28 Concepts in Plant Physiology

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Plant Cells

The Plant Kingdom

Since the time of Aristotle (384–322 B.C.E.), biologists have
sought to classify organisms. At first the purpose was ease
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of identification (“artificial” classification schemes). Carolus
Linnaeus (1707–1778), arguably the greatest of the pre-modern
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Naturalists, sought to classify plants and other organisms according

to affinity groups that reflected the mind of the Creator. Later,
after Darwin, the goal of classification was to show evolutionary
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relationships (“natural” classification schemes).

For the past 150 years, biologists have emphasized natural
systems of classification and have attempted to define
morphological criteria that reveal evolutionary relationships.
We now know that morphology, the form and structure
of organisms, is the end product of the actions of genes. Virtually
all of the information needed to form a complete organism
is encoded in its DNA sequences, both nuclear and cytoplasmic
(mitochondria and chloroplasts).
DNA sequence analysis has thus provided evolutionary
biologists with a powerful new tool for arriving at a truly natural
classification system.
On the basis of phylogenetic analyses of highly conserved
DNA sequences, living organisms have been divided into three
major domains: Bacteria, Archaea, and Eucarya.
1. The common ancestor of all the organisms first gave rise
to the Bacteria and the common ancestor of the Archaea
and the Eucarya.
 Plant Cells 29

2. The Archaea branch off from the Eucarya lineage.

3. The Eucarya common ancestor acquires mitochondrial
endosymbiont.
4. A heterogeneous group of eukaryotes called protists branch
off the lineage leading to plants, fungi, and animals.
5. The common ancestor of fungi and animals form a branch,
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followed by a divergence into the fungal and animal

lineages.
6. The common ancestor of plants, green algae, red algae,
and glaucophytes acquires chloroplast endosymbiont (a
cyanobacterium).
7. The three lineages of glaucophytes, red algae, and green
algae diverge.
8. Various lineages of protists acquire chloroplasts via green
or red algal endosymbionts.
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9. The earliest branch of green algae diverge.
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10. The later branch of green algae diverge.

11. The remaining lineage leads to plants.
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The Eucarya include the eukaryotes, organisms whose cells

contain a true nucleus. The Bacteria, or eubacteria, which include
the cyanobacteria, lack a true nucleus and are therefore prokaryotic.
The Archaea, or archaebacteria, are also prokaryotic, but they
differ from the Bacteria: Besides their morphological and
biochemical differences, they are often adapted to extreme
environments, such as sulfur hot springs or saline ponds.
Phylogenetic studies have indicated that the Archaea and Eucarya
split after the Bacteria separated from the common ancestor. Thus
Archaea and Eucarya represent sister groups. This closer relation
between Archaea and Eucarya is reflected in their similar promoter
structures and RNA polymerases, the presence of histones, and
many other characteristics.
Fungi were formerly classified as algal-like plants that had
lost their chloroplasts. However, fungi and animals branched off
from the Eucarya lineage before the appearance of choloroplasts.
They are thus more closely related to animals than to plants. Fungi
are heterotrophic; that is, they depend on other organisms for
their food, and they satisfy their nutritional needs by absorbing
 30 Concepts in Plant Physiology

inorganic ions and organic molecules from the external

environment. Most fungal species are filamentous and possess cell
walls made of chitin, the same substance that is found in insect
exoskeletons.
Bryophytes are small (rarely more than 4 cm in height), very
simple land plants, and the least abundant in terms of number of
species and overall population. Bryophytes do not appear to be
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in the direct line of evolution leading to the vascular plants; rather,

they seem to constitute a separate minor branch.
Bryophytes include mosses, liverworts, and hornworts. These
small plants have life cycles that depend on water during the
sexual phase. Water facilitates fertilization, the fusion of gametes
to produce a diploid zygote, a feature also seen in the algal
precursors of these plants.
Bryophytes are like algae in other respects as well: They have
neither true roots nor true leaves, they lack a vascular system, and
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they produce no hard tissues for structural support. The absence
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of these structures that are important for growth on land greatly

restricts the potential size of bryophytes, which, unlike algae, are
terrestrial rather than aquatic.
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The ferns represent the largest group of spore-bearing vascular

plants. In contrast to the bryophytes, ferns have true roots, leaves,
and vascular tissues, and they produce hard tissues for support.
These architectural features enable ferns to grow to the size
of small trees. Although ferns are better adapted to the drying
conditions of terrestrial life than bryophytes are, they still depend
on water as a medium for the movement of sperm to the egg. This
dependence on water during a critical stage of their life cycle restricts
the ecological range of ferns to relatively moist habitats.
The most successful terrestrial plants are the seed plants. Seed
plants have been able to adapt to an extraordinary range of habitats.
The embryo, protected and nourished inside the seed, is able to
survive in a dormant state during unfavourable growing conditions
such as drought. Seed dispersal also facilitates the dissemination
of the embryos away from the parent plant.
Another important feature of seed plants is their mode of
fertilization. Fertilization in seed plants is brought about by wind-
or insect-mediated transfer of pollen, the gamete-producing
structure of the male, to the sexual structure of the female, the
 Plant Cells 31

pistil. Pollination is independent of external water, a distinct

advantage in terrestrial environments. Many seed plants produce
massive amounts of woody tissues, which enable them to grow
to extraordinary heights. These features of seed plants have
contributed to their success and account for their wide range.
There are two categories of seed plants: gymnosperms (from
the Greek for “naked seed”) and angiosperms (based on the Greek
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for “vessel seed,” or seeds contained in a vessel). Gymnosperms

