2007). A variety of disease resistance genes
and abiotic stress tolerance genes have been transferred from wild Triticeae species into
wheat through the use of chromosome addition, substitution and translocation lines
(Ceoloni et al., 2015; Jiang et al., 1993; Liu et al., 2016; Mujeeb-Kazi and Rajaram, 2002;
Mujeeb-Kazi et al., 2013; Sharma and Gill, 1983). One of the most prominent examples of
the transfer of alien genetic variation into wheat is the 1BL/1RS translocation, in which
the short arm of rye chromosome one (1RS) was substituted for the long arm of Group 1
wheat Chromosome B (1BL) (Gupta and Vasistha, 2018).

Preferential elimination of constituent subgenome(s)

In some cases, the hybrid amphidiploids of some wide crosses preferentially eliminate
chromosomes from constituent subgenome(s) (Laurie, 1989; Laurie and Bennett, 1988b,
1986; Liu et al., 2016). For example, in the man-made octaploid Triticale that comprises

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  51

the R genome from its rye parent and A, B and D genomes from its wheat parent, either
the D genome or R genome was preferentially eliminated (Li et al., 2015; Liu et al., 2016).
The loss of D genome from octaploid Triticale led to the production of hexaploid Triticale
with complete A, B, and R genomes (Hao et al., 2013; Li et al., 2015) or hexaploid lines
with complete A and B genomes, and a composite genome consisting of the chromosomes
of D and R genomes (Dou et al., 2006). In F3 progenies derived from a cross between
wheat × Psathyrostachys huashanica amphiploid (2n = 56, ABBDDNsNs) and hexaploid
Triticale (2n = 42, BBAARR), it was found that both the A and B genomes remained
complete but the chromosomes of D, Ns and R genomes were eliminated. Comparatively,
the R genome chromosomes from rye were more likely to be retained in the progenies than
the Ns genome chromosomes from P. huashanica and D genome chromosomes from
wheat (Xie et al., 2012).

Uniparental genome elimination and polyhaploids

Unlike the normal F1 hybrids, which contains a complete haploid genome from each parent
species, some wide hybrids contain only one parent species’ haploid genome while the
other parent species’ haploid genome is eliminated, resulting in the production of haploid
hybrids (Mujeeb-Kazi, 1995). Such phenomenon of uniparental genome elimination has
been observed in a variety of crosses between Triticum and other genera within tribe
Triticeae, such as Hordeum, Elymus, and Agropyron as well as in wide crosses between
wheat and more distantly related species outside tribe Triticeae, such as Z. mays, S.
bicolor, Pennisetum glaucum, Coix lacryma-jobi, and Imperata cylindrica (Laurie, 1989;
Laurie and Bennett, 1988b, 1986; Liu et al., 2014, 2016; Mujeeb-Kazi, 1995). Chromosome
elimination of one parental genome after fertilisation can occur either spontaneously or
through modified pollination methods in vivo, or by in vitro culture of immature male or
female gametophytes in intraspecific, interspecific, intergeneric, or more distant hybrids in
many species (Dunwell, 2010). This uniparental genome elimination process leads to
the generation of polyhaploid wheat. For example, wide crosses between wheat and maize
has become a main approach for double haploid production in wheat (Niu et al., 2014).
No introgression of maize DNA into wheat has been found (Brazauskas, Pasakinskiene
and Jahoor, 2004). The polyhaploid wheat can then lead to the production of double
haploids through either spontaneous or artificial chromosome doubling. Double haploids in
wheat can benefit breeders to quickly fix genetic recombination and increase breeding
efficiency.
Genetic recombination (transfer)
Polyploidisation and genetic recombination are two important driving factors for speciation
and evolution of many plant species (De Storme and Mason, 2014; Gaeta and Chris Pires,
2010; Lambing et al., 2017; Pelé, Rousseau-Gueutin and Chèvre, 2018). From the
standpoint of crop genetic improvement, genetic recombination also plays an important
role in facilitating the transfer of desirable genes from wild relative species to wheat. This
is because wide hybrids and their derivatives with different ploidy levels of alien genomes
are usually associated with a number of undesirable characteristics, such as: poor fertility,
low yield and late maturation, to list a few, and genetic recombination between wheat and
its wild relative chromosomes will help remove or at least minimise these linkage drags
(Feuillet, Langridge and Waugh, 2008; Mujeeb-Kazi et al., 2013; Qi et al., 2007).
Homologous and homoeologous recombination are two major genetic recombination
mechanisms in allopolyploid species such as wheat (Jiang et al., 1993; Qi et al., 2007).
Chromosome pairs originating from a common ancestry are considered homologous,

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
52  ENV/CBC/MONO(2024)23

whereas chromosomes derived from different species are considered homoeologous

(Glover, Redstig and Dessimoz, 2016). While recombination predominantly occurs among
DNA sequences on homologous chromosomes, it may also occur among sequences on
homoeologous chromosomes in allopolyploids (Gaeta and Chris Pires, 2010).

Homologous recombination

As described above, wheat, an allohexaploid, comprises three subgenomes (A, B and D)

derived from three ancestral diploid progenitors. However, the allohexaploid wheat
behaves cytologically like a diploid during its meiosis process because only homologous
chromosomes can pair and the meiotic pairing of homoeologous chromosomes is
suppressed by a pairing homoeologous (Ph) gene system in wheat (Kimber and Sears,
1987; Sears, 1976). This Ph gene system includes a major gene called Ph1 on the long
arm of chromosome 5B (Kimber and Sears, 1987; Riley, 1966; Riley, Chapman and
Kimber, 1959), an intermediate effect gene, Ph2, on chromosome 3D and several minor
loci (Sears, 1976). The pairing suppression genes Ph1 and Ph2 are also shown to
suppress the pairing between wheat and alien chromosomes in wide cross hybrids (Liu
et al., 2014). Homologous chromosomes undergo crossing over (reciprocal exchange) and
non-crossover gene conversion events during meiosis, leading to novel genetic variations
(Lambing et al., 2017).
As previously described, common wheat and the species in primary and secondary gene
pools share at least one subgenome, and when brought together through wide crossing
these homologous subgenomes can exchange genetic information via homologous
recombination. A number of desirable genes have been successfully transferred from
primary and secondary gene pool species into wheat through the homologous
recombination between the shared subgenomes of wheat and its wild relative species (Liu
et al., 2016; Mujeeb-Kazi and Rajaram, 2002; Mujeeb-Kazi et al., 2013). Numerous
examples of alien genes transferred from different wheat wild relatives into wheat have
been summarized by (Molnár-Láng, Ceoloni and Doležel et al., 2015).

Homoeologous recombination

The species within the tertiary gene pool contain genomes that are non-homologous to the
wheat genomes, and thus their genes cannot be transferred to wheat by homologous
recombination, making the exploitation of tertiary gene species more difficult (Mujeeb-Kazi
and Rajaram, 2002). However, technologies that assist homoeologous gene transfers can
be achieved through techniques such as, genetic manipulations of Ph genes controlling
chromosome pairing, irradiation, and tissue culture-induced translocations (Griffiths et al.,
2006; Mujeeb-Kazi and Rajaram, 2002).
To achieve the transfer of desirable genes into wheat from a wild species with non
homologous genomes, chromosomes of such wild species must be able to pair with wheat
chromosomes. As described above, Ph1 and Ph1-like genes normally suppress
intergenomic chromosome paring. When the Ph1 gene is absent or inactivated,
considerable pairing can occur between homoeologous chromosomes (Mujeeb-Kazi and
Rajaram, 2002). The genetic manipulation of the Ph1 gene to promote homoeologous
chromosome paring and the subsequent recombination has been extensively used for
alien gene introgression (Liu et al., 2016; Mujeeb-Kazi and Rajaram, 2002; Qi et al., 2007).
Segmental introgression lines have been developed from the hybridisation between wheat
and perennial wheat relatives. As reviewed by Hegde and Waines (2004), the effect of
the Ph1 gene can also be suppressed under certain genetic backgrounds such as

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  53

the diploid Aegilops species (Chen, Tsujimoto and Gill, 1994; Hegde and Waines, 2004).
The facilitation of homoeologous chromosome pairing through manipulating Ph genes can
enhance intergenomic recombination between wheat chromosomes and their
homoeologous chromosomes in related species, promoting the transfer of the desired
gene from certain alien chromatin segments while reducing the amounts of unwanted alien
DNA (Baum, Lagudah and Appels, 1992; Liu et al., 2014). In addition to the manipulation
of Ph genes, irradiation mutation and tissue culture-mediated somaclonal variations also
have been used to induce chromosome breakage and rearrangement in wheat wide
hybridisation (Baum, Lagudah and Appels, 1992; Jauhar and Chibbar, 1999).
It is worth noting that while homoeologous recombination can generate novel gene
combinations and phenotypes, it may also destabilise the karyotype and lead to aberrant
meiotic behaviour and reduced fertility (De Storme and Mason, 2014; Gaeta and Chris
Pires, 2010). Selection, either naturally or artificially, plays a significant role to retain
the lines with the desirable fertility, stabilised chromosome inheritance, and advantageous
variations (Gaeta and Chris Pires, 2010).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
54  ENV/CBC/MONO(2024)23

SECTION 5. General interactions with

other organisms (ecology)
5.1. Interactions in agricultural ecosystems
5.1.1. Weeds
Competition with other plants for light, water and nutrition can reduce the yield of wheat
crops. There are a number of weeds which are associated with wheat crops, however not
all warrant control in wheat production or in all seasons. Weeds common in wheat crops
world-wide include wild oats (Avena fatua L.), fat hen (Chenopodium album), canary grass
(Phalaris minor) and cleavers (Galium aparine) (Jabran et al., 2017). Examples of weeds
affecting wheat in specific countries are shown in Table 5.1.. Herbicide resistance is an
important problem in the management of weeds and several of the most common weeds,
namely wild oats, canary grass and annual ryegrass exhibit resistance to herbicides
globally (Heap, 2018).
5.1.2. Vertebrate pests
Damage to wheat crops by birds has been noted around the world. Birds such as geese
(Cummings, 2016), crows (Kennedy and Connery, 2008), cockatoos (Temby and Marshall,
2003) and sparrows (Dawson, 1970) feed on seeds, dig and tear out plants, or otherwise
damage cereal crops. Birds may also damage grain stored on farm (GRDC, 2014, 2015).
Animals such as feral pigs, wild boars, kangaroos, rabbits, moose and deer can also cause
considerable damage to wheat plants by feeding on seedlings or trampling mature plants
(Amici et al., 2012; Gentle, Phinn and Speed, 2010).
Rodents cause significant losses to wheat crops worldwide both directly by gnawing and
feeding and indirectly by spoilage and contamination. This damage can be highly variable
and strongly dependent on rodent density (Brown et al., 2007). Rodents are opportunistic
feeders and their diet can include seeds, the pith of stems and other plant materials
(Caughley et al., 1998). Rodents may eat seeds at the seed source or they may hoard
seed (AGRI-FACTS, 2002). The dominant rodent pest in wheat crops depends on the
geographical location. For example, the lesser bandicoot rat (Bandicota bengalensis) in
India (Parshad, 1999), the montane vole (Microtus montanus) in North America (Witmer et
al., 2007), the common vole (Microtus arvalis) in Europe (Jacob and Tkadlec, 2010) and
the house mouse (Mus musculus) in Australia (ACIAR, 2003) are the predominating rodent
pests, respectively.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  55

Table 5.1. Common weeds in wheat crops

Location Scientific name Common name References
Australia Raphanus raphanistrum wild radish GRDC, 2014, 2015
Lolium rigidum Gaudin annual ryegrass
Phalaris paradoxa L. paradoxa grass
Echinochloa colona (L.) awnless barnyard
Link. grass
Conyza spp. fleabane
Bromus spp. brome grasses
Canada Polygonum convolvulus L. wild buckwheat Mason and Spaner, 2006
Stellaria media L. chickweed
Sinapsis arvensis L. wild mustard
Cirsium arvense L. Canada thistle
Europe Alopecurus myosuroides blackgrass Gianessi, Sankula and Reigner,
Agrostis spp. bentgrass 2003
Bromus sterilis sterile brome
Lolium spp. ryegrass
Poa annua annual meadowgrass
Tripleurospermum scentless chamomile
inodorum common chickweed
Stellaria media L. corn poppy
Papaver rhoeas speedwells
Veronica spp. Canada thistle
Cirsium arvense L. field pansy
Viola arvensis wild radish
Raphanus raphanistrum wild mustard
Sinapsis arvensis L. shepherds’ purse
Capsella bursa-pastoris bifora
Bifora spp. field bindweed
Convolvulus arvensis hemp nettle
Galeopsis spp. knotweed
Polygonum spp.
China (People’s Republic of) and Calystegia spp. bindweed Jabran et al., 2017
India Capsella bursa-pastoris shepherds’ purse
Cirsium spp. thistles
United States Kochia scoparia kochia Van Wychen, 2017
Avena fatua wild oat
Polygonum convolvulus wild buckwheat
Setaria spp. green foxtail
Chenopodium album common
Bromus spp. lambsquarters
Lamium spp. cheatgrass
henbit or deadnettle

5.1.1. Invertebrate pests

Many insects have been described to affect wheat worldwide. Although damage caused
by most of these insects is either insignificant or limited to isolated areas, some pests inflict
serious yield and forage losses. Some of these pest problems are directly linked to the
unique farming system employed in a particular area, while other pests are opportunistic
or generalist herbivores that do not specifically target wheat as a host (Miller and Pike,
2002). Insects usually do not cause major direct damage in wheat, unless populations
reach very high levels (Duveiller, Singh and Nicol, 2007). More information on insect pests
affecting wheat can be found in Miller and Pike (2002) and Annex A.
Wheat stored as grain is subject to a number of insect and mite pests, many of which have
developed cosmopolitan distributions over the years (Miller and Pike, 2002). Proper

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
56  ENV/CBC/MONO(2024)23

storage involves ensuring that stored grain is free from insects and then maintaining grain
moisture and temperature at sufficiently low levels to inhibit insect activity and
development. Treatment of contaminated grain with appropriate chemicals prevents pest
outbreaks. Many insect species have developed resistance to frequently used pesticides.
Current research on stored product integrated pest management seeks to develop
biological and cultural control methods for managing pests (Miller and Pike, 2002).
5.1.2. Pathogens
Wheat diseases can reduce the quantity and quality of grain yield. It has been estimated
that 12.4% of world-wide wheat yield is lost annually due to diseases (Oerke, 2006). The
incidence and impact of pathogens varies across and within wheat growing regions but
typically increases with the intensity of the crop productivity (Oerke, 2006). Foliar diseases
are the most important yield constraints in irrigated wheat systems that are characterised
by dense stands and high tiller density. In contrast, soil borne pathogens are more
frequently found in rain-fed wheat cropping (Duveiller, Singh and Nicol, 2007). Disease
management plans rely on cultural practices that break the disease cycles, the application
of biocide chemicals and planting disease resistant wheat varieties. Disease resistant
varieties are developed by breeding programs that survey for sources of genetic resistance
and combine a number of resistance factors in a single variety. There are also coordinated
international surveillance programs that monitor the progress of diseases worldwide and
guide management strategies (e.g. RustTracker.org 13).
Information on wheat diseases commonly found in agricultural systems can be found in
specialised compendia (Bockus et al., 2010; Mehta, 2014; Wiese, 1987) and Annex A.
Foliar diseases caused by fungi
Some of the most important wheat diseases caused by fungi are wheat rusts, the blotch
diseases and Fusarium head blight/scab disease.
Wheat rust diseases are caused by fungi belonging to the genus Puccinia. P. graminis f.
sp. tritici, P. striiformis f. sp. tritici and P. triticina are the causal agents of the stem, stripe
and leaf rusts, respectively. Wheat rust diseases cause substantial losses in global wheat
production annually. A large percentage of wheat varieties cultivated worldwide are
susceptible to these diseases and in some cases susceptibility leads to 80-100% yield
losses. The constant emergence of new virulent strains of wheat rusts constitutes a threat
to wheat production across the globe (Figueroa, Hammond-Kosack and Solomon, 2018).
Three different diseases caused by Ascomycete fungi are known as the blotch diseases:
Septoria tritici blotch (caused by Zymoseptoria tritici), Septoria nodorum blotch
(Parastagonospora nodorum) and tan spot (Pyrenophora tritici-repentis). Septoria tritici
blotch is the primary leaf disease of wheat in temperate growing regions and is regarded
as the primary threat to wheat production in Europe. Septoria nodorum blotch and tan spot
are prominent diseases in Australia (Figueroa, Hammond-Kosack and Solomon, 2018).
Tan spot is also common in other parts of the wheat-growing world. The use of minimum
or zero tillage practices may be increasing the incidence of tan spot.
Fusarium head blight disease, also known as wheat scab or ear blight, is the most serious
and hazardous floral disease of wheat. The disease is caused by the Ascomycete fungus
Fusarium graminearum and leads to the premature senescence of the wheat head. Wheat
crops are particularly prone to Fusarium head blight if rain prevails just prior to and during
crop flowering. Fusarium head blight causes a reduction in grain yield and quality, but also