are the less advanced type; about 700 species of gymnosperms are
known. The largest group of gymnosperms is the conifers (“cone-
bearers”), which include such commercially important forest trees
as pine, fir, spruce, and redwood.
Two types of cones are present: male cones, which produce
pollen, and female cones, which bear ovules. The ovules are located
on the surfaces of specialized structures called cone scales. After
wind-mediated pollination, the sperm reaches the egg via a pollen
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tube, and the fertilized egg develops into an embryo. Upon
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maturation, the cone scales, which are appressed during early

development, separate from each other, allowing the naked seeds
to fall to the ground.
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Angiosperms, the more advanced type of seed plant, first

became abundant during the Cretaceous period, about 100 million
years ago. Today, they dominate the landscape, easily outcompeting
their cousins, the gymnosperms. About 250,000 species are known,
but many more remain to be characterized. A typical angiosperm
life cycle, that of Zea mays (corn). The major innovation of the
angiosperms is the flower; hence they are referred to as flowering
plants. There are other anatomical differences between angiosperms
and gymnosperms, but none so crucial and far-reaching as the
mode of reproduction.

Flower Structure and the Angiosperm Life Cycle

The flower consists of several leaflike structures attached to
a specialized region of the stem called the receptacle. Sepals and
petals are the most leaflike. Petals have the primary function of
attracting insects to serve as pollinators, accounting for their often
showy and brightly coloured appearance.
The stamen is the male sexual structure, and the pistil is the
female sexual structure. The pistil is composed of one or more
 32 Concepts in Plant Physiology

united carpels; the pistil, or in some flowers a whorl of pistils, is

sometimes referred to as the gynoecium. The stamen consists of
a narrow stalk called the filament and a chambered structure
called the anther. The anther contains tissue that gives rise to
pollen grains. The pistil consists of the stigma (the tip where
pollen lands during pollination), the style (an elongated structure),
and the ovary. The ovary, the hollow basal portion of the pistil,
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completely encloses one or more ovules. Each ovule, in turn,

contains an embryo sac, the structure that gives rise to the female
gamete, the egg.
After landing on the stigma, the pollen grain germinates to
form a long pollen tube, which penetrates the tissues of the style
and ultimately enters the cavity of the ovary, which houses the
ovule. Within the ovary, the pollen tube enters the ovule and
deposits two haploid sperm cells in the embryo sac. One sperm
cell fuses with the egg to produce the zygote; the other typically
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fuses with the two polar nuclei to produce a specialized storage
tissue termed the endosperm, which provides nutrients to the
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growing embryo.
Endosperm tissue also provides the bulk of the worldrquotes
food supply in the form of cereal grains. As in conifers, in
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angiosperms the outer tissues of the ovule harden into a protective

seed coat. Angiosperm seeds have a second layer of protective
tissues, the fruit. The fruit consists of the ovary wall and, in some
cases, receptacle tissue.
Angiosperms are divided into two major groups, dicotyledons
(dicots) and monocotyledons (monocots). This distinction is based
primarily on the number of cotyledons, or seed leaves. In addition,
the two groups differ with respect to other anatomical features,
such as the arrangement of their vascular tissues, and their floral
structure.
As the dominant plant group on Earth, and because of their
great economic and agricultural importance, angiosperms have
been studied much more intensively than other types of plants.
Plant physiologists have focused on a relatively small number of
species that represent convenient experimental systems for the
study of specific phenomena.
Therefore, while we focus on these famous few, it is important
to keep in mind the tremendous diversity of form and function
 Plant Cells 33

that exists within the angiosperms, and the even greater diversity
of form and function that is found within the plant kingdom as
a whole.

Plant Tissue Systems: Dermal, Ground, and Vascular

Dermal tissue. The epidermis is the dermal tissue of young
plants undergoing primary growth. It is generally composed of
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specialized, flattened polygonal cells that occur on all plant surfaces.

Shoot surfaces are usually coated with a waxy cuticle to prevent
water loss and are often covered with hairs, or trichomes, which
are epidermal cell extensions.
Pairs of specialized epidermal cells, the guard cells, are found
surrounding microscopic pores in all leaves. The guard cells and
pores are called stomata (singular stoma), and they permit gas
exchange (water loss, CO2 uptake, and O2 release or uptake)
between the atmosphere and the interior of the leaf.
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The root epidermis is adapted for absorption of water and
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minerals, and its outer wall surface typically does not have a waxy
cuticle. Extensions from the root epidermal cells, the root hairs,
increase the surface area over which absorption can take place.
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Ground tissue. Making up the bulk of the plant are cells

termed the ground tissue. There are three types of ground tissue:
parenchyma, collenchyma, and sclerenchyma.
• Parenchyma, the most abundant ground tissue, consists
of thin-walled, metabolically active cells that carry out a
variety of functions in the plant including photosynthesis
and storage
• Collenchyma tissue is composed of narrow, elongated
cells with thick primary walls. Collenchyma cells provide
structural support to the growing plant body, particularly
shoots, and their thickened walls are nonlignified so they
can stretch as the organ elongates. Collenchyma cells are
typically arranged in bundles or layers near the periphery
of stems or leaf petioles.
• Sclerenchyma consists of two types of cells, sclereids and
fibres Both have thick secondary walls and are frequently
dead at maturity. Sclereids occur in a variety of shapes,
ranging from roughly spherical to branched, and are
widely distributed throughout the plant. In contrast, fibres
 34 Concepts in Plant Physiology

are narrow, elongated cells that are commonly associated

with vascular tissues. The main function of sclerenchyma
is to provide mechanical support particularly to parts of
the plant that are no longer elongating.
In the stem, the pith and the cortex make up the ground tissue.
The pith is located within the cylinder of vascular tissue, where
it often exhibits a spongy texture because of the presence of large
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intercellular air spaces.