13 https://rusttracker.cimmyt.org/

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  57

leads to the accumulation in the grain of various mycotoxins that represent a major food
safety risk and health hazard to humans and animals. In many countries, legal limits are in
place on the permitted mycotoxin levels for the various end-uses (Figueroa, Hammond-
Kosack and Solomon, 2018).
Soilborne pathogens
Soilborne pathogens have a global distribution and cause economic yield losses in areas
of the world where cereals dominate the cropping system and result in suboptimal growing
conditions. Soilborne disease pathogens of cereals invade the crown and root tissues,
diminishing their capacity for efficient nutrient and water uptake (Singleton, 2002). In
dryland areas, a complex of soilborne fungi and nematodes cause root rot diseases. Some
of the most important diseases caused by soilborne pathogens include the root lesion and
the cereal cyst nematode complex, caused by the nematodes Pratylenchus thornei and
Heterodera spp. respectively. Similarly, soilborne fungi Fusarium graminearum and
Bipolaris sorokiniana cause crown rot and common root rot diseases, respectively.
Viral pathogens
A number of viral diseases of wheat are present regionally and globally; refer to Annex A
for a list. Viruses have high potential for economic impact, capable of causing high (up to
96%) localised yield losses when coinfection by multiple viruses occurs (Byamukama et
al., 2014). Viral diseases may become more prevalent with climate change, as elevated
CO2 levels can increase viral titres (Trębicki et al., 2015).
The predominant and most economically relevant virus in North America is the Wheat
streak mosaic virus (WSMV, genus Tritimovirus) (Byamukama et al., 2014), which can
contribute to significant losses. WSMV is increasingly globally relevant, found in the
Americas, Australia, Europe, Asia, and North Africa (Brunt et al., 1996; Dwyer et al., 2007;
Singh et al., 2018). Barley yellow dwarf virus (BYDV, genus Luteovirus) is another
significant viral disease (Choudhury et al., 2019) that is found globally. In individual wheat
fields prone to infection, it can cause average yield loss of between 11% and 33%
(reviewed by Walls III, Rajotte and Rosa, 2019).
An important vector of the WSMV and at least four other viruses is wheat curl mite (WCM),
Aceria tosichella (reviewed by Skoracka, Rector and Hein, 2018). Transmission occurs by
wind dispersion of viruliferous WCMs from infected plants (Singh et al., 2018). Other
transmission vectors include aphids (e.g. for BYDV transmission) and seeds, with vectors
and viruses benefitting from ‘green bridges’ between growing seasons where viruses may
be harboured by alternate host organisms (Dwyer et al., 2007; Singh et al., 2018).
Infected wheat displays symptoms that are dependent on the virus. Typically, chlorosis
and streaking of leaves are observed for mosaic viruses, and purpling for yellow dwarf
viruses. Mixed infections produce more severe symptoms, but no additional unique
symptoms (Burrows et al., 2009). Symptoms are fewer and milder as plants mature and
they become more resistant to infection (Somsen and Sill, 1970).
5.1.3. Non-domesticated animals and incidental human contact
Wheat is generally considered non-toxic to animals, supported by the diversity of pest
species (see Sections 5.1.2. and 5.1.3.) and its use as food and feed. Wheat pollen is able
to induce grass pollen allergic reactions (Constantin et al., 2009). High exposure is limited
to peak flowering season (Andersson and Lidholm, 2003) and by the limited travelling
distance of wheat pollen (see Section 2.2.). Details on food allergens and the use of wheat
in food and feed can be found in the OECD ‘Consensus document on compositional

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
58  ENV/CBC/MONO(2024)23

considerations for new varieties of bread wheat (Triticum aestivum): key food and feed
nutrients, anti-nutrients and toxicants’ (OECD, 2003).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  59

References
Abbott, R.J. (1992), “Plant invasions, interspecific hybridization and the evolution of new plant taxa”, Trends in
Ecology & Evolution, Vol. 7, pp. 401-405, https://doi.org/10.1016/0169-5347(92)90020-c.
Acevedo, E., P. Silva, and H. Silva (2002), “Wheat growth and physiology”, in Bread Wheat: Improvement and
Production, B.C. Curtis, S. Rajaram, and H. Gómez Macpherson (eds), Food and Agriculture Organization of the
United Nations, Rome, ISBN: 92-5-104809-6.
ACIAR (2003), Rodents: Losses and Control in Primary Produce, Report No. 64, Australian Centre for International
Agricultural Research.
AGRI-FACTS (2002), “Mice and their control”, Report No. Agdex 683, Alberta Agriculture, Food and Rural
Development.
Agriculture Victoria (2012a), “Growing wheat”, Department of Environment and Primary Industries, State of Victoria.
Agriculture Victoria (2012b), “Identification of cereal seedlings”, Department of Environment and Primary Industries,
State of Victoria.
Allan, R.E. (1980), “Wheat”, in Hybridization of Crop Plants, W.R. Fehr and H.H. Hadley (eds.), American Society of
Agronomy, Crop Science Society of America, pp. 709-720, https://doi.org/10.2135/1980.hybridizationofcrops.c51.
Allard, R.W. (1999), Principles of Plant Breeding, 2nd ed., John Wiley & Sons, Inc., New York, ISBN: 978-0-471-
02309-8.
Amici, A. et al. (2012), “Increase in crop damage caused by wild boar (Sus scrofa L.): the ‘refuge effect’”, Agronomy
for Sustainable Development, Vol. 32, pp. 683-692, https://doi.org/10.1007/s13593-011-0057-6.
Anderson, E. (1949), Introgressive Hybridization, John Wiley & Sons, Inc., New York.
Anderson, E., and L. Hubricht (1938), “Hybridization in Tradescantia. III. The evidence for introgressive
hybridization”, American Journal of Botany, Vol. 25, pp. 396-402, https://doi.org/10.1002/J.1537-
2197.1938.TB09237.X.
Anderson, R.L., and D.C. Nielsen (1996), “Emergence pattern of five weeds in the central Great Plains”, Weed
Technology, Vol. 10, pp. 744-749, https://doi.org/10.1017/S0890037X00040756.
Anderson, R.L., and G. Soper (2003), “Review of volunteer wheat (Triticum aestivum) seedling emergence and seed
longevity in soil”, Weed Technology, Vol. 17, pp. 620-626, https://doi.org/10.1614/0890-
037X(2003)017[0620:ROVWTA]2.0.CO;2.
Anderson, W., and A. Impiglia (2002), “Management of dryland wheat”, in Bread Wheat: Improvement and
Production, B.C. Curtis, S. Rajaram, and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the
United Nations, Rome, pp. 407-432.
Andersson, K., and J. Lidholm (2003), “Characteristics and immunobiology of grass pollen allergens”, International
Archives of Allergy and Immunology, Vol. 130, pp. 87-107, https://doi.org/10.1159/000069013.
Anzooman, M. et al. (2018), “Selection for rapid germination and emergence may improve wheat seedling
establishment in the presence of soil surface crusts”, Plant and Soil, Vol. 426, pp. 227-239,
https://doi.org/10.1007/s11104-018-3609-6.
Arrigo, N. et al. (2011), “Gene flow between wheat and wild relatives: empirical evidence from Aegilops geniculata,
Ae. neglecta and Ae. triuncialis”, Evolutionary Applications, Vol. 4, pp. 685-695, https://dx.doi.org/10.1111/j.1752-
4571.2011.00191.x.
Asker, S. (1979), “Progress in apomixis research”, Hereditas, Vol. 91, pp. 231-240, https://doi.org/10.1111/j.1601-
5223.1979.tb01665.x.
Asseng, S. et al. (2012), “Wheat”, in Crop Yield Response to Water, P. Steduto et al. (eds.), Food and Agriculture
Organization of the United Nations, Rome, pp. 92-103.
Baenziger, P.S., and R.M. DePauw (2009), “Wheat breeding: procedures and strategies”, in Wheat Science and
Trade, B.F. Carver (ed.), Wiley-Blackwell,pp. 273-308.
Bakker, J.P. et al. (1996), “Seed banks and seed dispersal: important topics in restoration ecology”, Acta Botanica
Neerlandica, Vol. 45, pp. 461-490, https://doi.org/10.1111/j.1438-8677.1996.tb00806.x.
Barkworth, M.E. and R. von Bothmer (2009), “Scientific names in the Triticeae”, in Genetics and Genomics of the
Triticeae, G.J. Muehlbauer and C. Feuillet (eds.), Springer, New York, pp. 3-30, https://doi.org/10.1007/978-0-
387-77489-3_1.
Bassi, F.M. et al. (2016), “Breeding schemes for the implementation of genomic selection in wheat (Triticum spp.)”,
Plant Science, Vol. 242, pp. 23-36, https://doi.org/10.1016/j.plantsci.2015.08.021.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
60  ENV/CBC/MONO(2024)23

Baum, M., E.S. Lagudah and R. Appels (1992), “Wide crosses in cereal”, Annual Review of Plant Physiology and
Plant Molecular Biology, Vol. 43, pp. 117-143, https://doi.org/10.1146/annurev.pp.43.060192.001001.
Beckie, H.J., and M.D.K. Owen (2007), “Herbicide-resistant crops as weeds in North America”, CAB Reviews:
Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, Vol. 2, 22,
https://doi.org/10.1079/PAVSNNR20072044.
Bell, J. et al. (2016), “The Importance of Controlling Volunteer Wheat”, Report No. SCS-2016-28, Texas A&M
AgriLife Extension Service.
Beri, S.M. and S.C. Anand (1971), “Factors affecting pollen shedding capacity in wheat”, Euphytica, Vol. 20, pp. 327-
332, https://doi.org/10.1007/BF00056096.
Bernard, S. and M. Bernard (1987), “Creating new forms of 4x, 6x and 8x primary triticale associating both complete
R and D genomes”, Theoretical and Applied Genetics, Vol. 74, pp. 55-59, https://doi.org/10.1007/bf00290083.
Bernhardt, N. (2015), “Taxonomic treatments of Triticeae and the wheat genus Triticum”, in Alien Introgression in
Wheat: Cytogenetics, Molecular Biology, and Genomics, M. Molnár-Láng, C. Ceoloni and J. Doležel (eds.),
Springer, Cham, pp. 1-19, https://doi.org/10.1007/978-3-319-23494-6_1.
Bhaskaran, S. and R.H. Smith (1990), “Regeneration in cereal tissue culture: a review”, Crop Science, Vol. 30,
pp. 1328-1337, https://doi.org/10.2135/cropsci1990.0011183X003000060034x.
Bicknell, R.A. and A.M. Koltunow (2004), “Understanding apomixis: recent advances and remaining conundrums”,
The Plant Cell, Vol. 16, pp. S228-S245, https://dx.doi.org/10.1105/tpc.017921.
Bockus, W.W. et al. (2010), Compendium of Wheat Diseases and Pests, American Phytopathological Society, St.
Paul, Minnesota, https://doi.org/10.1094/9780890546604.
Bonjean, A.P. and W.J. Angus (2001), The World Wheat Book: A History of Wheat Breeding, Vol. 1, Lavoisier.
Bonjean, A.P., W.J. Angus and M. van Ginkel (2011), The World Wheat Book: A History of Wheat Breeding, Vol. 2,
Lavoisier.
Bowden, W.M. (1959), “The taxonomy and nomenclature of the wheats, barleys, and ryes and their wild relatives”,
Canadian Journal of Botany, Vol. 37, pp. 657-684, https://doi.org/10.1139/b59-053.
Bowran, D. (2000), “Weed control in wheat”, in The Wheat Book Principles and Practice, W.K. Anderson and J.R.
Garlinge (eds.), Department of Primary Industries and Regional Development, Western Australia, Perth, Bulletin
4443, pp. 245-258.
Braun, H.J. and N.N. Sãulescu (2002), “Breeding winter and facultative wheat”, in Bread Wheat, B.C. Curtis, S.
Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the United Nations, Rome.
Brazauskas, G., I. Pasakinskiene and A. Jahoor (2004), “AFLP analysis indicates no introgression of maize DNA in
wheat x maize crosses”, Plant Breeding, Vol. 123, pp. 117-121, https://doi.org/10.1046/j.1439-
0523.2003.00927.x.
Brenchley, R., et al. (2012), “Analysis of the bread wheat genome using whole-genome shotgun sequencing”,
Nature, Vol. 491, pp. 705-710, https://doi.org/10.1038/nature11650.
Briggle, L.W. (1969), “Triticale - a review”, Crop Science, Vol. 9, pp. 197-202,
https://doi.org/10.2135/cropsci1969.0011183X000900020026x.
Briggle, L.W. and B.C. Curtis (1987), “Wheat worldwide”, in Wheat and Wheat Improvement, 2nd ed., E.G. Heyne
(ed.), American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, pp. 1-
32, https://doi.org/10.2134/agronmonogr13.2ed.c1.
Brown, P.R. et al. (2007), “Relationship between abundance of rodents and damage to agricultural crops”,
Agriculture, Ecosystems & Environment, Vol. 120, pp. 405-415, https://doi.org/10.1016/j.agee.2006.10.016.
Brumlop, S., T. Pfeiffer and M.R. Finckh (2017), “Evolutionary effects on morphology and agronomic performance of
three winter wheat composite cross populations maintained for six years under organic and conventional
conditions”, Organic Farming, Vol. 3, pp. 34-50, https://doi.org/10.12924/of2017.03010034.
Brunt, A. et al. (1996), Viruses of Plants. Descriptions and Lists from the VIDE Database, CAB International,
Wallingford, U.K., ISBN: 9780851987941.
Burrows, M. et al. (2009), “Occurrence of viruses in wheat in the Great Plains region, 2008”, Plant Health Progress,
Vol. 10, https://doi.org/10.1094/PHP-2009-0706-01-RS.
Byamukama, E. et al. (2014), “Quantification of yield loss caused by Triticum mosaic virus and Wheat streak mosaic
virus in winter wheat under field conditions”, Plant Disease, Vol. 98, pp. 127-133, https://doi.org/10.1094/pdis-04-
13-0419-re.
Calflora (2019), Information on wild California Plants, https://www.calflora.org/ (accessed 10 October 2019).
Castillo, A., S.G. Atienza and A.C. Martin (2014), “Fertility of CMS wheat is restored by two Rf loci located on a
recombined acrocentric chromosome”, Journal of Experimental Botany, Vol. 65, pp. 6667-6677,
https://doi.org/10.1093/jxb/eru388.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  61