If the growth of the pith fails to keep up with that of the
surrounding tissues, the pith may degenerate, producing a hollow
stem. In general, roots lack piths, although there are exceptions
to this rule. In contrast, the cortex, which is located between the
epidermis and the vascular cylinder, is present in both stems and
roots.
At the boundary between the ground tissue and the vascular
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tissue in roots, and occasionally in stems, is a specialized layer of
cortex known as the endodermis. This single layer of cells originates
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from cortical tissue at the innermost layer of the root cortex and
forms a cylinder that surrounds the central vascular tissue, or
stele.
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Early in root development, a narrow band composed of the

waxy substance suberin is formed in the cell walls circumscribing
each endodermal cell. These suberin deposits, called Casparian
strips, form a barrier in the endodermal walls to the intercellular
movement of water, ions, and other water-soluble solutes to the
vascular cells. Leaves have two interior layers of ground tissue
that are collectively known as the mesophyll.
The palisade parenchyma consists of closely spaced, columnar
cells located beneath the upper epidermis. There is usually one
layer of palisade parenchyma in the leaf. Palisade parenchyma
cells are rich in chloroplasts and are a primary site of photosynthesis
in the leaf. Below the palisade parenchyma are i rregularly shaped,
widely spaced spongy mesophyll cells.
The spongy mesophyll cells are also photosynthetic, and the
large spaces between these cells allow diffusion of carbon dioxide.
The spongy mesophyll also contributes to leaf flexibility in the
wind, and this flexibility facilitates the movement of gases within
the leaf.
 Plant Cells 35

Vascular tissues: xylem and phloem. The vascular tissue is

composed of two major conducting systems: the xylem and the
phloem. The xylem transports water and mineral ions from the
root to the rest of the plant.
The phloem distributes the products of photosynthesis and a
variety of other solutes throughout the plant. The tracheids and
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vessel elements are the conducting cells of the xylem.

Both of these cell types have elaborate secondary-wall
thickenings and lose their cytoplasm at maturity; that is, they are
dead when functional. Tracheids overlap each other, whereas vessel
elements have open end walls and are arranged end to end to form
a larger unit called a vessel. Other cell types present in the xylem
include parenchyma cells, which are important for the storage of
energy-rich molecules and phenolic compounds, and sclerenchyma
fibres.
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The sieve elements and sieve cells are responsible for sugar
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translocation in the phloem. The former are found in angiosperms;

the latter perform the same function in gymnosperms. Like vessel
elements, sieve elements are often stacked in vertical rows, forming
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larger units called sieve tubes, whereas sieve cells form overlapping
arrays. Both types of conducting cells are living when functional,
but they lack nuclei and central vacuoles and have relatively few
cytoplasmic organelles.
Substances are translocated from sieve cell to sieve cell laterally
through circular or oval zones containing enlarged pores, called
sieve areas. In contrast, sieve tubes translocate substances through
large pores in the end walls of the sieve elements, called sieve
plates. Sugar movement through sieve tubes is more efficient and
rapid than through sieve cells and represents a more evolutionarily
advanced mechanism.
Sieve elements are associated with, and depend on, densely
cytoplasmic parenchyma cells called companion cells. The
analogous cells adjacent to the sieve cells of gymnosperms are
called albuminous cells. Companion cells provide proteins and
metabolites necessary for the functions of the sieve tube elements.
In addition, the phloem frequently contains storage parenchyma
and fibres that provide mechanical support.
 36 Concepts in Plant Physiology

The Structures of Chloroplast Glycosylglycerides

The structures of the three plant glycosylglycerides, mono-
galactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol
(DGDG) and sulfoquinovosyldiacylglycerol (SQDG). The two
galactolipids, MGDG and DGDG, are uncharged but polar; the
sulfolipid, SQDG, is negatively charged as well as being polar.
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The Multiple Steps in Construction of the Cell Plate

Following Mitosis
During Mitosis in plants, the daughter chromosomes separate
during anaphase and migrate toward their respective poles. In
contrast to animal cells, the spindle poles of plant cells is quite
diffuse, so that the chromosomes move in nearly parallel fashion.
At the end of teleophase, cytokinesis begins with the formation
of the cell plate. This process involves fusion of many small
secretory vesicles, and attachment of the resulting structure to the
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plasma membrane. A diagram of the steps involved in cell plate
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formation.
In the first step, Golgi vesicles, some of which are
interconnected via fusion tubes, aggregate in the spindle midzone
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area. This structure is called the fusion tube network (FTN). The
transition from the first to the second stage of cell plate formation
can be inhibited by caffeine. (B) Formation of a tubulo-vesicular
network (TVN).
The contents of the vesicles, mainly pectins, represent the
precursors from which the new middle lamella is assembled outside
the cell. In the next stage, vesicle fusion increases, forming a
tubulo-vesicular network (TVN), and the membranes become
coated with either clathrin or other proteins. (C) In the third stage,
the central region of the growing cell plate forms a tubular network
(TN), with vesicle fusion occurring at the growing edges where
the remaining microtubules are located. (D) In the final stage, the
cell plate contacts and adheres to the plasma membrane of the
parent cell.
At the same time the tubular network expands to form a
fenestrated sheet. (E) At the end of mitosis, the phragmoplast
disappears, the cell enters interphase, and microtubules reappear
in the cytosol near the plasma membrane, where they play a role
in the deposition of cellulose microfibrils during cell wall growth.
 Plant Cells 37

(F) Electron micrograph of a cell plate forming in a root tip of a

beet, (Beta vulgaris) (10,000×) MT, microtubule; VE, vesicles; N,
nucleus; NE, nuclear envelope; P, cell plate.