Caughley, J. et al. (1998), Managing Vertebrate Pests: Rodents, Bureau of Rural Sciences, Canberra, ISBN:
9780644292405.
Ceoloni, C. et al. (2015), “Wheat-perennial Triticeae introgressions: major achievements and prospects”, in Alien
Introgression in Wheat: Cytogenetics, Molecular Biology, and Genomics, M. Molnár-Láng, C. Ceoloni and J.
Doležel (eds.), Springer International Publishing, Cham, pp. 273-313, https://doi.org/10.1007/978-3-319-23494-
6_11.
Chandra, S. and S.P. Bhatnagar (1974), “Reproductive biology of Triticum. II. Pollen germination, pollen tube growth,
and its entry into the ovule”, Phytomorphology, Vol. 24, pp. 211-217.
Chapman, V., T.E. Miller and R. Riley (1976), “Equivalence of the A genome of bread wheat and that of Triticum
urartu”, Genetical Research, Vol. 27, pp. 69-76, https://doi.org/10.1017/S0016672300016244.
Chen, P.D., H. Tsujimoto and B.S. Gill (1994), “Transfer of PhI genes promoting homoeologous pairing from Triticum
speltoides to common wheat”, Theoretical and Applied Genetics, Vol. 88, pp. 97-101,
https://doi.org/10.1007/bf00222400.
Chhokar, R.S. et al. (2007), “Effect of tillage and herbicides on weeds and productivity of wheat under rice–wheat
growing system”, Crop Protection, Vol. 26, pp. 1689-1696, http://dx.doi.org/10.1016/j.cropro.2007.01.010.
Choudhury, S. et al. (2019), “Barley yellow dwarf virus infection affects physiology, morphology, grain yield and flour
pasting properties of wheat”, Crop and Pasture Science, Vol. 70, pp. 16-25, https://doi.org/10.1071/CP18364.
Clarke, J.M. (1985), “Harvesting losses of spring wheat in windrower/combine and direct combine harvesting
systems”, Agronomy Journal, Vol. 77, pp. 13-17, https://doi.org/10.2134/agronj1985.00021962007700010004x.
Clayton, W.D. et al. (2015), World Checklist of Poaceae, the Royal Botanic Gardens, Kew,
https://wcsp.science.kew.org/ (accessed June 2015).
Constantin, C. et al. (2009), “Micro-arrayed wheat seed and grass pollen allergens for component-resolved
diagnosis”, Allergy, Vol. 64, pp. 1030-1037, https://doi.org/10.1111/j.1398-9995.2009.01955.x.
Coutts, B. et al. (2008), “The epidemiology of Wheat streak mosaic virus in Australia: case histories, gradients, mite
vectors, and alternative hosts”, Australian Journal of Agricultural Research, Vol. 59, pp. 844-853,
https://doi.org/10.1071/AR07475.
Crane, C.F. and J.G. Carman (1987), “Mechanisms of apomixis in Elymus rectisetus from Eastern Australia and New
Zealand”, American Journal of Botany, Vol. 74, pp. 477-496, https://doi.org/10.1002/j.1537-2197.1987.tb08668.x.
Cummings, J. (2016), Geese, Ducks and Coots, Wildlife Damage Management Technical Series, U.S. Department
of Agriculture,
https://www.aphis.usda.gov/wildlife_damage/reports/Wildlife%20Damage%20Management%20Technical%20Ser
ies/GeeseDucksCoots-WDM-Technical-Series.pdf.
Curtis, B.C., S. Rajaram and H. Gómez Macpherson (2002), Bread Wheat - Improvement and Production, Food and
Agriculture Organization of the United Nations, Rome, ISBN: 92-5-104809-6.
D’Souza, L. (1970), “Investigations concerning the suitability of wheat as pollen-donor for cross-pollination by wind
as compared to rye, Triticale and Secalotricum”, Zeitschrift fur Pflanzenzuechtung, Vol. 63, pp. 246-269 (in
German).
DAF (2012), “Wheat – planting information”, Department of Agriculture and Fisheries, Queensland.
Davies, S.J.J.F. (1978), “The food of emus”, Australian Journal of Ecology, Vol. 3, pp. 411-422,
https://doi.org/10.1111/j.1442-9993.1978.tb01189.x.
Dawson, D.G. (1970), “Estimation of grain loss due to sparrows (Passer domesticus) in New Zealand”, New Zealand
Journal of Agricultural Research, Vol. 13, pp. 681-688, https://doi.org/10.1080/00288233.1970.10421615.
De Corby, K.A. et al. (2007), “Emergence timing and recruitment of volunteer spring wheat”, Weed Science, Vol. 55,
pp. 60-69, https://doi.org/10.1614/WS-06-102.1.
De Storme, N. and A. Mason (2014), “Plant speciation through chromosome instability and ploidy change: Cellular
mechanisms, molecular factors and evolutionary relevance”, Current Plant Biology, Vol. 1, pp. 10-23,
https://doi.org/10.1016/j.cpb.2014.09.002.
De Vries, A.P. (1971), “Flowering biology of wheat, particularly in view of hybrid seed production – A review”,
Euphytica, Vol. 20, pp. 152-170, https://doi.org/10.1007/BF00056076.
Donald, W.W. and A.G. Ogg, Jr. (1991), “Biology and control of jointed goatgrass (Aegilops cylindrica), a review”,
Weed Technology, Vol. 5, pp. 3-17, https://doi.org/10.1017/S0890037X00033170.
Dong, S. et al. (2016), “Investigating pollen and gene flow of WYMV-resistant transgenic wheat N12-1 using a dwarf
male-sterile line as the pollen receptor”, PLoS ONE, Vol. 11, e0151373,
https://doi.org/10.1371/journal.pone.0151373.
Dorofeev, V.F. (1969), “Spontaneous hybridization in wheat populations of Transcaucasia”, Euphytica, Vol. 18,
pp. 406-416, https://doi.org/10.1007/BF00397790.
Dou, Q.W. et al. (2006), “Molecular cytogenetic analyses of hexaploid lines spontaneously appearing in octoploid

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
62  ENV/CBC/MONO(2024)23

Triticale”, Theoretical and Applied Genetics, Vol. 114, pp. 41-47, https://doi.org/10.1007/s00122-006-0408-x.
Dubcovsky, J. and J. Dvorak (2007), “Genome plasticity a key factor in the success of polyploid wheat under
domestication”, Science, Vol. 316, pp. 1862-1866, https://doi.org/10.1126/science.1143986.
Dunwell, J.M. (2010), “Haploids in flowering plants: origins and exploitation”, Plant Biotechnology Journal, Vol. 8,
pp. 377-424, https://doi.org/10.1111/j.1467-7652.2009.00498.x.
Duveiller, E., R.P. Singh and J.M. Nicol (2007), “The challenges of maintaining wheat productivity: pests, diseases,
and potential epidemics”, Euphytica, Vol. 157, pp. 417-430, https://doi.org/10.1007/s10681-007-9380-z.
Dvořák, J. (1976), “The relationship between the genome of Triticum urartu and the A and B genomes of Triticum
aestivum”, Canadian Journal of Genetics and Cytology, Vol. 18, pp. 371-377, https://doi.org/10.1139/g76-045.
Dvorak, J. et al. (2006), “Molecular characterization of a diagnostic DNA marker for domesticated tetraploid wheat
provides evidence for gene flow from wild tetraploid wheat to hexaploid wheat”, Molecular Biology and Evolution,
Vol. 23, pp. 1386-1396, https://doi.org/10.1093/molbev/msl004.
Dwyer, G.I. et al. (2007), “Wheat streak mosaic virus in Australia: Relationship to isolates from the Pacific Northwest
of the USA and its dispersion via seed transmission”, Plant Disease, Vol. 91, pp. 164-170,
https://doi.org/10.1094/pdis-91-2-0164.
Endo, T.R. and B.S. Gill (1996), “The deletion stocks of common wheat”, Journal of Heredity, Vol. 87, pp. 295-307,
https://doi.org/10.1093/oxfordjournals.jhered.a023003.
European Biomass Industry Association (2017), “Bioethanol in the world”, European Biomass Industry Association.
Evans, L., I. Wardlaw and R. Fischer (1975), “Wheat”, in Crop Physiology: Some Case Histories, L. Evans (ed.)
Cambridge University Press, Cambridge, pp. 101-149, ISBN: 978-0521204224.
Falk, D.E. and K.J. Kasha (1981), “Comparison of the crossability of rye (Secale cereale) and Hordeum bulbosum
onto wheat (Triticum aestivum) ”, Canadian Journal of Genetics and Cytology, Vol. 23, pp. 81-88,
https://doi.org/10.1139/g81-010.
FAO (2021), Food Outlook: Biannual Report on Global Food Markets. June 2021, Food and Agriculture Organization
of the United States, Rome, https://reliefweb.int/report/world/food-outlook-biannual-report-global-food-markets-
june-2021.
FAO (2014), Wheat - the Largest Primary Commodity, Food and Agriculture Organization of the United States,
https://www.fao.org/assets/infographics/FAO-Infographic-wheat-en.pdf.
Felber, F. et al. (2007), “Genetic and ecological consequences of transgene flow to the wild flora”, in Green Gene
Technology: Research in an Area of Social Conflict, A. Fiechter and C. Sautter (eds.), Advances in Biochemical
Engineering/Biotechnology, Vol. 107, Springer, Berlin, Heidelberg, pp. 173-205,
https://doi.org/10.1007/10_2007_050.
Feldman, M. and A.A. Levy (2005), “Allopolyploidy–a shaping force in the evolution of wheat genomes”, Cytogenetic
and Genome Research, Vol. 109, pp. 250-258, https://doi.org/10.1159/000082407.
Feldman, M. and A.A. Levy (2012), “Genome evolution due to allopolyploidization in wheat”, Genetics, Vol. 192,
pp. 763-774, https://doi.org/10.1534/genetics.112.146316.
Feldman, M. and A.A. Levy (2015), “Origin and evolution of wheat and related Triticeae species”, in Alien
Introgression in Wheat: Cytogenetics, Molecular Biology, and Genomics, M. Molnár-Láng, C. Ceoloni and J.
Doležel (eds.), Springer International Publishing, Cham, pp. 21-76, https://doi.org/10.1007/978-3-319-23494-6_2.
Feuillet, C., Langridge, P. and R. Waugh (2008), “Cereal breeding takes a walk on the wild side”, Trends in Genetics,
Vol. 24, pp. 24-32, https://doi.org/10.1016/j.tig.2007.11.001.
Figueroa, M., K.E. Hammond-Kosack and P.S. Solomon (2018), “A review of wheat diseases–a field perspective”,
Molecular Plant Pathology, Vol. 19, pp. 1523-1536, https://doi.org/10.1111/mpp.12618.
Friesen, L. et al. (1990), “Effects of volunteer wheat and barley on the growth and yield of flax”, Canadian Journal of
Plant Science, Vol. 70, pp. 1115-1122, https://doi.org/10.4141/cjps90-134.
Fuller, D.Q. and R. Allaby (2009), “Seed dispersal and crop domestication: Shattering, germination and seasonality
in evolution under cultivation”, Fruit Development and Seed Dispersal, L. Østergaard (ed.), Annual Plant
Reviews, Vol. 38, pp. 238-295, https://doi.org/10.1002/9781119312994.apr0414.
Gaeta, R.T. and J. Chris Pires (2010), “Homoeologous recombination in allopolyploids: the polyploid ratchet”, New
Phytologist, Vol. 186, pp. 18-28, https://doi.org/10.1111/j.1469-8137.2009.03089.x.
Gaines, T.A. et al. (2008), “Jointed goatgrass (Aegilops cylindrica) by imidazolinone-resistant wheat hybridization
under field conditions”, Weed Science, Vol. 56, pp. 32-36, https://doi.org/10.1614/WS-07-033.1.
Gale, M.D. and T.E. Miller (1987), “The introduction of alien genetic variation in wheat”, in Wheat Breeding: Its
Scientific Basis, F.G.H. Lupton (ed.), Springer, Dordrecht, pp. 173-210, https://doi.org/10.1007/978-94-009-3131-
2_7.
Gandhi, H.T. et al. (2006), “Hybridization between wheat and jointed goatgrass (Aegilops cylindrica) under field
conditions”, Weed Science, Vol. 54, pp. 1073-1079, https://doi.org/10.1614/WS-06-078R1.1.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  63

Garber, R.J. and K.S. Quesenberry (1923), “Natural crossing in winter wheat”, Agronomy Journal, Vol. 15, pp. 508-
512, https://doi.org/10.2134/AGRONJ1923.00021962001500120005X.
Gatford, K.T. et al. (2006), “Gene flow from transgenic wheat and barley under field conditions”. Euphytica, Vol. 151,
pp. 383-391, http://dx.doi.org/10.1007/s10681-006-9160-1.
Geisler G. (1991) Farbatlas Landwirtschaftlicher Kulturpflanzen. Eugen Ulmer Verlag, Stuttgart, Germany.
Gentle, M., S. Phinn and J. Speed (2010), “Assessing pig damage in agricultural crops with remote sensing”,
Australian Pest Animal Management Program Final Report, Bureau of Rural Sciences,
https://pestsmart.org.au/wp-content/uploads/sites/3/2020/06/Gentle2011_APARPfinalreport.pdf.
Gianessi, L., S. Sankula and N. Reigner (2003), Plant Biotechnology: Potential Impact for Improving Pest
Management in European Agriculture: a Summary of Nine Case Studies, The National Center for Food and
Agricultural Policy, Washington, DC, http://www.ncfap.org/documents/ExecutiveSummaryDecember.pdf.
Gill, B.S. (2015). Wheat chromosome analysis. in Advances in Wheat Genetics: From Genome to Field, Proceedings
of the 12th International Wheat Genetics Symposium Y. Ogihara, S. Takumi, H. Handa (eds),
Springer Japan, Tokyo, https://doi.org/10.1007/978-4-431-55675-6
Gill, B.S., B, Friebe and T.R. Endo (1991), “Standard karyotype and nomenclature system for description of
chromosome bands and structural aberrations in wheat (Triticum aestivum)”, Genome, Vol. 34, pp. 830-839,
https://doi.org/10.1139/g91-128.
Glover, N.M., H. Redestig and C. Dessimoz (2016), “Homoeologs: what are they and how do we infer them?”,
Trends in Plant Science, Vol. 21, pp. 609-621, https://doi.org/10.1016/j.tplants.2016.02.005.
Goff, S.A. et al. (2002), “A draft sequence of the rice genome (Oryza sativa L. ssp. japonica) ”, Science, Vol. 296,
pp. 92-100, https://doi.org/10.1126/science.1068275.
GRDC (2012), Irrigated Wheat: Best Practice Guidelines in Cotton Farming Systems, Grains Research and
Development Corporation, Canberra, ISBN: 978-0-9872308-1-2.
GRDC (2014), Wheat: Northern Region, GrowNotes™, Grains Research and Development Corporation, Canberra,
https://grdc.com.au/GN-Wheat-North.
GRDC (2015), Wheat: Northern Region, GrowNotes™, Grains Research and Development Corporation, Canberra,
https://grdc.com.au/GN-Wheat-North.
Griffiths, S. et al. (2006), “Molecular characterization of Ph1 as a major chromosome pairing locus in polyploid
wheat”, Nature, Vol. 439, pp. 749-752, https://doi.org/10.1038/nature04434.
Gupta, P.K., and N.K. Vasistha (2018), “Wheat cytogenetics and cytogenomics: the present status”, The Nucleus,
Vol. 61, pp. 195-212, https://doi.org/10.1007/s13237-018-0243-x.
Gupta, V., C.N. Mishra and S. Kumar (2018), “Uses and importance of wide hybridization in crop improvement”, in
Advanced Molecular Plant Breeding: Meeting the Challenge of Food Security, D.N. Bharadwaj (ed.), CRC Press,
ISBN: 9781774631515.
Hanks, R.J., and F.C. Thorp (1956), “Seedling emergence of wheat as related to soil moisture content, bulk density,
oxygen diffusion rate, and crust strength”, Soil Science Society of America Journal, Vol. 20, pp. 307-310,
https://doi.org/10.2136/sssaj1956.03615995002000030003x.
Hanna, W.W. and E.C. Bashaw (1987), “Apomixis: its identification and use in plant breeding”, Crop Science,
Vol. 27, pp. 1136-1139, https://doi.org/10.2135/cropsci1987.0011183X002700060010x.
Hanson, B.D. et al. (2005a), “Interspecific hybridization: potential for movement of herbicide resistance from wheat to
jointed goatgrass (Aegilops cylindrica)”, Weed Technology, Vol. 19, pp. 674-682, https://doi.org/10.1614/WT-04-
217R.1.
Hanson, B.D. et al. (2005b), “Pollen-mediated gene flow from blue aleurone wheat to other wheat cultivars”, Crop
Science, Vol. 45, pp. 1610-1617, https://doi.org/10.2135/cropsci2004.0443.
Hao, M. et al (2013), “Production of hexaploid triticale by a synthetic hexaploid wheat-rye hybrid method”, Euphytica,
Vol. 193, pp. 347-357, https://doi.org/10.1007/s10681-013-0930-2.
Harker, K.N. et al. (2005), “Glyphosate-resistant wheat persistence in western Canadian cropping systems”, Weed
Science, Vol. 53, pp. 846-859, https://doi.org/10.1614/WS-05-068R1.1.
Harlan, J.R. and J.M.J. de Wet, J.M.J. (1971), “Toward a rational classification of cultivated plants”, Taxon, Vol. 20,
pp. 509-517, https://doi.org/10.2307/1218252.
Harrington, J.B. (1932), “Natural crossing in wheat, oats, and barley at Saskatoon, Saskatchewan”, Scientific
Agriculture, Vol. 12, pp. 470-483, https://cdnsciencepub.com/doi/10.4141/sa-1932-0032.
Hayward, A.C. et al. (2015), “Molecular marker applications in plants”, in Plant Genotyping: Methods and Protocols,
J. Batley (ed.) Springer, New York, pp. 13-27, https://doi.org/10.1007/978-1-4939-1966-6_2.
Heap, I. (2018), The International Herbicide-Resistant Weed Database, https://www.weedscience.org (accessed 1
October 2018).
Hegde, S.G. and J.G. Waines (2004), “Hybridization and introgression between bread wheat and wild and weedy