Water and Plant Cells

Calculating Capillary Rise
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We learn on page 36 of the textbook that the water properties

of cohesion, adhesion and surface tension, give rise to the
phenomenon of capillarity, the movement of water for small
distances up a capillary tube. The smaller the tube radius, the
higher the capillary rise. How far the water will move may be
calculated using the following formula:

14.9 ×10−6 m 2
Capillary rise =
radius
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where both capillary rise and radius are expressed in meters.
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For a xylem vessel with 25 µm radius, the capillary rise is

about 0.6 m. This distance is much too small to be significant for
water transport up tall trees.
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Fibrous materials such as cell walls can act like wicks to draw
water by capillarity from nearby xylem.
This capillary action ensures that cell wall surfaces that are
directly exposed to the air, such as those in leaf mesophyll, remain
wetted and do not dry out. Because the cell wall capillaries have
a tiny radius, about 10 m–8, very large physical forces can be generated
in the water just below the evaporative surfaces of cell walls.

Calculating Half-Times of Diffusion

Extremely Slow over Long Distances
From Fick’s law, one can derive an expression for the time it
takes for a substance to diffuse a particular distance.
If one defines conditions such that all the solute molecules are
concentrated at the starting position, then the concentration front
moves away from the starting position, as shown for a later time
point. As the substance diffuses away from the starting point, the
concentration gradient becomes less steep (Dcs decreases) and
thus net movement becomes slower.
 38 Concepts in Plant Physiology

The time it takes for the substance at any given distance from
the starting point to reach one-half of the concentration at the
starting point (tc = ½) is given by the following equation:

(dis tan ce) 2

t c =1/ 2 = K
DS
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where K is a constant and Ds is the diffusion coefficient. The above

equation shows that the time required for a substance to diffuse
a given distance increases in proportion to the square of that distance.
Let us consider two numerical examples. First, how long it would
take a small molecule to diffuse across a typical cell? The diffusion
coefficient for a small molecule like glucose is about 10–9 m2 s–1,
and the cell length may be 50 µm. Thus, for this example:

(50 + 10−6 m) 2
t c =1/ 2 = = 2.5s
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10−9 m 2s −1
This calculation shows that small molecules diffuse over
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cellular dimensions rapidly. What about diffusion over longer

distances? Calculating the time needed for the same substance to
move a distance of 1 m (e.g., the length of a corn leaf), we find:
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(1m) 2
t c =1/ 2 = = 109 s ≈ 32 year
10−9 m 2s −1
a value that exceeds by orders of magnitude the life span of a corn
plant, which lives only a few months. This shows that diffusion
in solutions can be effective within cellular dimensions but is far
too slow for mass transport over long distances. Diffusion is of
great importance as a driving force for the water vapour lost from
leaves, because the diffusion coefficient for a molecule in air is
much greater than in aqueous solutions.
Alternative Conventions for Components of Water Potential
Students planning further study of plant water relations should
note that the components of water potential defined in the text are
sometimes given different names and symbols. In particular, the
equation

ψ w = ψs + ψ p
 Plant Cells 39

is often replaced by the following equivalent equation:

ψw = π + P
In this alternative convention, P is the same as Yp. It is the
hydrostatic pressure of the solution, and may be positive, as in
turgid cells, or negative, as in xylem water.
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The symbol p is called osmotic pressure and is the negative

of Ys. That is, p has positive values, and Ys has negative values.
“Osmotic pressure” is the term that physical chemists, zoologists,
and many others use to denote the effect of dissolved solutes on
the free energy of water. Most handbooks of physics and chemistry
use the term “osmotic pressure” and the symbol π.
The negative sign in front of p in the equation above accounts
for the reduction in water potential (Yw) by dissolved solutes. Thus
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Ys = – p. A very interesting, if somewhat unconventional, account
of the history and physical meaning of osmotic pressure is given
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by Hammel and Scholander (1976).

Unfortunately, some authors have mixed the conventions for
Ys and p, leading to unnecessary confusion about what is meant
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by the symbol p. Thus, p is sometimes incorrectly called osmotic

potential instead of osmotic pressure, and it may be used either
as a positive quantity or as a negative quantity.
Regarding matric potential, it is usually designated by the
symbol t.

Can Negative Turgor Pressures Exist in Living Cells?

One of the challenging aspects of understanding plant water
relations is the remarkable range of pressures, both positive and
negative, that occur within the bodies of plants.
Negative pressures (or tensions), which depend upon the
cohesive strength of water coupled with the strength of lignified
cell walls to resist deformation, play an important role in water
transport through the xylem. Positive pressures, which depend
upon the semipermeable nature of the plasma membrane and the
elastic nature of primary cell walls, occur in all hydrated living
plants cells but can be especially large in sieve tubes and guard
cells.
 40 Concepts in Plant Physiology

Living plant cells are typically assumed to have only positive

pressures. However, there appears to be no reason that negative
pressures could not also occur within the cytoplasm of living plant
cells. This web topic explores the existence and potential role of
negative turgor pressures in plants.
When a cell loses water in air, turgor declines and solute
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concentrations increase. At the turgor loss point (yp = 0), the

hydrostatic pressure in the cell sap is equal to atmospheric pressure,
meaning that no net force is exerted on the cell wall.
If water continues to be lost from the cell, the pressure within
the cytoplasm drops below atmospheric pressure, resulting in a
force imbalance that collapses the cell wall. The deformation of
living cells upon desiccation is called cytorrhysis. Note that the
plasma membrane is pressed against the cell wall throughout
desiccation (i.e., plasmolysis does not occur) because the hydrostatic
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pressure in cytoplasm remains greater than the hydrostatic pressure
in apoplast.
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In the living cells described above, a decrease in cell water

potential below the turgor loss point is balanced by an increase
in solute concentration as the volume of the cell decreases.
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Thus, true “negative” pressures do not develop. In contrast,

xylem conduits have rigid cell walls that resist deformation,
allowing them to sustain negative pressures without imploding.
This raises the question of what happens when water is lost from
living cells that have thick walls or are embedded in a rigid matrix
of cells (e.g., xylem parenchyma). Might these cells resist
deformation and thus develop negative turgor pressures?
It is important to emphasize that this is a controversial area,
due in large part to the absence of direct measurements of negative
turgor pressures within cells. However, before reviewing the
evidence for negative turgor pressures, it is worthwhile to consider
what physiological effects might result from the development of
such negative pressures in living cells.
The major outcome of negative turgor is that it allows stiff-
walled cells to decrease in water potential without undergoing
major changes in cell volume or osmotic concentration. Because
cytorrhysis might cause physical damage to the wall and/or cell
membranes, while the high concentrations of solutes resulting
 Plant Cells 41

from the reduction in cell volume might adversely affect the

conformation of membranes and proteins, it is possible to imagine
a physiological role or benefit for the development of negative
turgor pressures in plant cells.
We now turn this around and ask if there are any downsides
to the generation of negative turgor pressures in living cells? In
the xylem, the primary issue associated with negative pressures
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is cavitation; could this also be a concern for living cells?

The primary mechanism for cavitation in plants is air seeding,
reflecting the fact that the probability of the de novo formation
of gas voids in water (either by homogeneous or heterogeneous
nucleation) is extremely low. In air seeding, air is sucked in through
the cell wall, where it then “nucleates” the transition to the vapour phase.
For air seeding to occur in a living cell experiencing negative
pressures, air would have to be pulled through the very small
pores of the cell wall. While one can imagine this happening, the
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movement of air across the cell wall would result in plasmolysis
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and thus the immediate release of any tension in the cytoplasm

because the plasma membrane is not capable of withstanding any
significant pressure.
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Other costs associated with the ability of living cells to generate

negative pressures include the metabolic costs of producing rigid
cell walls, as well as any limitations lignified walls might place
on physiological function. In addition, a strategy for avoiding cell
damage due to desiccation via the generation of negative pressures
might impose limitations on cell size. The strength of a cell to
withstand collapse (and thus generate negative pressures) is
inversely proportional to cell size.
Evidence for negative turgor pressure in plants is limited, in
part reflecting the fact that few researchers have devoted much
attention to this issue. Living cells with flexible (unlignified) cell
walls deform relatively easily upon desiccation and thus do not
appear to support negative pressures. However, measurements of
the forces needed to collapse cell walls suggest that living cells
with thick walls can withstand forces in the range of 1.0 MPa.
Visual examination of tissues adapted to withstand very cold
temperatures also provides indirect evidence for negative turgor.
For example, frozen ray parenchyma cells often do not exhibit
significant deformation, despite the very strong desiccatory effects
 42 Concepts in Plant Physiology

(low water potential) of extracellular ice. One can hypothesize that

the existence of negative pressures within these cells acts to balance
the water potential gradient across the cell membrane (i.e., between
the cytoplasm and ice formed within the apoplast).
In summary, negative turgor pressure remains an intriguing
but little-studied area of plant water relations.
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While it does not appear to form in cells such as the leaf

mesophyll, that can deform easily, its existence in living cells with
either lignified cell walls or where the cell is embedded in a matrix
of lignified tissues (e.g., living cells within wood) cannot be ruled
out. The potential benefits of negative turgor pressures in terms
of preventing mechanical and osmotic damage associated with
severe desiccation makes.
The Matric Potential
In discussions of soils, seeds, and cell walls, one often finds
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reference to yet another component of Yw: the matric potential
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(Ym).
The matric potential is used to account for the reduction in
free energy of water when it exists as a thin surface layer, one or
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two molecules thick, adsorbed onto the surface of relatively dry

soil particles, cell walls, and other materials.
The matric potential does not represent a new force acting on
water, because the effect of surface interactions can theoretically
be accounted for by an effect on Ys and Yp. In dry materials,
however, this surface interaction effect often cannot easily be
separated into Yp and Ys components in dry materials, so they are
frequently bulked together and designated as the matric potential.
It is generally not valid to add Ym to independent measurements
of Ys and Yp to arrive at a total water potential.
This is particularly true for water inside hydrated cells and
cell walls, where matric effects are either negligible or they are
accounted for by a reduction in Yp.
For instance, the negative pressure in water held by cell wall
microcapillaries at the evaporative surfaces of leaves, is sometimes
described as a wall matric potential. Care is needed to avoid
inconsistencies when accounting for this physical effect in
definitions of Yp, Ys, and Ym.
 Plant Cells 43

Measuring Water Potential

Plant scientists have expended considerable effort in devising
accurate and reliable methods for evaluating the water status of
a plant. Four instruments that have been used extensively to
measure Ψw, Ψs, and Ψp are described here: psychrometer, pressure
chamber, cryoscopic osmometer, and pressure probe.
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Psychrometer (Yw Measurement)