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
64  ENV/CBC/MONO(2024)23

relatives in North America”, Crop Science, Vol. 44, pp. 1145-1155, https://doi.org/10.2135/cropsci2004.1145.
Heslop-Harrison, J.S. (1992), “Molecular cytogenetics, cytology and genomic comparisons in the Triticeae”,
Hereditas, Vol. 116, pp. 93-99, https://doi.org/10.1111/j.1601-5223.1992.tb00805.x.
Heun, M. et al. (1997), “Site of einkorn wheat domestication identified by DNA fingerprinting”, Science, Vol. 278,
pp.1312-1314.
Heuzé, V., G. Tran and R. Baumont (2015), “Wheat forage”, Feedipedia, INRAE, CIRAD, AFZ and FAO,
https://feedipedia.org/node/363.
Heuzé, V. et al. (2015), “Wheat grain”, Feedipedia, INRAE, CIRAD, AFZ and FAO,
https://www.feedipedia.org/node/223.
Heyne, E.G. (ed.) (1987), Wheat and Wheat Improvement, 2nd ed., American Society of Agronomy, Crop Science
Society of America, Soil Science Society of America, ISBN: 9780891180913.
Huang, S. et al. (2002), “Genes encoding plastid acetyl-CoA carboxylase and 3-phosphoglycerate kinase of the
Triticum/Aegilops complex and the evolutionary history of polyploid wheat”, Proceedings of the National
Academy of Sciences of the United States of America, Vol. 99, pp. 8133-8138,
https://doi.org/10.1073/pnas.072223799.
Hucl, P. (1996), “Out-crossing rates for 10 Canadian spring wheat cultivars”, Canadian Journal of Plant Science,
Vol. 76, pp. 423-427, https://doi.org/10.4141/cjps96-075.
Hucl, P. and M. Matus-Cádiz (2001), “Isolation distances for minimizing out-crossing in spring wheat”, Crop Science,
Vol. 41, pp. 1348-1351, https://doi.org/10.2135/cropsci2001.4141348x.
Illinois Wildflowers (2019), “Wheat Triticum aestivum”, Illinois Wildflowers,
http://www.illinoiswildflowers.info/grasses/plants/wheat.html (accessed 10 November 2019).
IGC (2022), Supply & Demand data, International Grains Council, Market Information, IGC London,
https://www.igc.int/en/default.aspx (accessed 17 February 2022).
IWGSC (International Wheat Genome Sequencing Consortium) (2014), “A chromosome-based draft sequence of the
hexaploid bread wheat (Triticum aestivum) genome”, Science, Vol. 345, 1251788,
https://doi.org/10.1126/science.1251788.
Jabran, K. et al. (2017), “Weed dynamics and management in wheat”, in Advances in Agronomy, Vol. 145, D.L.
Sparks (ed.), Academic Press, pp. 97-166, ISBN: 978-0-12-812417-8.
Jacob, J. and E. Tkadlec (2010), “Rodent outbreaks in Europe: dynamics and damage”, in Rodent Outbreaks:
Ecology and Impacts, G.R. Singleton et al. (eds.), International Rice Research Institute, pp. 207-223, SBN 978-
971-22-0257-5.
Jacot, Y. et al. (2004), “Hybridization between wheat and wild relatives, a European Union research programme”, in
Introgression from Genetically Modified Plants into Wild Relatives, H.C.M. den Nijs, D. Bartch and J. Sweet
(eds.), CABI Publishing, pp. 63-73, http://dx.doi.org/10.1079/9780851998169.0063.
Jain, S.M. (2001), “Tissue culture-derived variation in crop improvement”, Euphytica, Vol. 118, pp. 153-166,
https://doi.org/10.1023/A:1004124519479.
Jarvis, D.I. and T. Hodgkin (1999), “Wild relatives and crop cultivars: detecting natural introgression and farmer
selection of new genetic combinations in agroecosystems”, Molecular Ecology, Vol. 8, pp. S159-S173,
https://doi.org/10.1046/j.1365-294X.1999.00799.x.
Jarvis, R.J. et al. (2000), “Tillage”, in The Wheat Book: Principles and Practice, W.K. Anderson and J.R. Garlinge
(eds.), Agriculture Western Australia, pp. 175-200.
Jauhar, P.P. and R.N. Chibbar (1999), “Chromosome-mediated and direct gene transfers in wheat”, Genome,
Vol. 42, pp. 570-583, https://doi.org/10.1139/g99-045.
Jenkins, J.E.E. and L. Lescar (1980), “Use of foliar fungicides on cereals in Western Europe”, Plant Disease, Vol. 64,
987.
Jia, J. et al. (2013), “Aegilops tauschii draft genome sequence reveals a gene repertoire for wheat adaptation”,
Nature, Vol. 496, pp. 91-95, https://doi.org/10.1038/nature12028.
Jiang, J., B. Friebe and B.S. Gill (1993), “Recent advances in alien gene transfer in wheat”, Euphytica, Vol. 73,
pp. 199-212, https://doi.org/10.1007/BF00036700.
Joppa, L.R. (1987), “Aneuploid analysis in tetraploid wheat”, in Wheat and Wheat Improvement, E.G. Heyne (ed.),
2nd ed., American Society of Agronomy, Crop Science Society of America, Soil Science Society of America,
pp. 255-267, https://doi.org/10.2134/agronmonogr13.2ed.c11.
Joppa, L.R., F.H. McNeal and M.A. Berg (1968), “Pollen production and pollen shedding of hard red spring (Triticum
aestivum L. em Thell.) and durum (T. durum Desf.) wheats”, Crop Science, Vol. 8, pp. 487-490,
https://doi.org/10.2135/cropsci1968.0011183X000800040028x.
Jørgensen, L.N. et al. (2008), “Controlling cereal disease with reduced agrochemical inputs - a challenge for both
growers and advisers”, in Cereal Pathosystems, G. Jellis and C. Edwards (eds.), British Society for Plant

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  65

Pathology.
Keeler, K.H. (1989), “Can genetically engineered crops become weeds?”, Bio/Technology, Vol. 7, pp. 1134-1139,
https://doi.org/10.1038/nbt1189-1134.
Kempe, K., M. Rubtsova and M. Gils (2014), “Split-gene system for hybrid wheat seed production”. Proceedings of
the National Academy of Science of the United States of America, Vol. 111, pp. 9097-9102,
https://doi.org/10.1073/pnas.1402836111.
Kennedy, T.F. and J. Connery (2008), “An investigation of seed treatments for the control of crow damage to newly-
sown wheat”, Irish Journal of Agricultural and Food Research, Vol. 47, pp. 79-91,
http://hdl.handle.net/11019/630.
Khan, M.N., E.G. Heyne and A.L. Arp (1973), “Pollen distribution and the seedset on Tritieum aestivum L.”, Crop
Science, Vol. 13, pp. 223-226, https://doi.org/10.2135/cropsci1973.0011183X001200020022x.
Kihara, H. (1944), “Discovery of the DD-analyzer, one of the ancestors of Triticum vulgare”, Agriculture and
Horticulture Vol. 19, pp. 889-890 (in Japanese).
Kilian, B., Martin, W., and Salamini, F. (2010). Genetic Diversity, Evolution and Domestication of Wheat and Barley
in the Fertile Crescent. In Evolution in Action: Case studies in Adaptive Radiation, Speciation and the Origin of
Biodiversity, M. Glaubrecht (ed.) Springer Berlin Heidelberg, pp. 137-166, https://doi.org/10.1007/978-3-642-
12425-9
Kilian, B. et al. (2006), “Independent Wheat B and G genome origins in outcrossing Aegilops progenitor haplotypes”,
Molecular Biology and Evolution, Vol. 24, pp 217-227, https://doi.org/10.1093/molbev/msl151
Kilian, B. et al. (2009), “Domestication of the Triticeae in the Fertile Crescent”, in Genetics and Genomics of the
Triticeae, G.J. Muehlbauer and C. Feuillet (eds.), Springer, New York, pp. 81-119, http://dx.doi.org/10.1007/978-
0-387-77489-3_3.
Kimber, G. and M. Feldman (1987), Wild Wheat - an Introduction, Special Report 353, College of Agriculture,
University of Missouri, Columbia.
Kimber, G. and E.R. Sears (1987), “Evolution in the genus Triticum and the origin of cultivated wheat”, in Wheat and
Wheat Improvement, E.G. Heyne (ed.), 2nd ed., American Society of Agronomy, Crop Science Society of
America, Soil Science Society of America, pp. 154-164, https://doi.org/10.2134/agronmonogr13.2ed.c6.
Kirby, E.J.M. (2002), “Botany of the wheat plant”, in Bread Wheat: Improvement and Production, B.C. Curtis, S.
Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the United Nations, Rome,
ISBN: 92-5-104809-6.
Klein, R. et al. (2016), “Control of volunteer winter wheat and other weeds now to increase and protect 2017 yields,
income” , CropWatch, University of Nebraska-Lincoln Extension, https://cropwatch.unl.edu/2016/control-
volunteer-winter-wheat-and-other-weeds-now-increase-and-protect-2017-yields-income.
Knapp, S. et al. (2019), “Natural selection towards wild-type in composite cross populations of winter wheat”,
Frontiers in Plant Science, Vol. 10, 1757, https://doi.org/10.3389/fpls.2019.01757.
Koltunow, A.M. and U. Grossniklaus (2003), “Apomixis: a developmental perspective”, Annual Review of Plant
Biology, Vol. 54, pp. 547-574, https://doi.org/10.1146/annurev.arplant.54.110901.160842.
Komatsuzaki, M. and O. Endo (1996), “Seed longevity and emergence of volunteer wheat in upland fields”, Journal
of Weed Science and Technology, Vol. 41, pp. 197-204, https://doi.org/10.3719/weed.41.197.
Korber-Grohne, U. (1988), Nutzpflanzen in Deutschland - Kulturgeschichte und Biologie, Theiss Verlag, Stuttgart,
Germany.
Krasileva, K.V. et al. (2017), “Uncovering hidden variation in polyploid wheat”, Proceedings of the National Academy
of Sciences of the United States of America, Vol. 114, pp. E913-E921, https://doi.org/10.1073/pnas.1619268114.
Kumlehn, J. et al. (2001), “Parthenogenetic egg cells of wheat: cellular and molecular studies”, Sexual Plant
Reproduction, Vol. 14, pp. 239-243, https://doi.org/10.1007/s00497-001-0115-3.
Lacy, J. and K. Giblin (2006), “Growing eight tonnes a hectare of irrigated wheat in southern NSW”, Primefact, 197,
NSW Department of Primary Industries.
Laffan, J. (1999), Cropping Systems for Sustainable Wheat Production, NSW Department of Primary Industries,
ISBN: 0731305299.
Lafond, G.P. and B.D. Fowler (1989), “Soil temperature and water content, seeding depth, and simulated rainfall
effects on winter wheat emergence”, Agronomy Journal, Vol. 81, pp. 609-614,
https://doi.org/10.2134/agronj1989.00021962008100040012x.
Lambing, C., F.C.H. Franklin and C.-J.R. Wang (2017), “Understanding and manipulating meiotic recombination in
plants”, Plant Physiology, Vol. 173, pp. 1530-1542, https://doi.org/10.1104/pp.16.01530.
Larkin, P.J. et al. (1984), “Heritable somaclonal variation in wheat”, Theoretical and Applied Genetics, Vol. 67,
pp. 443-455, https://doi.org/10.1007/bf00263410.
Larkin, P.J. and W.R. Scowcroft (1981), “Somaclonal variation — a novel source of variability from cell cultures for