Psychrometry (the prefix “psychro-” comes from the Greek
word psychein, “to cool”) is based on the fact that the vapour
pressure of water is lowered as its water potential is reduced.
Psychrometers measure the water vapour pressure of a solution
or plant sample, on the basis of the principle that evaporation of
water from a surface cools the surface.
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Figure: Diagram illustrating the use of isopiestic psychrometry to

measure the water potential of a plant tissue.
One psychrometric technique, known as isopiestic psychrometry,
has been used extensively by John Boyer and coworkers and is
illustrated in above Figure. Investigators make a measurement by
placing a piece of tissue sealed inside a small chamber that contains
a temperature sensor (in this case, a thermocouple) in contact with
a small droplet of a standard solution of known solute concentration
(known Ys and thus known Yw).
 44 Concepts in Plant Physiology

If the tissue has a lower water potential than that of the droplet,
water evaporates from the droplet, diffuses through the air, and
is absorbed by the tissue. This slight evaporation of water cools
the drop.
The larger the difference in water potential between the tissue
and the droplet, the higher the rate of water transfer and hence
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the cooler the droplet. If the standard solution has a lower water
potential than that of the sample to be measured, water will diffuse
from the tissue to the droplet, causing warming of the droplet.
Measuring the change in temperature of the droplet for several
solutions of known Yw makes it possible to calculate the water
potential of a solution for which the net movement of water between
the droplet and the tissue would be zero signifying that the droplet
and the tissue have the same water potential.
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Figure: The temperature of the sensor in the psychrometer shown in

above figure depends on the water potential of the solution relative to
the water potential of the tissue sample.
Psychrometers can be used to measure the water potentials
of both excised and intact plant tissue. Moreover, the method can
be used to measure the Ys of solutions.
This can be particularly useful with plant tissues. For example,
the Yw of a tissue is measured with a psychrometer, and then the
tissue is crushed and the Ys value of the expressed cell sap is
measured with the same instrument. By combining the two
 Plant Cells 45

measurements, researchers can estimate the turgor pressure that

existed in the cells before the tissue was crushed (Yp = Yw – Ys).
A major difficulty with this approach is the extreme sensitivity
of the measurement to temperature fluctuations. For example,
a change in temperature of 0.01°C corresponds to a change in
water potential of about 0.1 MPa. Thus, psychrometers must be
operated under constant temperature conditions. For this reason,
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the method is used primarily in laboratory settings. There are

many variations in psychrometric technique; interested readers
should consult Brown and Van Haveren 1972, Slavik 1974, and
Boyer 1995.

Pressure Chamber
A relatively quick method for estimating the water potential
of large pieces of tissues, such as leaves and small shoots, is by
use of the pressure chamber. This method was pioneered by Henry
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Dixon at Trinity College, Dublin, at the beginning of the twentieth
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century, but it did not come into widespread use until P. Scholander
and coworkers at the Scripps Institution of Oceanography improved
the instrument design and showed its practical use.
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In this technique, the organ to be measured is excised from

the plant and is partly sealed in a pressure chamber.
Before excision, the water column in the xylem is under tension.
When the water column is broken by excision of the organ (i.e., its
tension is relieved allowing its Yp to rise to zero), water is pulled
rapidly from the xylem into the surrounding living cells by osmosis.
The cut surface consequently appears dull and dry. To make
a measurement, the investigator pressurizes the chamber with
compressed gas until the distribution of water between the living
cells and the xylem conduits is returned to its initial, pre-excision, state.
This can be detected visually by observing when the water
returns to the open ends of the xylem conduits that can be seen
in the cut surface. The pressure needed to bring the water back
to its initial distribution is called the balance pressure and is readily
detected by the change in the appearance of the cut surface, which
becomes wet and shiny when this pressure is attained.
The pressure chamber is often described as a tool to measure
the tension in the xylem.
 46 Concepts in Plant Physiology

Rubber Gasket

Lid

Pressure
gauge

TP < 0 P P
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(A) (B) (C)

Water Water Water column
column column when
in xylem after pressure
before excision balance (P)
Compressed
excision is reached
gas
Chamber cylinder
Figure: The pressure chamber method for measuring plant water
potential. The diagram at left shows a shoot sealed into a chamber,
which may be pressurized with compressed gas. The diagrams at right
show the state of the water columns within the xylem at three points in
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time: (A) The xylem is uncut and under a negative pressure, or
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tension. (B) The shoot is cut, causing the water to pull back into the
tissue, away from the cut surface, in response to the tension in the
xylem. (C) The chamber is pressurized, bringing the xylem sap back to
the cut surface.
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However, this is only strictly true for measurements made on

a non-transpiring leaf or shoot (for example, one that has been
previously enclosed in a plastic bag). When there is no transpiration,
the water potential of the leaf cells and the water potential in the
xylem will come into equilibrium.
The balancing pressure measured on such a non-transpiring
shoot is equal in magnitude but opposite in sign to the pressure
in the xylem (Yp). Because the water potential of our non-transpiring
leaf is equal to the water potential of the xylem, one can calculate
the water potential of the leaf by adding together Yp and Ys of the
xylem, provided one collects a sample of xylem sap for
determination of Ys.
Luckily Ys of the xylem is usually small (> -0.1 MPa) compared
to typical midday tensions in the xylem (Yp of –1 to –2 MPa). Thus,
correction for the Ys of the xylem sap is frequently omitted.
Balancing pressure measurements of transpiring leaves are
more difficult to interpret. The fact that water is flowing from the
 Plant Cells 47

xylem to the leaf means that differences in water potential must

exist.
When the transpiring leaf or shoot is cut off, the tension in
the xylem is instantly relieved and water is drawn into the leaf
cells until the water potentials of the xylem and the leaf cells come
into equilibrium. Because the total volume of the leaf cells is much
larger than the volume of sap in the xylem, this equilibrium water
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potential will be heavily weighted towards that of the leaf.

Thus, any measurement of the balancing pressure on such a
leaf or shoot will result in a value that is approximately the water
potential of the leaf, rather than the tension of the xylem. (To be
exact, one would have to add the Ys of the xylem sap to the
negative of the balancing pressure to get the leaf water potential.)
One can explore the differences between the water potential of the
xylem and the water potential of a transpiring leaf by comparing
balancing pressures measured on covered (i.e., non-transpiring)
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versus uncovered (transpiring) leaves.
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Pressure chamber measurements provide a quick and accurate

way of measuring leaf water potential. Because the pressure
chamber method does not require delicate instrumentation or
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temperature control, it has been used extensively under field

conditions. For a more complete description of the theory and
operation of the pressure chamber.
Cryoscopic Osmometer

The cryoscopic osmometer measures the osmotic potential of

a solution by measuring its freezing point.
Solutions have colligative properties that collectively depend
on the number of dissolved particles and not on the nature of the
solute. For example, solutes reduce the vapour pressure of a
solution, raise its boiling point, and lower its freezing point.
The specific nature of the solute does not matter. One of the
colligative properties of solutions is the decrease in the freezing
point as the solute concentration increases. For example, a solution
containing 1 mol of solutes per kilogram of water has a freezing
point of –1.86°C, compared with 0°C for pure water.
Various instruments can be used to measure the freezing-
point depression of solutions. With a cryoscopic osmometer,
 48 Concepts in Plant Physiology

solution samples as small as 1 nanoliter (10–9 L) are placed in an

oil medium located on the temperature-controlled stage of a
microscope. The very small sample size allows sap from single
cells to be measured and permits rapid thermal equilibration with
the stage.
To prevent evaporation, the investigator suspends the samples
in oil-filled wells in a silver plate (silver has high thermal
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conductivity).
Solid Frozen Sample

Liquid Sample

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Liquid Sample Temperature-

containing controlled
diminishing sample
ice crystals holder

Temperature-
measuring
device

Figure: A cryoscopic osmometer measures the concentration of total

dissolved solutes by measuring the freezing-point depression of a
solution. (A) Very small liquid samples are loaded onto the
temperature-controlled stage of a microscope. (B) When the
temperature is quickly reduced, the samples supercool and freeze.
(C) Slowly warming the stage causes the samples to thaw. The
temperature at which the last ice crystal melts provides a measure of
the melting point of the sample.
 Plant Cells 49

The temperature of the stage is rapidly decreased to about –

30° C, which causes the sample to freeze. The temperature is then
raised very slowly, and the melting process in the sample is
observed through the microscope. When the last ice crystal in the
sample melts, the temperature of the stage is recorded (note that
the melting and freezing points are the same).
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It is straightforward to calculate the solute concentration from

the freezing-point depression; and from the solute concentration
(cs), Ys is calculated as –RTcs. This technique has been used to
measure droplets extracted from single cells.
Pressure Probe
If a cell were as large as a watermelon or even a grape, measuring
its hydrostatic pressure would be a relatively easy task. Because
of the small size of plant cells, however, the development
of methods for direct measurement of turgor pressure has been
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slow.
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Using a micromanometer, Paul Green at the University of

Pennsylvania developed one of the first direct methods for
measuring turgor pressure in plant cells (Green and Stanton 1967).
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In this technique, an air-filled glass tube sealed at one end is

inserted into a cell.
The high pressure in the cell compresses the trapped gas, and
from the change in volume one can readily calculate the pressure
of the cell from the ideal gas law (pressure × volume = constant).
This method works only for cells of relatively large volume, such
as the giant cell of the filamentous green alga Nitella. For smaller
cells, the loss of cell sap into the glass tube is sufficient to deflate
the cell and this yields artifactually low pressures.
For higher plant cells, which are several orders of magnitude
smaller in volume than Nitella, a more sophisticated device, the
pressure probe, was developed by Ernest Steudle, Ulrich
Zimmermann, and their colleagues in Germany. This instrument
is similar to a miniature syringe.
A glass microcapillary tube is pulled to a fine point and is
inserted into a cell. The microcapillary is filled with silicone oil,
a relatively incompressible fluid that can be readily distinguished
from cell sap under a microscope.
 50 Concepts in Plant Physiology

When the tip of the microcapillary is first inserted into the cell,
cell sap begins to flow into the capillary because of the initial low
pressure of that region. Investigators can observe such movement
of sap under the microscope and counteract it by pushing on the
plunger of the device, thus building up a pressure. In such fashion
the boundary between the oil and the cell sap can be pushed back
to the tip of the microcapillary.
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When the boundary is returned to the tip and is held in a

constant position, the initial volume of the cell is restored and the
pressure inside the cell is exactly balanced by the pressure in the
capillary. This pressure is measured by a pressure sensor in the
device. Thus the hydrostatic pressure of individual cells may be
measured directly.
This method has been used to measure Yp and other parameters
of water relations in cells of both excised and intact tissues of a
variety of plant species. The primary limitation of this method is
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that some cells are too small to measure.
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Furthermore, some cells tend to leak after being stabbed with

the capillary, and others plug up the tip of the capillary, thereby
preventing valid measurements. The pressure probe has also been
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adapted to measure positive and negative values of Yp in the

xylem. However, technical problems with cavitation limit the
measurement of negative Yp by this technique.