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
66  ENV/CBC/MONO(2024)23

plant improvement”, Theoretical and Applied Genetics, Vol. 60, pp. 197-214,
https://doi.org/10.1007/BF02342540.
Laurie, D.A. (1989), “The frequency of fertilization in wheat × pearl millet crosses”, Genome, Vol. 32, pp. 1063-1067,
https://doi.org/10.1139/g89-554.
Laurie, D.A. and M.D. Bennett (1986), “Wheat × maize hybridization”, Canadian Journal of Genetics and Cytology,
Vol. 28, pp. 313-316, https://doi.org/10.1139/g86-046.
Laurie, D.A. and M.D. Bennett (1988a), “Cytological evidence for fertilization in hexaploid wheat × sorghum crosses”,
Plant Breeding, Vol. 100, pp. 73-82, https://doi.org/10.1111/j.1439-0523.1988.tb00220.x.
Laurie, D.A. and M.D. Bennett (1988b), “The production of haploid wheat plants from wheat x maize crosses”,
Theoretical and Applied Genetics, Vol. 76, pp. 393-397, https://doi.org/10.1007/BF00265339.
Leff, B., N. Ramankutty and J.A. Foley (2004), “Geographic distribution of major crops across the world”, Global
Biogeochemical Cycles, Vol. 18, GB1009, https://doi.org/10.1029/2003GB002108.
Leighty, C.E. (1915), “Natural wheat-rye hybrids”, Journal of the American Society of Agronomy, Vol. 7, pp. 209-216,
https://doi.org/10.2134/agronj1915.00021962000700050002x.
Leighty, C.E. and W.J. Sando (1924), “The blooming of wheat flowers”, Journal of Agricultural Research, Vol. 27,
pp. 231-244.
Leighty, C.E. and W.J. Sando (1928), “Natural and artificial hybrids of a chinese wheat and rye”, Journal of Heredity,
Vol. 19, pp. 23-27, https://doi.org/10.1093/oxfordjournals.jhered.a102911.
Lein, A. (1943), “The genetical basis of the crossability between wheat and rye”, Zeitschrift für Induktive
Abstammungs- und Vererbungslehre, Vol. 81, pp. 28-61, https://doi.org/10.1007/BF01847441 (in German).
Lemerle, D. et al. (2016), “Seeding rate and cultivar effects on canola (Brassica napus) competition with volunteer
wheat (Triticum aestivum)”, Crop and Pasture Science, Vol. 67, pp. 857-863, http://dx.doi.org/10.1071/CP16159.
Lersten, N.R. (1987), “Morphology and anatomy of the wheat plant”, in Wheat and Wheat Improvement, E.G. Heyne
(ed.), American Society of Agronomy, Crop Science Society of America, Soil Science Society of America, pp. 33-
75, https://doi.org/10.2134/agronmonogr13.2ed.c2.
Li, H. et al. (2015), “Spontaneous and divergent hexaploid triticales derived from common wheat × rye by complete
elimination of D-genome chromosome”, PLoS ONE, Vol. 10, e0120421,
https://doi.org/10.1371/journal.pone.0120421.
Ling, H.-Q. et al. (2013), “Draft genome of the wheat A-genome progenitor Triticum urartu”, Nature, Vol. 496, pp. 87-
90, https://doi.org/10.1038/nature11997.
Liu, D. et al. (2016), “Allopolyploidy and interspecific hybridization for wheat improvement”, in Polyploidy and
Hybridization for Crop Improvement, A.S. Mason (ed.), CRC Press, pp. 27-53,
https://doi.org/10.1201/9781315369259.
Liu, D. et al. (2014), “Distant hybridization: a tool for interspecific manipulation of chromosomes”, in Alien Gene
Transfer in Crop Plants, Volume 1, A. Pratap and J. Kumar (eds.), Springer, New York, pp. 25-42,
https://doi.org/10.1007/978-1-4614-8585-8_2.
Liu, Z.-W., R.R.-C. Wang and J.G. Carman (1994), “Hybrids and backcross progenies between wheat (Triticum
aestivum L.) and apomictic Australian wheatgrass [Elymus rectisetus (Nees in Lehm.) A. Löve & Connor]:
karyotypic and genomic analyses”, Theoretical and Applied Genetics, Vol. 89, pp. 599-605,
https://doi.org/10.1007/BF00222454.
Lloveras, J. et al. (2004), “Seeding rate influence on yield and yield components of irrigated winter wheat in a
mediterranean climate”, Agronomy Journal, Vol. 96, pp. 1258-1265, http://dx.doi.org/10.2134/agronj2004.1258.
Longin, C.F.H. et al. (2012), “Hybrid breeding in autogamous cereals”, Theoretical and Applied Genetiecs, Vol. 125,
pp. 1087-1096, https://doi.org/10.1007/s00122-012-1967-7.
Loureiro, I. et al. (2009), “Spontaneous wheat-Aegilops biuncialis, Ae. geniculata and Ae. triuncialis amphiploid
production, a potential way of gene transference”, Spanish Journal of Agricultural Research, Vol. 7, pp. 614-620,
http://dx.doi.org/10.5424/sjar/2009073-445.
Loureiro, I. et al. (2006), “Evidence of natural hybridization between Aegilops geniculata and wheat under field
conditions in Central Spain”, Environmental Biosafety Research, Vol. 5, pp. 105-109,
http://dx.doi.org/10.1051/ebr:2006020.
Loureiro, I. et al. (2007a), “Hybridization between wheat (Triticum aestivum) and the wild species Aegilops geniculata
and A. biuncialis under experimental field conditions”, Agriculture, Ecosystems & Environment, Vol. 120, pp. 384-
390, https://doi.org/10.1016/j.agee.2006.10.015.
Loureiro, I. et al. (2007b), “Wheat pollen dispersal under semiarid field conditions: potential outcrossing with Triticum
aestivum and Triticum turgidum”, Euphytica, Vol. 156, pp. 25-37, http://dx.doi.org/10.1007/s10681-006-9345-7.
Loureiro, I. et al. (2012), “Pollen-mediated gene flow in wheat (Triticum aestivum L.) in a semiarid field environment
in Spain”, Transgenic Research, Vol. 21, pp. 1329-1339, https://doi.org/10.1007/s11248-012-9619-x.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  67

Luo, M. (1989), “The crossability of landraces of common wheat in Sichuan with Aegilops tauschii and Secale
cereale L. ”, Journal of Sichuan Agricultural University, Vol. 2, pp. 71-76 (in Chinese).
Maan, S.S. (1987), “Interspecific and intergeneric hybridization in wheat”, in Wheat and Wheat Improvement, E.G.
Heyne (ed.) American Society of Agronomy, Crop Science Society of America, Soil Science Society of America,
pp. 453-461, https://doi.org/10.2134/agronmonogr13.2ed.c26.
Macdonald, A.J. (ed.) (2018) “Broadbalk winter wheat”, Rothamsted Long-Term Experiments: Guide to the Classical
and Other Long-Term Experiments, Datasets and Sapmle Archive, Rothamsted Research, Harpenden, U.K.,
pp. 7-18, https://doi.org/10.23637/ROTHAMSTED-LONG-TERM-EXPERIMENTS-GUIDE-2018.
Macholdt, J. et al. (2020), “The effects of cropping sequence, fertilization and straw management on the yield
stability of winter wheat (1986–2017) in the Broadbalk Wheat Experiment, Rothamsted, UK”, The Journal of
Agricultural Science, Vol. 158, pp. 65-79, https://doi.org/10.1017/S0021859620000301.
Makałowski, W. (2001), “The human genome structure and organization”, Acta Biochimica Polonica, Vol. 48,
pp. 587-598.
Malo, J.E. and F. Suárez (1995), “Herbivorous mammals as seed dispersers in a Mediterranean dehesa”, Oecologia,
Vol. 104, pp. 246-255, https://doi.org/10.1007/bf00328589.
Mandy G. (1970) “Pflanzenzüchtung - Kurz und bündig.” VEB Deutscher Landwirtschafts-verlag, Berlin.
Marcussen, T. et al. (2014), “Ancient hybridizations among the ancestral genomes of bread wheat”, Science,
Vol. 345, 1250092, https://doi.org/10.1126/science.1250092.
Marshall, G. et al. (1989), “Effects of ‘volunteer’ wheat and barley on the growth and yield of rapeseed”, Canadian
Journal of Plant Science, Vol. 69, pp. 445-453, https://doi.org/10.4141/cjps89-056.
Martin, T.J. (1990), “Outcrossing in twelve hard red winter wheat cultivars”, Crop Science Vol. 30, pp. 59-62,
https://doi.org/10.2135/cropsci1990.0011183X003000010013x.
Mason, H.E. and D. Spaner (2006), “Competitive ability of wheat in conventional and organic management systems:
A review of the literature”, Canadian Journal of Plant Science, Vol. 86, pp. 333-343, https://doi.org/10.4141/P05-
051.
Matsuoka, Y. (2011), “Evolution of polyploid Triticum wheats under cultivation: The role of domestication, natural
hybridization and allopolyploid speciation in their diversification”, Plant and Cell Physiology, Vol. 52, pp. 750-764,
https://doi.org/10.1093/pcp/pcr018.
Matus-Cádiz, M.A., P. Hucl and B. Dupuis (2007), “Pollen-mediated gene flow in wheat at the commercial scale”,
Crop Science, Vol. 47, pp. 573-579, https://doi.org/10.2135/cropsci06.07.0441.
Matus-Cádiz, M.A. et al. (2004), “Gene flow in wheat at the field scale”, Crop Science, Vol. 44, pp. 718-727,
https://doi.org/10.2135/cropsci2004.7180.
Matzk, F. (1996), “The ‘salmon system’ of wheat — a suitable model for apomixis research”, Hereditas, Vol. 125,
pp. 299-304, http://dx.doi.org/10.1111/j.1601-5223.1996.00299.x.
May, C.E. and R. Appels (1987), “The molecular genetics of wheat: Toward an understanding of 16 billion base pairs
of DNA”, in Wheat and Wheat Improvement, E.G. Heyne (ed.), American Society of Agronomy, Crop Science
Society of America, Soil Science Society of America, pp. 165-198,
https://doi.org/10.2134/agronmonogr13.2ed.c7.
McFadden, E.S. and E.R. Sears (1946), “The origin of Triticum spelta and its free-threshing hexaploid relatives”,
Journal of Heredity, Vol. 37, pp. 81-89, https://doi.org/10.1093/oxfordjournals.jhered.a105590.
McIntosh, R.A. (1987 ), “Gene location and gene mapping in hexaploid wheat”, in Wheat and Wheat Improvement,
E.G. Heyne (ed.), American Society of Agronomy, Crop Science Society of America, Soil Science Society of
America, pp. 269-287, https://doi.org/10.2134/agronmonogr13.2ed.c12.
Mehta, Y.R. (2014), Wheat Diseases and Their Management, Springer, ISBN: 978-3-319-06465-9.
Meister, G.K. (1921), “Natural hybridization of wheat and rye in Russia”, Journal of Heredity, Vol. 12, pp. 467-470,
https://doi.org/10.1093/oxfordjournals.jhered.a102049.
Miller, R., and Pike, K. (2002), Insects in wheat-based systems. In Bread Wheat: Improvement and Production, B.C.
Curtis, S. Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the United Nations,
Rome, pp. 367-393.
Molnár-Láng, M., C. Ceoloni and J. Doležel (eds.) (2015), Alien Introgression in Wheat: Cytogenetics, Molecular
Biology, Genomics, ISBN: 978-3-319-23494-6.
Molnár-Láng, M., G. Linc and É. Szakács (2013), “Wheat–barley hybridization: the last 40 years”, Euphytica,
Vol. 195, pp. 315-329, https://doi.org/10.1007/s10681-013-1009-9.
Monfreda, C., N. Ramankutty and J.A. Foley (2008), “Farming the planet: 2. Geographic distribution of crop areas,
yields, physiological types, and net primary production in the year 2000”, Global Biogeochemical Cycles, Vol. 22,
GB1022, http://dx.doi.org/10.1029/2007GB002947.
Morris, C.F. and G.M. Paulsen (1985), “Preharvest sprouting of hard winter wheat as affected by nitrogen nutrition”,

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
68  ENV/CBC/MONO(2024)23

Crop Science, Vol. 25, pp. 1028-1031, https://doi.org/10.2135/cropsci1985.0011183X002500060032x.

Morrison, L.A., L.C. Crémieux and C.A. Mallory-Smith (2002), “Infestations of jointed goatgrass (Aegilops cylindrica)
and its hybrids with wheat in Oregon wheat fields”, Weed Science, Vol. 50, pp. 737-747,
https://doi.org/10.1614/0043-1745(2002)050[0737:IOJGAC]2.0.CO;2.
Mujeeb-Kazi, A. (1981), “Apomictic progeny derived from intergeneric Hordeum-Triticum hybrids”. Journal of
Heredity, Vol. 72, pp. 284-285, https://doi.org/10.1093/oxfordjournals.jhered.a109498.
Mujeeb-Kazi, A. (1995), “Introduction: 15 years of progress in wheat wide crosses at CIMMYT”, Utilizing Wild Grass
Biodiversity in Wheat Improvement: 15 Years of Wide Cross Research at CIMMYT, A. Mujeeb-Kazi and G.P.
Hettel (eds), CIMMYT Research Report, No. 2, International Maize and Wheat Improvement Center (CIMMYT),
Mexico, pp. 1-4.
Mujeeb-Kazi, A. and G.P. Hettel, G.P. (1995), Utilizing Wild Grass Biodiversity in Wheat Improvement: 15 Years of
Wide Cross Research at CIMMYT, CIMMYT Research Report, No. 2, International Maize and Wheat
Improvement Center (CIMMYT), Mexico.
Mujeeb-Kazi, A. and S. Rajaram (2002), “Transferring alien genes from related species and genera for wheat
improvement”, in Bread Wheat: Improvement and Production, B.C. Curtis, S. Rajaram, and H. Gómez
Macpherson (eds.), Food and Agriculture Organization of the United Nations, Rome.
Mujeeb-Kazi, A. et al. (2013), “Genetic diversity for wheat improvement as a conduit to food security”, in Advances in
Agronomy, Volume 122, D.L. Sparks (ed.), Academic Press, Burlington, pp. 179-257.
Muntrzing, R. (1974), “Historic review of the development if Triticale”, in Triticale : Proceedings of an International
Symposium, El Batán, Mexico, 1-3 October, 1973, R. Maclntyre and M. Campbell (eds.) International
Development Research Centre, Ottawa, pp. 13-30.
Murai, K. (2013), “Homeotic genes and the ABCDE model for floral organ formation in wheat”, Plants, Vol. 2,
pp. 379-395, https://dx.doi.org/10.3390%2Fplants2030379.
Myers, J.A. et al. (2004), “Seed dispersal by white-tailed deer: implications for long-distance dispersal, invasion, and
migration of plants in eastern North America”, Oecologia, Vol. 139, pp. 35-44, https://doi.org/10.1007/s00442-
003-1474-2.
Nathan, R. and H.C. Muller-Landau (2000), “Spatial patterns of seed dispersal, their determinants and
consequences for recruitment”. Trends in Ecology & Evolution, Vol. 15, pp. 278-285,
https://doi.org/10.1016/S0169-5347(00)01874-7.
Nathan, R. et al. (2008), “Mechanisms of long-distance seed dispersal”, Trends in Ecology & Evolution, Vol. 23,
pp. 638-647, https://doi.org/10.1016/j.tree.2008.08.003.
Nesbitt, M. and D. Samuel (1996), “From staple crop to extinction? The archaeology and history of the hulled
wheats”, Hulled Wheats, S. Padulosi, K. Hammer and J. Heller (eds.), Vol. 4, pp. 41-100.
Newman, S. (1990), Pollen Transfer from Transgenic Plants: Concepts for Risks Assessment and Data
Requirements, AAAS/EPA Environmental Science and Engineering Fellow.
Nigam, J.N., 2001. Ethanol production from wheat straw hemicellulose hydrolysate by Pichia stipitis. Journal of
biotechnology, 87(1), pp.17-27.
Niu, Z. et al. (2014), “Review of doubled haploid production in durum and common wheat through wheat × maize
hybridization”, Plant Breeding, Vol. 133, pp. 313-320, https://doi.org/10.1111/pbr.12162.
Nuttonson, M.Y. (1955), Wheat-climate Relationships and the Use of Phenology in Ascertaining the Thermal and
Photo-thermal Requirements of Wheat Based on Data of North America and of Some Thermally Analogous
Areas of North America in the Soviet Union and in Finland, American Institute of Crop Ecology, Washington, DC.,
https://doi.org/10.2307/1292056
Nyachiro, J.M. et al. (2002), “Temperature effects on seed germination and expression of seed dormancy in wheat”,
Euphytica, Vol. 126, pp. 123-127, https://doi.org/10.1023/A:1019694800066.
O’Donovan, J.T., K.J. Kirkland and A.K. Sharma (1989), “Canola yield and profitability as influenced by volunteer
wheat infestations”, Canadian Journal of Plant Science, Vol. 69, pp. 1235-1244, https://doi.org/10.4141/cjps89-
145.
OECD (2022), OECD Schemes for the Varietal Certification or the Control of Seed Moving in International Trade,
Organisation for Economic Co-operation and Development, Paris, https://www.oecd.org/agriculture/seeds/
OECD (2021), Crop production, indicator, Organisation for Economic Co-operation and Development, Paris,
https://data.oecd.org/agroutput/crop-production.htm (accessed 18 February 2022).
OECD (2003), “Consensus Document on Compositional Considerations for New Varieties of Bread Wheat (Triticum
aestivum): Key Food and Feed Nutrients, Anti-nutrients and Toxicants”, Series on the Safety of Novel Foods and
Feeds, No. 7, OECD, Paris, https://www.oecd.org/env/ehs/biotrack/46815206.pdf.
Oerke, E.-C. (2006), “Crop losses to pests”, The Journal of Agricultural Science, Vol. 144, pp. 31-43,
https://doi.org/10.1017/S0021859605005708.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  69

Oerke, E.-C. and H.-W. Dehne (1997), “Global crop production and the efficacy of crop protection - current situation
and future trends”, European Journal of Plant Pathology, Vol. 103, pp. 203-215,
https://doi.org/10.1023/A:1008602111248.
Oettler, G. (2005), “The fortune of a botanical curiosity – Triticale: past, present and future”, The Journal of
Agricultural Science, Vol. 143, pp. 329-346, https://doi.org/10.1017/S0021859605005290.
Ogbonnaya, F.C. et al. (2013), “Synthetic hexaploids: harnessing species of the primary gene pool for wheat
improvement”, in Plant Breeding Reviews, J. Janick (ed.), Wiley-Blackwellpp. 35-122,
https://doi.org/10.1002/9781118497869.ch2.
OGTR (2017), The Biology of Triticum aestivum L. (Bread Wheat), Version 3.1, Office of the Gene Technology
Regulator, Australia.
Ortiz-Monasterio, J.I. (2002), “Nitrogen management in irrigated spring wheat”, in Bread Wheat: Improvement and
Production, B.C. Curtis, S. Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the
United Nations, Rome.
Ozkan, H. et al. (2007), “Natural variation and identification of microelements content in seeds of einkorn wheat
(Triticum Monococcum)”, Wheat Production in Stressed Environments, H.T. Buck, J.E. Nisi and N. Salomón
(eds.), Springer, Dordrecht, https://doi.org/10.1007/1-4020-5497-1_55.
Pajkovic, M. et al. (2014), “Wheat alleles introgress into selfing wild relatives: empirical estimates from approximate
Bayesian computation in Aegilops triuncialis”, Molecular Ecology, Vol. 23, pp. 5089-5101,
https://doi.org/10.1111/mec.12918.
Parshad, V.R. (1999), “Rodent control in India”, Integrated Pest Management Reviews, Vol. 4, pp. 97-126,
https://doi.org/10.1023/a:1009622109901.
Payne, T.S. (2002), “Harvest and storage management of wheat”, in Bread Wheat: Improvement and Production,
B.C. Curtis, S. Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the United
Nations, Rome.
Peel, M.D. et al. (1997), “Meiotic anomalies in hybrids between wheat and apomictic Elymus rectisetus (Nees in
Lehm.) A. Löve & Connor”, Crop Science, Vol. 37, pp. 717-723,
https://doi.org/10.2135/cropsci1997.0011183X003700030005x.
Pelé, A., M. Rousseau-Gueutin and A.-M. Chèvre (2018), “Speciation success of polyploid plants closely relates to
the regulation of meiotic recombination”, Frontiers in Plant Science, Vol. 9, 907,
https://doi.org/10.3389/fpls.2018.00907.
Peña, R.J. (2002), “Wheat for bread and other foods”, in Bread Wheat: Improvement and Production, B.C. Curtis, S.
Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the United Nations, Rome, pp.
483-542.
Percival, J. (1921), The Wheat Plant: a Monograph, Duckworth and Co., London.
Perrino, P. (1996), “Ecogeographical distribution of hulled wheat species”, Hulled Wheats, S. Padulosi, K. Hammer
and J. Heller (eds.), Vol. 4, pp. 101-119.
Petersen, G. et al. (2006), “Phylogenetic relationships of Triticum and Aegilops and evidence for the origin of the A,
B, and D genomes of common wheat (Triticum aestivum)”, Molecular Phylogenetics and Evolution, Vol. 39,
pp. 70-82, https://doi.org/10.1016/j.ympev.2006.01.023.
Pingali, P.L. (2012), “Green revolution: impacts, limits, and the path ahead”, Proceedings of the National Academy of
Sciences of the United States of America, Vol. 109, pp. 12302-12308, https://doi.org/10.1073/pnas.0912953109.
Poehlman, J.M. (1959a), Breeding Field Crops, Holt, Rinehart and Winston, Inc., New York.
Poehlman, J.M. (1959b), “Breeding wheat”, in Breeding Field Crops, Holt, Rinehart and Winston, Inc., New York,
pp. 102-127.
Pomeranz, Y. (1987), Modern Cereal Science and Technology, VCH Publishers Inc., New York, ISBN:
9780895733269.
Qi, L. et al. (2007), “Homoeologous recombination, chromosome engineering and crop improvement”, Chromosome
Research, Vol. 15, pp. 3-19, https://doi.org/10.1007/s10577-006-1108-8.
Reddy, L.V., R.J. Metzger and T.M. Ching (1985), “Effect of temperature on seed dormancy of wheat”, Crop
Science, Vol. 25, pp. 455-458, https://doi.org/10.2135/cropsci1985.0011183X002500030007x.
Ribeiro-Carvalho, C. et al. (1997), “Wheat–rye chromosome translocations involving small terminal and intercalary
rye chromosome segments in the Portuguese wheat landrace Barbela”, Heredity, Vol. 78, pp. 539-546,
https://doi.org/10.1038/hdy.1997.84.
Ribeiro-Carvalho, C. et al. (2001), “Introgression of rye chromatin on chromosome 2D in the Portuguese wheat
landrace ‘Barbela’”, Genome, Vol. 44, pp. 1122-1128, https://doi.org/10.1139/g01-088.
Rieben, S. et al. (2011), “Gene flow in genetically modified wheat”, PLoS ONE, Vol. 6, e29730,
https://doi.org/10.1371/journal.pone.0029730.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
70  ENV/CBC/MONO(2024)23

Rieseberg, L.H. and J.F. Wendel (1993), “Introgression and its consequences in plants”, in Hybrid Zones and the
Evolutionary Process, R.G. Harrison (ed.), Oxford University Press, pp. 70-109, ISBN: 0-19-506917-X.
Riley, R. (1966), “The genetic regulation of meiotic behaviour in wheat and its relatives”, Proceedings of the 2 nd
International Wheat Symposium, Lund, Sweden, August, 1963, Hereditas, Supplementary Vol. 2, pp. 395-408.
Riley, R. and Chapman, V. (1958), “Genetic control of the cytologically diploid behaviour of hexaploid wheat”,
Nature, Vol. 182, pp. 713-715, https://doi.org/10.1038/182713a0.
Riley, R. V. Chapman and G. Kimber (1959), “Genetic control of chromosome pairing in intergeneric hybrids with
wheat”, Nature, Vol. 183, pp. 1244-1246, https://doi.org/10.1038/1831244a0.
Rawson, H. and E. Gómez Macpherson (2000), Irrigated Wheat, Food and Agriculture Organization of the United
Nations, Rome, ISBN: 92-5-104488-0.
Sallares, R. and T.A. Brown (2004), “Phylogenetic analysis of complete 5′ external transcribed spacers of the 18S
ribosomal RNA genes of diploid Aegilops and related species (Triticeae, Poaceae)”, Genetic Resources and
Crop Evolution, Vol. 51, pp. 701-712, https://doi.org/10.1023/B:GRES.0000034576.34036.a1.
Sarkar, P. and G.L. Stebbins (1956), “Morphological evidence concerning the origin of the B genome in wheat”,
American Journal of Botany, pp. 297-304, https://doi.org/10.2307/2438947.
Saunders, J., M. Izydorczyk and D.B. Levin (2011), “Limitations and challenges for wheat-based bioethanol
production”, in Economic Effects of Biofuel Production, M.A.d.S. Bernardes (ed.), InTech,
https://doi.org/10.5772/20258.
Savidan, Y. (2001), “Transfer of apomixis through wide crosses”, in The Flowering of Apomixis : From Mechanisms
To Genetic Engineering, Y. Savidan, J.G. Carman and T. Dresselhaus (eds.), Mexico, D.F.: ClMMYT, IRD,
European Commission DG VI (FAIR), ISBN: 970-648-074-9.
Sayre, K.D. (2002), “Management of irrigated wheat”, In Bread Wheat: Improvement and Production, B.C. Curtis, S.
Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the United Nations, Rome.
Schnable, P.S. et al. (2009), “The B73 maize genome: complexity, diversity, and dynamics”, Science, Vol. 326,
pp. 1112-1115, https://doi.org/10.1126/science.1178534.
Schneider, A., I. Molnár and M. Molnár-Láng (2008), “Utilisation of Aegilops (goatgrass) species to widen the genetic
diversity of cultivated wheat”, Euphytica, Vol. 163, pp. 1-19, https://doi.org/10.1007/s10681-007-9624-y.
Sears, E.R. (1954), The Aneuploids of Common Wheat, Research Bulletin, No. 572, Agricultural Experiment Station,
University of Missouri, Columbia.
Sears, E.R. (1976), “Genetic control of chromosome pairing in wheat”, Annual Review of Genetics, Vol. 10, pp. 31-
51, https://doi.org/10.1146/annurev.ge.10.120176.000335.
Seefeldt, S.S. et al. (1998), “Production of herbicide-resistant jointed goatgrass (Aegilops cylindrica) × wheat
(Triticum aestivum) hybrids in the field by natural hybridization”, Weed Science, Vol. 46, pp. 632-634,
http://doi.org/10.1017/S004317450008961X.
Setter, T.L. and G. Carlton (2000a), “Germination, vegetative and reproductive growth”, in The Wheat Book:
Principles and Practice, W.K. Anderson and J.R. Garlinge (eds.), Department of Agriculture, Western Australia,
Bulletin 4443, pp. 37-54.
Setter, T.L. and G. Carlton (2000b), “The structure and development of the cereal plant”, in The Wheat Book:
Principles and Practice, W.K. Anderson and J.R. Garlinge (eds.), Department of Agriculture, Western Australia,
Bulletin 4443, pp. 23-36.
Sharma, H.C. and B.S. Gill (1983), “Current status of wide hybridization in wheat”, Euphytica, Vol. 32, pp. 17-31,
https://doi.org/10.1007/BF00036860.
Shevkani, K. et al. (2017), “Wheat starch production, structure, functionality and applications-a review”. International
Journal of Food Science & Technology, Vol. 52, pp. 38-58, https://doi.org/10.1111/ijfs.13266.
Silkova, O.G., A.I. Shchapova and L.A. Kravtsova (2003), “Mechanisms of meiotic restitution and their genetic
regulation in wheat–rye polyhaploids”, Russian Journal of Genetics, Vol. 39, pp. 1271-1280,
https://doi.org/10.1023/B:RUGE.0000004143.45700.13.
Simmonds, D.H. (1989), Wheat and Wheat Quality in Australia, Commonwealth Scientific and Industrial Research
Organization (CSIRO), Australia, ISBN: 0-643-04799-9.
Simmonds, N.W. (1986), Principles of Crop Improvement, 2nd ed., Longman Science and Technology, New York.
Singh, K. et al. (2018), “Wheat streak mosaic virus: a century old virus with rising importance worldwide”, Molecular
Plant Pathology, Vol. 19, pp. 2193-2206, https://doi.org/10.1111/mpp.12683.
Singleton, L.L. (2002), “Diseases of roots and crowns”, in Bread Wheat: Improvement and Production, B.C. Curtis, S.
Rajaram and H. Gómez Macpherson (eds.) Food and Agriculture Organization of the United Nations, Rome.
Skoracka, A., B.G. Rector and G.L. Hein (2018), “The interface between wheat and the wheat curl mite, Aceria
tosichella, the primary vector of globally important viral diseases”, Frontiers in Plant Science, Vol. 9, 1098,
https://doi.org/10.3389/fpls.2018.01098.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  71

Smith, C.W. (1995), “Wheat”, in Crop Production: Evolution, History, and Technology, C.W. Smith (ed.), John Wiley
& Sons, New York, pp. 57-127, ISBN: 978-0-471-07972-9.
Smith, D.C. (1942), “Intergeneric hybridization of cereals and other grasses”, Journal of Agricultural Research,
Vol. 64, pp. 33-45.
Somsen, H.W. and W.H. Sill, Jr. (1970), The Wheat Curl Mite, Aceria tulipae Keifer, in Relation to Epidemiology and
Control of Wheat streak mosaic, Agricultural Experimental Station, Kansas State University of Agriculture and
Applied Science, Manhattan.
Sorensen, A.E. (1986), “Seed dispersal by adhesion”, Annual Review of Ecology and Systematics, Vol. 17, pp. 443-
463, https://doi.org/10.1146/annurev.es.17.110186.002303.
Sparks Companies Inc. (2002), New and Improved Wheat Uses Audit, Prepared for: National Association of Wheat
Growers.
St-Martin, A. et al. (2017), “Diverse cropping systems enhanced yield but did not improve yield stability in a 52-year
long experiment”, Agriculture, Ecosystems & Environment, Vol. 247, pp. 337-342,
https://doi.org/10.1016/j.agee.2017.07.013.
Steinmann, H.-H. and E.S. Dobers (2016), “Spatio-temporal analysis of crop rotations and crop sequence patterns in
Northern Germany: potential implications on plant health and crop protection”, Journal of Plant Diseases and
Protection. Vol. 120, pp. 85-94, https://doi.org/10.1007/BF03356458.
Stewart, W.M. et al. (2005), “The contribution of commercial fertilizer nutrients to food production”, Agronomy
Journal, Vol. 97, pp. 1-6, https://doi.org/10.2134/agronj2005.0001.
Stone, A.E. and T.F. Peeper (2004), “Characterizing jointed goatgrass (Aegilops cylindrica) × winter wheat hybrids in
Oklahoma”, Weed Science, Vol. 52, pp. 742-745, https://doi.org/10.1614/WS-03-119R1.
Stoyanov, H. (2013), “Characteristics of natural hybrids between Aegilops cylindrica host and common winter wheat
(Triticum aestrivum L.)”, AgroLife Scientific Journal, Vol. 2, pp. 66-71.
Suneson, C.A. (1956), “An evolutionary plant breeding method”, Agronomy Journal, Vol. 48, pp. 188-191,
https://doi.org/10.2134/agronj1956.00021962004800040012x.
Tadesse, W. et al. (2013), Methods and Applications of Doubled Haploid Technology in Wheat Breeding, ICARDA
(International Center for Agricultural Research in the Dry Areas), Aleppo, Syria, ISBN: 92-9127-450-X.
Tanzarella, O.A. and B. Greco (1990), “Clonal propagation of wheat”, in Wheat, Y.P.S. Bajaj (ed.) Springer, Berlin,
Heidelberg, pp. 98-108, https://doi.org/10.1007/978-3-662-10933-5_6.
Tasmanian Institute of Agriculture (2014), Wheat Market Profile, Tasmanian Institute of Agriculture,
https://nre.tas.gov.au/Documents/Wheat%20Profile%20updated%20March%202014.pdf. (accessed July 2017).
Temby, I. and D. Marshall (2003), “Reducing cockatoo damage to crops”, Landcare Notes, LC0009, Department of
Sustainability and Environment, State of Victoria.
The Decolonial Atlas (2016). Average regional output of wheat. Available online at
https://decolonialatlas.wordpress.com/2016/10/09/agricultural-maps-of-the-world/. (accessed March 18, 2022).
Thomas, J.A., G.L. Hein and D.J. Lyon (2004), “Spread of wheat curl mite and Wheat streak mosaic virus is
influenced by volunteer wheat control methods”, Plant Health Progress, Vol. 5, https://doi.org/10.1094/PHP-
2004-1206-01-RS.
Trębicki, P. et al. (2015), “Virus disease in wheat predicted to increase with a changing climate”, Global Change
Biology, Vol. 21, pp. 3511-3519, https://doi.org/10.1111/gcb.12941.
Tsunewaki, K. (1964), “Genetic studies of a 6x-derivative from an 8x Triticale”, Canadian Journal of Genetics and
Cytology, Vol. 6, pp. 1-11, https://doi.org/10.1139/g64-001.
Tsunewaki, K. (1988), “Cytoplasmic variation in Triticum and Aegilops”, Proceedings of the Seventh International
Wheat Genetics Symposium, Cambridge.
Tutin, T.G. (1980), Flora Europaea, Vol. 5, Cambridge University Press.
Twigg, L.E. et al. (2009), “The potential of seed-eating birds to spread viable seeds of weeds and other undesirable
plants”, Austral Ecology, Vol. 34, pp. 805-820, https://doi.org/10.1111/j.1442-9993.2009.01992.x.
Uauy, C. (2017), “Wheat genomics comes of age”, Current Opinion in Plant Biology, Vol. 36, pp. 142-148,
https://doi.org/10.1016/j.pbi.2017.01.007.
Ueno, K., R. Fujita, R. and K. Yamazaki (1999), “Factors relating to seedling emergence in spring wheat”, Plant
Production Science, Vol. 2, pp. 235-240, https://doi.org/10.1626/pps.2.235.
Ueno, K., and H. Itoh (1997), “Cleistogamy in wheat: genetic control and the effect of environmental conditions”,
Cereal Research Communications, Vol. 25, pp. 185-189, https://doi.org/10.1007/BF03543455.
van Gastel, A.J.G., Z. Bishaw and B.R. Gregg (2002), “Wheat seed production”, in Bread Wheat: Improvement and
Production, B.C. Curtis, S. Rajaram and H. Gómez Macpherson (eds.), Food and Agriculture Organization of the
United Nations, Rome.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
72  ENV/CBC/MONO(2024)23

van Slageren, M.W. (1994), Wild Wheats: A Monograph of Aegilops L. and Amblypyrum (Jaub. & Spach) Eig
(Poaceae), Wageningen Agricultural University, Wageningen, the Netherlands, ICARDA (International Center for
Agricultural Research in the Dry Areas), Aleppo, Syria.
Van Wychen, L. (2017), 2017 Survey of the Most Common and Troublesome Weeds in Grass Crops, Pasture and
Turf in the United States and Canada, Weed Science Society of America National Weed Survey Dataset.
Vasil, I.K. (1987), “Developing cell and tissue culture systems for the improvement of cereal and grass crops”,
Journal of Plant Physiology, Vol. 128, pp. 193-218, https://doi.org/10.1016/S0176-1617(87)80234-1.
Virmani, S.S. and I.B. Edwards (1983), “Current status and future prospects for breeding hybrid rice and wheat”, in
Advances in Agronomy, Vol. 36, N.C. Brady (ed.), Academic Press, pp. 145-214, https://doi.org/10.1016/S0065-
2113(08)60354-5.
Waines, J.G. and S.G. Hegde (2003), “Intraspecific gene flow in bread wheat as affected by reproductive biology and
pollination ecology of wheat flowers”, Crop Science, Vol. 43, pp. 451-463,
https://doi.org/10.2135/cropsci2003.4510.
Walls III, J., E. Rajotte and C. Rosa (2019), “The past, present, and future of Barley yellow dwarf management”,
Agriculture, Vol. 9, 23, https://doi.org/10.3390/agriculture9010023.
Wang, R.R.-C. (2011), “Agropyron and Psathyrostachys”, in Wild Crop Relatives: Genomic and Breeding Resources,
Cereals, C. Kole (ed.), Springer, Berlin, Heidelberg, pp. 77-108, https://doi.org/10.1007/978-3-642-14228-4_2.
Wang, R.R.C. and B. Lu (2014), “Biosystematics and evolutionary relationships of perennial Triticeae species
revealed by genomic analyses”, Journal of Systematics and Evolution, Vol. 52, pp. 697-705,
https://doi.org/10.1111/jse.12084.
Wang et al. (2018) Gene editing and mutagenesis reveal inter-cultivar differences and additivity in the contribution
of TaGW2 homoeologues to grain size and weight in wheat. Theor Appl Genet 131:2463–
2475. https://doi.org/10.1007/s00122-018-3166-7.
Wang, R.R.-C. et al. (1994), “Genome symbols in the Triticeae (Poaceae) ”, in Triticeae, R.R.-C. Wang (ed.),
Herbarium Publications, Logan, Utah, pp. 29-34.
Warburton, M.L. et al. (2006), “Bringing wild relatives back into the family: recovering genetic diversity in CIMMYT
improved wheat germplasm”, Euphytica, Vol. 149, pp. 289-301, https://doi.org/10.1007/s10681-005-9077-0.
Wicks, G.A. et al. (2000), “Changes in fallow weed species in continuous wheat in northern New South Wales 1981-
1990”, Australian Journal of Experimental Agriculture, Vol. 40, pp. 831-842, https://doi.org/10.1071/EA99105.
Wiese, M.V. (1987), Compendium of Wheat Diseases, 2nd ed., American Phytopathological Society, St. Paul,
Minnesota, ISBN: 9780890540763.
Willenborg, C.J. and R.C. Van Acker (2008), “The biology and ecology of hexaploid wheat (Triticum aestivum L.) and
its implications for trait confinement”, Canadian Journal of Plant Science, Vol. 88, pp. 997-1013,
https://doi.org/10.4141/CJPS07144.
Wilson, H.K. and C.F. Hottes (1927), “Wheat germination studies with particular reference to temperature and
moisture relationships”, Agronomy Journal, Vol. 19, pp. 181-190,
https://doi.org/10.2134/agronj1927.00021962001900020010x.
Wilson, A.S. (1873), “II. Wheat and rye hybrids”, Transactions of the Botanical Society of Edinburgh, Vol. 12,
pp. 286-288, https://doi.org/10.1080/03746607309469536.
Witmer, G. et al. (2007), “Ecology and management of rodents in no‐till agriculture in Washington, USA”, Integrative
Zoology, Vol. 2, pp. 154-164, http://dx.doi.org/10.1111/j.1749-4877.2007.00058.x.
Wolff, J. (1987), Schädliche Unkraut- und Grassamen. Die Mühle und Mischfuttertechnik 87, 579-584.
Xie, Q. et al. (2012), “Wheat lines derived from trigeneric hybrids of wheat–rye–Psathyrostachys huashanica, the
potential resources for grain weight improvement”, Australian Journal of Crop Science, Vol. 6, pp. 1550-1557.
Yen, C., J.-L. Yang and Y. Yen (2005), “Hitoshi Kihara, Áskell Löve and the modern genetic concept of the genera in
the tribe Triticeae (Poaceae”, Acta Phytotaxonomica Sinica, Vol. 43, pp. 82-93.
Yoshioka, M. et al. (2017), “Three dominant awnless genes in common wheat: Fine mapping, interaction and
contribution to diversity in awn shape and length”, PLoS ONE, Vol. 12, e0176148,
https://doi.org/10.1371/journal.pone.0176148.
Zaharieva, M. and P. Monneveux (2006), “Spontaneous hybridization between bread wheat (Triticum aestivum L.)
and its wild relatives in Europe”, Crop Science, Vol. 46, pp. 512-527, https://doi.org/10.2135/cropsci2005.0023.
Zaharieva, M., Ayana, N.G., Hakimi, A.A., Misra, S.C., and Monneveux, P. (2010). Cultivated emmer wheat
(Triticum dicoccon Schrank), an old crop with promising future: a review. Genetic Resources and Crop Evolution
57, 937-962, https://doi.org/10.1007/s10722-010-9572-6.
Zemetra, R.S., J. Hansen and C.A. Mallory-Smith (1998), “Potential for gene transfer between wheat (Triticum
aestivum) and jointed goatgrass (Aegilops cylindrica)”, Weed Science, Vol. 46, pp. 313-317,
https://doi.org/10.1017/S0043174500089475.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  73

Zeven, A.C. (1987), “Crossability percentages of some 1400 bread wheat varieties and lines with rye”, Euphytica,
Vol. 36, pp. 299-319, https://doi.org/10.1007/bf00730677.
Zhang, P. et al. (2015), “Wheat–Aegilops introgressions”, in Alien Introgression in Wheat: Cytogenetics, Molecular
Biology, and Genomics, M. Molnár-Láng, C. Ceoloni and J. Doležel (eds.), Springer, Cham, pp. 221-243,
https://doi.org/10.1007/978-3-319-23494-6_9.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
74  ENV/CBC/MONO(2024)23

Annex A. Most common diseases and pests in

Triticum aestivum
Specialised Compendia
Bockus et al. (2010), Chelkowski (1991), Mehta (2014), Wiese (1987).
Online databases
● Wheat Diseases and Pests: a guide for field identification (CIMMYT) 14
● GrainGenes database (USDA) 15
● Wheat Doctor (CIMMYT)16
● Diseases of wheat (Triticum spp. L.) (The American Phytopathological Society) 17
● European and Mediterranean Plant Protection Organization (EPPO) database 18

Table A.1. Viruses, mycoplasms

Disease Agent
Agropyron mosaic virus Agropyron mosaic virus (AgMV). Geographic occurrence, e.g. in Eurasia, Canada and the United States
Barley stripe mosaic Barley stripe mosaic hordeivirus (BSMV). Geographic occurrence, e.g. in Eurasia, Northern America and the
hordeivirus Pacific
Barley yellow dwarf virus Barley yellow dwarf virus (BYDV). Geographic occurrence world-wide; wheat varieties show different tolerance
level (Baltenberger, Ohm and Foster, 1987); tolerance level had been increased through cross breeding with
resistant Agropyron varieties (Goulart et al., 1993; Sharma et al., 1989).
Barley yellow streak mosaic Barley yellow streak mosaic virus. Geographic occurrence, e.g. in Canada and the United States
virus
Barley yellow striate mosaic Barley yellow striate mosaic cytorhabdovirus (BYSMV). Geographic occurrence, e.g. in Africa, Eurasia, Middle
cytorhabdovirus East and Pacific
Brome mosaic virus Brome mosaic virus (BMV). Geographic occurrence, e.g. in Eurasia, Australia, South Africa and the United
States
European wheat striate European wheat striate mosaic tenuivirus (EWSMV). Geographic occurrence e.g. in Czechia, Poland,
mosaic tenuivirus Romania, Denmark, Finland, Sweden, Germany, United Kingdom and Spain
Wheat American striate Wheat American striate mosaic cytorhabdovirus (WASMV). Geographic occurrence, e.g. in Canada and the
mosaic virus United States
Wheat dwarf virus Wheat dwarf virus (WDV). Geographic occurrence, e.g. in Bulgaria, Czechia, Slovak Republic, Hungary,
former USSR, France and Sweden
Wheat soilborne mosaic Wheat soilborne mosaic virus. Geographic occurrence, e.g. in China, Japan, Italy and the United States
virus
Wheat spindle streak Wheat spindle streak mosaic virus (WSSMV). Geographic occurrence, e.g. in France, Germany, Italy, India,
mosaic virus Japan, China, and United States
Wheat streak mosaic virus Wheat streak mosaic virus (WSMV). Geographic occurrence e.g. in Canada, the United States, Romania and
Jordan
Wheat yellow leaf virus Wheat yellow leaf virus (WYLV). Geographic occurrence, e.g. in Japan and Italy
Wheat yellow mosaic Wheat yellow mosaic bymovirus (WYMV). Geographic occurrence e.g. in China, Japan, Korea, Canada and
bymovirus France

Source: Brunt et al. (1996).

14 https://repository.cimmyt.org/xmlui/handle/10883/1115
15 https://graingenes.org/GG3/
16 http://wheatdoctor.cimmyt.org/seed

17 https://www.apsnet.org/edcenter/resources/commonnames/Pages/Wheat.aspx
18 https://gd.eppo.int/

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  75

Table A.2. Bacteria

Disease Agent
Basal glume rot Pseudomonas syringae pv. atrofaciens (McCulloch)
Black chaff, Bacterial leaf Xanthomonas translucens pv. undulosa and Xanthomonas translucens pv. translucens. This pathogen causes
streak disease mainly in wheat, but can infect a number of other Poaceae species, such as barley, triticale and rye

Source: Brunt et al. (1996)

Table A.3. Fungi

Disease Agent
Ergot Claviceps purpurea: infects florets and produces grain-like sclerotia containing mycotoxins (ergot alkaloids).
The fungal grains are harvested with the wheat grains and, if not removed, mycotoxin contamination of
products occurs
Eyespot, stembreak, straw Oculimacula yallundae (Wallwork and Spooner). Syn: Pseudocercosporella herpotrichoides (Fron.) Deight.
breaker
Fusarium diseases Numerous Fusarium species play a part in the pathology of the cereal fusaria. The major species are:
– Fusarium culmorum (W.G. Smith) Sacc. var. culmorum
– Fusarium avenaceum (Fr.) Sacc. var. avenaceum
– Fusarium graminearum Schwabe (perfect form: Gibberella zeae (Schw.) Petch): widespread, especially
harmful not only to wheat but also to maize.
– Fusarium poae (Peck) Wollenw.: occurs sporadically, often in conjunction with the grass mite (Siteroptes
graminum [Reuter]), which feeds on the fungus and helps it to proliferate.
– Other species found in wheat include:
Fusarium acuminatum Ell. et Kellerm. (Gibberella acuminata Wollenw.),
Fusarium dimerum Penzig,
Fusarium equiseti (Corda) Sacc. (Gibberella intricans Wollenw.),
Fusarium sporotrichioides Sherb.,
Fusarium tricinctum (Corda) Sacc. and,
Fusarium moniliforme Sheldon sensu Wollenw. et Reinking.
Mould Aspergillus ssp./Penicillium ssp. can proliferate during storage. Both are potential mycotoxin producers
(Ochratoxin A).
Phoma leaf spot Phoma glomerata (Cda.) Wr. et Hochaf.
Powdery mildew of cereals Blumeria graminis (DC) Speer f. sp. tritici. Syn: Erysiphe graminis DC. f. sp. tritici March
Resistance genes, e.g. Mlk, Pm1 to Pm9, M1Ax, U1 and U2, can be found in different wheat varieties and
related species (Heun and Fischbeck, 1987, 1989; Hovmøller, 1989; Zeller, Lutz and Stephan, 1993).
Rhizoctonia root rot Rhizoctonia spp., Thanatephorus cucumeris (Frank) Donk.
Rusts
- Yellow/stripe rust Puccinia striiformis f. sp. tritici West., Syn.: Puccinia glumarum Erikss. et Henn.
Formation of pathotypes which specialize in wheat or barley. In exceptional cases wheat stripe rust strains may
attack highly susceptible barley varieties or vice versa.
- Leaf rust of wheat Puccinia triticina Erikss., Syn.: Puccinia recondita Rob. ex Desm. f. sp. tritici, Syn.: Puccinia rubigovera Wint.
Formation of pathotypes, alternate host Thalictrum spp.
- Black stem rust of wheat Puccinia graminis Pers. f. sp. tritici. Development of formae speciales specialised in rye, barley, oats, wheat
and grasses. Numerous pathotypes formed.
Septoria nodorum blotch, Parastagonospora nodorum (Berk.), Syn.: Leptosphaeria nodorum (E. Müll.), conidial form Septoria nodorum
Stagonospora nodorum Berk., Syn.: Phaesopheria nodorum (E. Müll.) Hejarude.
blotch Only partial resistance in wheat found (Bostwick, Ohm and Shaner, 1993; Jeger et al., 1983).
Septoria tritici blotch Zymoseptoria tritici. Syns: Mycosphaerella graminicola (Fckl.) Sanderson, conidial form: Septoria tritici Rob. ex
Desm.
Smuts
- Loose smut of wheat Ustilago tritici (Pers.) Rostr.
- Stinking smut (Common Various Tilletia species with different sori, including:
bunt) – Tilletia caries (DC.) Tul. Syn.: Tilletia tritici (Bjerk.) Wint.
– Tilletia foetida (Wallr.) Liro, Syn.: Tilletia laevis Kühn or Tilletia foetens (Bjerk. Et Curt.) Schroet.
– Tilletia intermedia (Gassner) Savul. Syn.: Tilletia tritici f. sp. intermedia Gassner
- Dwarf bunt of wheat Tilletia controversa Kühn

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
76  ENV/CBC/MONO(2024)23

- Karnal bunt Tilletia indica Mitra. Syn: Neovossia indica (Mit.) Mund.
- Stripe/flag smut Urocystis agropyri (Preuss.) Schroet.
Take-all Gaeumannomyces graminis (Sacc.) v. Arx. et Olivier var. tritici Walker
Several varieties with overlapping hosts, var. tritici attacks wheat, triticale, barley and rye.
Tan spot Pyrenophora tritici-repentis (Died.) Drechsl. Syns: Drechslera tritici-repentis (Died.) Shoem., perfect form:
Pyrenophora trichostoma (Fr.) Fckl.

Source: Brunt et al. (1996)

Table A.4. Animals

Pest Agent
Aphids:
- Grain aphids Macrosiphum avenae (Fabr.), Syn.:
Sitobion avenae (Fabr.)
Also in barley, oats, rye, maize, fodder grasses. Aphid species which does not alternate hosts.
- Oat or bird cherry aphid Rhopalosiphum padi (L.)
Alternate-host aphid with broad host plant profile among cereal and grass species, e.g. barley, oats, maize,
fodder grasses.
- Rose grain aphid Metopolophium dirhodum (Walk.)
Alternate-host aphid (also in barley, oats, rye, maize, fodder grasses).
- Apart from the above- Bromegrass aphid (Diuraphis bromicola [H.R.L.]),
mentioned species of cat's-tail aphid (Diuraphis mühlei [Börn.]),
aphid, the following corn leaf aphid (Rhopalosiphum maidis [Fitch.]),
species may cause yellow cherry/reed canary grass aphid (Rhopalomyzus lonicerae [Siebold], Rhopalomyzus poae [Gill.]),
damage to cereals, maize cocksfoot aphid (Hyalopteroides humilis [Walk.]),
and grasses:
Laingia psammae (Theob.),
Schizaphis nigerrima H.R.L.,
Metopolophium festucae (Theob.),
green grain aphid (Schizaphis graminum [Rond.]),
grain aphid (Sitobion granarium [Kirby]),
cob aphid (Sipha maydis [Pass.], Sipha glyeriae [Kalt.]),
black (bean) aphid (Aphis fabae Scop.),
green peach aphid (Myzus persicae [Sulz.])
Cereal cyst nematodes, Heterodera avenae Woll.
cereal stem eelworm Also attacks barley, oats, rye, fodder grasses. Several biotypes distinguished by their host profile.
Cereal leaf beetle Red-throated cereal leaf beetle (Oulema melanopus [L.], Syn.: Lema melanopa [L.]),
Blue cereal leaf beetle (Oulema lichenis [Voet], Syn.: Lema lichenis [Voet])
Corn beetle Zabrus tenebroides Goeze (corn ground beetle)
Also found in barley, oats, rye, maize, fodder grasses.
Crane-fly larvae Larvae of the marsh crane-fly (Pales (Tipula) paludosa Meig.),
Common crane-fly (Pales (Tipula) oleracea L.),
Autumn crane-fly (Pales (Tipula) czizeki de Jong).
Also in barley, oats, rye, maize, fodder grasses.
March fly larvae Bibio hortulans (L.), Bibio marci (L.), Bibio johannis (L.), Bibio clavipes (Meig.)
Also in barley, oats, rye, maize, fodder grasses.
Myriapods Various species of myriapods, notably the common millipedes Cylindroiulus teutonicus (Pocock) and Blaniulus
guttulatus (Bosc.)
Also in barley, oats, rye, maize, fodder grasses.
Root aphids Anoecia corni (Fabr.), Anoecia vagans (Koch), Aploneura graminis (Buckt.), Aploneura lentisci Pass.,
Byrsocrypta personata Börner, Forda marginata Koch, Forda formicaria V. Heyden, Geoica discreta Börner,
Tetraneura ulmi (L.)
Also in barley, oats, rye, maize, fodder grasses.
Slugs Various species of slug, notably the field slug (Deroceras reticulatum O.F. Müll., Deroceras agreste L.), the
garden/blackfield slug (Arion hortensis [Fér.], Arion rufus [L.]).
Also in barley, oats, rye, maize, fodder grasses.
Wheat and grass bugs Wheat and grass bugs are a non-homogeneous group of pests. The greatest economic damage is caused by
wheat bugs (Eurygaster spp.).

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  77

Also in barley, oats, rye, maize, fodder grasses.

Wheat seed gall nematode Anguina tritici (Steinbuch) Filipjev.

Source: Brunt et al. (1996)

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
78  ENV/CBC/MONO(2024)23

Annex B. Biotechnology applications for wheat

improvement
Wheat breeding, like other crop breeding, has relied heavily on spontaneously occurring
natural genetic variations and induced genetic variations using radiation such as gamma
rays or mutagenic chemicals such as ethyl methanesulfonate (EMS). Biotechnological
tools provide an expanded toolset for wheat breeders to introduce novel genetic variation
into wheat plants for breeding selection.
One way to introduce a new trait in wheat is through genetic engineering or modification.
Typically, this process allows the introduction of a DNA sequence from one or more
organisms into the genome of a recipient organism. Because genetic engineering in wheat
involves a phase of tissue culture, only the few wheat varieties that are amenable to this
process can be genetically engineered (also named ‘genetically modified’ or ‘transgenic’).
These varieties can then be crossed with elite cultivars to transfer the new trait. Argentina
has authorised one transgenic drought and herbicide tolerant wheat variety known as HB4
for commercial planting. After Argentina in 2020, Brazil approved HB4 wheat for cultivation
in March 2023, following feed and food use approvals in 2021. Several countries have
approved HB4 wheat for feed and food uses since 2020 (ISAAA, 2023). Field trials of
transgenic bread wheat been undertaken in Argentina19, Australia20, the Europe Union21,
Canada 22 and the United States 23 with traits including herbicide tolerance, pathogen
resistance, insect resistance, abiotic stress tolerance, yield enhancement and improved
nutritional quality.
The development of emerging genome-editing technologies provides another way to
introduce new traits into wheat, and/or alter existing traits. The method known as clustered
regulatory interspaced short palindromic repeats (CRISPR/Cas9) has been used to create
genetic and phenotypic variation in wheat related to disease resistance (Wang et al., 2014;
Zhang et al., 2017; Su et al., 2019), yield (Zhang et al., 2016; Wang et al., 2018), herbicide
tolerance (Zhang et al., 2019), and quality traits (Li et al., 2020; Sanchez-Leon et al., 2018).
An advantage of genome editing for wheat is the potential for altering multiple genes
simultaneously. Wheat has a large and complex hexaploidy genomes with high functional
redundancy and complementarity among it’s A, B, and D subgenomes, resulting in six
alleles for most wheat genes. This genome complexity makes it challenging to alter multiple
or all copies of a target gene through naturally occurring mutations or conventional
mutation breeding methods. One example of overcoming this challenge is the use of
multiplexed CRISPR/Cas9 for the simultaneous mutation of up to 35 of the 45 different
α-gliadin genes (Sanchez-Leon et al., 2018). It is also possible to use CRISPR in wheat
without integrating heterologous DNA into the genome, because the vectors are either not

19 National Bioeconomy Directorate at the Ministry of Agriculture, Livestock and Fisheries, Argentina,
https://www.argentina.gob.ar/agricultura/alimentos-y-bioeconomia/ogm-vegetal-eventos-con-autorizacion-
comercial.
20 Office of Gene Technology Regulator, Australia. http://www.ogtr.gov.au/.

21 European Commission GMO Register, https://ec.europa.eu/food/plant/gmo/eu_register_en.

22 Canadian Food Inspection Agency, http://www.inspection.gc.ca/.

23 United States Department of Agriculture, https://www.aphis.usda.gov/aphis/home/.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  79

integrated into the wheat genome (Zhang et al., 2016; Liang et al., 2017) or are segregated
out through crossing (Wang et al., 2017).

While the biotechnologies described above have been applied to wheat improvement by
introducing new genetic variations and traits into wheat, these technologies are
continuously being refined (Puchta 2017; Nasti and Voytas 2021) and new technologies
and approaches are expected to emerge. Collectively these tools are expected to expand
opportunities for wheat improvement.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
80  ENV/CBC/MONO(2024)23

References in Annexes
Anzalone, A.V. et al. (2019), “Search-and-replace genome editing without double-strand breaks or donor DNA”,
Nature, Vol. 576, pp. 149-157, https://doi.org/10.1038/s41586-019-1711-4.
Baltenberger, D.E., H.W. Ohm, and J.E. Foster (1987), “Reactions of oat, barley, and wheat to infection with Barley
yellow dwarf virus isolates”, Crop Science, Vol. 27, pp. 195-198,
https://doi.org/10.2135/cropsci1987.0011183X002700020010x.
Bibikova, M. et al. (2003), “Enhancing gene targeting with designed zinc finger nucleases”, Science, Vol. 300, p. 764,
https://doi.org/10.1126/science.1079512.
Bockus, W.W. et al. (2010), Compendium of Wheat Diseases and Pests, American Phytopathological Society, St.
Paul, Minnesota, https://doi.org/10.1094/9780890546604.
Bogdanove, A.J. and D.F. Voytas (2011), “TAL effectors: customizable proteins for DNA targeting”, Science,
Vol. 333, pp. 1843-1846. https://doi.org/10.1126/science.1204094.
Bostwick, D.E., H.W. Ohm and G. Shaner (1993), “Inheritance of Septoria glume blotch resistance in wheat”, Crop
Science, Vol. 33, pp. 439-443, https://doi.org/10.2135/cropsci1993.0011183X003300030005x.
Brunt, A. et al. (1996), Viruses of Plants. Descriptions and Lists from the VIDE Database, CAB International,
Wallingford, U.K., ISBN: 9780851987941.
Chelkowski, J. (1991), Cereal Grain: Mycotoxins, Fungi and Quality in Drying and Storage, Developments in Food
Sceince, Vol 26, 1st ed., Elsevier Science, Amsterdam, ISBN: 978-0444885548.
Chen K. et al. (2019), “CRISPR/Cas genome editing and precision plant breeding in agriculture”, Annual Review of
Plant Biology, Vol. 70, pp. 667-697, https://doi.org/10.1146/annurev-arplant-050718-100049.
Doudna, J.A. and E. Charpentier (2014), “Genome editing. The new frontier of genome engineering with CRISPR-
Cas9”, Science, Vol. 346, 1258096. https://doi.org/10.1126/science.1258096.
Gaudelli, N.M. e al. (2017), “Programmable base editing of A*T to G*C in genomic DNA without DNA cleavage”,
Nature, Vol. 551, pp. 464-471, https://doi.org/10.1038/nature24644.
Goulart, L.R. et al.(1993), “Barley yellow dwarf virus resistance in a wheat × wheatgrass population”, Crop Science,
Vol. 33, pp. 595-599, https://doi.org/10.2135/CROPSCI1993.0011183X003300030035X.
Heun, M. and G. Fischbeck (1987), “Genes for powdery mildew resistance in cultivars of spring wheat”, Plant
Breeding, Vol. 99, pp. 282-288, https://doi.org/10.1111/j.1439-0523.1987.tb01183.x.
Heun, M. and G. Fischbeck (1989), “Inheritance of the powdery mildew resistance Mlk in wheat”, Plant Breeding,
Vol. 103, pp. 262-264, https://doi.org/10.1111/j.1439-0523.1989.tb00383.x.
Hovmøller, M.S. (1989), “Race specific powdery mildew resistance in 31 Northwest European wheat cultivars”, Plant
Breeding, Vol. 103, pp. 228-234, http://dx.doi.org/10.1111/j.1439-0523.1989.tb00376.x.
ISAAA, 2023, “GM Approval Database, Event Name: HB4 Wheat”, available at:
https://www.isaaa.org/gmapprovaldatabase/event/default.asp?EventID=574 (Accessed 07 July 2023)
Jeger, M.J., D. Gareth Jones and E. Griffiths (1983), “Components of partial resistance of wheat seedlings to
Septoria nodorum”, Euphytica, Vol. 32, pp. 575-584, https://doi.org/10.1007/BF00021470.
Jinek, M. et al. (2012), “A programmable dual-RNA-guided DNA endonuclease in adaptive bacterial immunity”,
Science, Vol. 337, pp. 816-821. https://doi.org/10.1126/science.1225829.
Kawall, K. (2019), “New possibilities on the horizon: genome editing makes the whole genome accessible for
changes”, Frontiers in Plant Science, Vol. 10, 525, https://doi.org/10.3389/fpls.2019.00525.
Komor, A.C. et al. (2016), “Programmable editing of a target base in genomic DNA without double-stranded DNA
cleavage”, Nature, Vol. 533, pp. 420-424, https://doi.org/10.1038/nature17946.
Li, J. et al. (2020), “Modification of starch composition, structure and properties through editing of TaSBEIIa in both
winter and spring wheat varieties by CRISPR/Cas9”, Plant Biotechnology Journal, Vol. 19, pp. 937-951,
https://doi.org/10.1111/pbi.13519.
Liang, Z. et al. (2017), “Efficient DNA-free genome editing of bread wheat using CRISPR/Cas9 ribonucleoprotein
complexes”, Nature Communications, Vol. 8, 14261, https://doi.org/10.1038/ncomms14261.
Mehta, Y.R. (2014), Wheat Diseases and Their Management, Springer, ISBN: 978-3-319-06465-9.
Nasti, R.A. and D.F. Voytas (2021), “Attaining the promise of plant gene editing at scale”, Proceedings of the
National Academy of Sciences of the United States of America, Vol. 118, e2004846117,
https://doi.org/10.1073/pnas.2004846117.
Puchta, H. (2017), “Applying CRISPR/Cas for genome engineering in plants: the best is yet to come”, Current

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified
 ENV/CBC/MONO(2024)23  81

Opinion in Plant Biology, Vol. 36, pp. 1-8, https://doi.org/10.1016/j.pbi.2016.11.011.

Sánchez-León, S. et al. (2018), “Low-gluten, nontransgenic wheat engineered with CRISPR/Cas9”, Plant
Biotechnology Journal, Vol. 16, pp. 902-910, https://doi.org/10.1111/pbi.12837.
Sharma, H.C. et al. (1989), “Response of wheatgrasses and wheat × wheatgrass hybrids to Barley yellow dwarf
virus”, Theoretical and Applied Genetics, Vol. 77, pp. 369-374, https://doi.org/10.1007/bf00305830.
Su, Z. et al. (2019), “A deletion mutation in TaHRC confers Fhb1 resistance to Fusarium head blight in wheat”,
Nature Genetics, Vol. 51, pp. 1099-1105, https://doi.org/10.1038/s41588-019-0425-8.
Wang, K. et al. (2017), “Generation of marker-free transgenic hexaploid wheat via an Agrobacterium-mediated co-
transformation strategy in commercial Chinese wheat varieties”, Plant Biotechnology Journal, Vol. 15, pp. 614-
623, https://doi.org/10.1111/pbi.12660.
Wang, Y. et al. (2014), “Simultaneous editing of three homoeoalleles in hexaploid bread wheat confers heritable
resistance to powdery mildew”, Nature Biotechnology, Vol. 32, pp. 947-951, https://doi.org/10.1038/nbt.2969.
Wiese, M.V. (1987), Compendium of Wheat Diseases, 2nd ed., American Phytopathological Society, St. Paul,
Minnesota, ISBN: 9780890540763.
Zhang, R. et al. (2019), “Generation of herbicide tolerance traits and a new selectable marker in wheat using base
editing”, Nature Plants, Vol. 5, pp. 480-485, https://doi.org/10.1038/s41477-019-0405-0.
Zhang, Y. et al. (2016), “Efficient and transgene-free genome editing in wheat through transient expression of
CRISPR/Cas9 DNA or RNA”, Nature Communications, Vol. 7, 12617, https://doi.org/10.1038/ncomms12617.
Zhang, Y. et al. (2017), “Simultaneous modification of three homoeologs of TaEDR1 by genome editing enhances
powdery mildew resistance in wheat”, Plant Journal, Vol. 91, pp. 714-724, https://doi.org/10.1111/tpj.13599.
Zeller, F.J., J. Lutz and U. Stephan (1993), “Chromosome location of genes for resistance to powdery mildew in
common wheat (Triticum aestivum L.) 1. Mlk and other alleles at the Pm3 locus”, Euphytica Vol. 68, pp. 223-229,
https://doi.org/10.1007/BF00029876.

REVISED CONSENSUS DOCUMENT ON THE BIOLOGY OF WHEAT (TRITICUM AESTIVUM L.)

Unclassified