Understanding Hydraulic Conductivity

Consider a cell with an initial water potential of –0.2 MPa,
submerged in pure water. From this information we know that
water will flow into the cell and that the driving force is DYw =
0.2 MPa, but what is the initial rate of movement? The rate depends
on permeability of the membrane to water, a property usually
called the hydraulic conductivity (Lp) of the membrane.
Driving force, membrane permeability, and flow rate are related
by the following equation:
Flow rate = driving force × hydraulic conductivity
Hydraulic conductivity expresses how readily water can move
across a membrane and has units of volume of water per unit area
of membrane per unit time per unit driving force (for instance,
m3 m –2 s–1 MPa–1 or m s–1 MPa–1). The larger the hydraulic
conductivity, the larger the flow rate. The hydraulic conductivity
 Plant Cells 51

of the membrane is 10–6 m s–1 MPa–1. The transport (flow) rate (Jv)
can then be calculated from the following equation:
J v = (Lp(∆ψ w )
where Jv is the volume of water crossing the membrane per
unit area of membrane and per unit time (m3 m–2 s–1 or, equivalently,
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m s–1). Please note that this equation assumes that the membrane
is ideal—that is, that solute transport is negligible and water
transport is equally sensitive to DYs and DYp across the membrane.
Nonideal membranes require a more complicated equation that
separately accounts for water flow induced by DYs and by DYp.
In our example, Jv has a value of 0.2 × 10–6 m s–1. Note that Jv
has the physical meaning of a velocity. We can calculate the flow
rate in volumetric terms (m3 s–1) by multiplying Jv by the surface
area of the cell.
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The resulting value is the initial rate of water transport. As
water is taken up, cell Yw increases and the driving force (DYw)
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decreases. As a result, water transport slows with time. As

elaborated in p. 43 of the textbook, the rate approaches zero in an
exponential manner.
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Wilting and Plasmolysis

Plasmolysis is the separation of plant cell cytoplasm from the
cell wall as a result of water loss. It is unlikely to occur in nature,
except in severe conditions. Plasmolysis is induced in the laboratory
by immersing a plant cell in a strongly saline or sugary solution,
so that water is lost by osmosis.
If onion epidermal tissue is immersed in a solution of calcium
nitrate, cells rapidly lose water by osmosis and the protoplasm of
the cells shrinks. This occurs because the calcium and nitrate ions
freely permeate the cell wall and encounter the selectively
permeable plasma membrane.
The large vacuole in the centre of the cell originally contains
a dilute solution with much lower osmotic pressure than that of
the calcium nitrate solution on the other side of the membrane.
The vacuole thus loses water and becomes smaller. The space
between the cell membrane and the cell wall enlarges and the
plasma membrane and the protoplasm within it contract to the
 52 Concepts in Plant Physiology

centre of the cell. Strands of cytoplasm extend to the cell wall

because of plasma membrane-cell wall attachment points.
Plasmolysed cells die unless they are transferred quickly from the
salt or sugar solution to water.

Mineral Nutrition
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Symptoms of Deficiency in Essential Minerals

Visual nutrient deficiency symptoms can be a very powerful
diagnostic tool for evaluating the nutrient status of plants. One
should keep in mind, however, that a given individual visual
symptom is seldom sufficient to make a definitive diagnosis of a
plant’s nutrient status.
Many of the classic deficiency symptoms such as tip burn,
chlorosis and necrosis are characteristically associated with more
than one mineral deficiency and also with other stresses that by
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themselves are not diagnostic for any specific nutrient stress.
However, their detection is extremely useful in making an
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evaluation of nutrient status.

In addition to the actual observations of morphological and
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spectral symptoms, knowing the location and timing of these

symptoms is a critical aspect of any nutrient status evaluation.
Plants do not grow in isolation, they are part of the overall
environment and as such they respond to environmental changes
as that affect nutrient availability. Also, plants do influence their
environment and can contribute to environmental changes, which
in turn can affect the nutrient status of the plant.
Sources of Visual Symptoms
Stresses such as salinity, pathogens, and air pollution induce
their own characteristic set of visual symptoms. Often, these
symptoms closely resemble those of nutrient deficiency.
Pathogens often produce an interveinal chlorosis, and air
pollution and salinity stress can cause tip burn. Although at first
these symptoms might seem similar in their general appearance
to nutrient deficiency symptoms, they do differ in detail and/or
in their overall developmental pattern.
Pathological symptoms can often be separated from nutritional
symptoms by their distribution in a population of affected plants.
 Plant Cells 53

If the plants are under a nutrient stress, all plants of a given type and
age in the same environment tend to develop similar symptoms
at the same time.
However if the stress is the result of pathology, the development
of symptoms will have a tendency to vary between plants until
a relatively advanced stage of the pathology is reached.
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Environmental Associations
Plants remove substantial amounts of nutrients from the soil
during their normal growth cycle and many long-term
environmental changes occur as a result of this process. Effects on
the soil go considerably beyond the straight removal or depletion
of nutrients. Charge balance must be maintained in the plant-soil
system during nutrient uptake. Charge balance is usually achieved
by the excretion of proton and/or hydroxyl ions by the plant to
replace the absorbed nutrient cations or anions.
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For example when plants are fertilized with ammonia, they
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acquire most of their nitrogen in the form of the ammonium

cation, rather than from the usual nitrate anion. Because nitrate
is the only anion used by the plant in large amounts, the net result
of this change is that during normal nutrient uptake the proton
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excretion will far exceed that of hydroxyl ions.

In the case of vigorously growing plants, the amount of excreted
protons can be sufficiently large as to decrease the pH of the soil
by several pH units. Changes in soil pH of such magnitude can
have large implications for a number of soil processes such as soil
structure, nutrient availability and leaching of nutrients.
The immediate effect on the soil may be favourable for some
plants, especially acid-loving plants, in that it tends to make iron
more available. However, in the long run, lowering the soil pH
can be deleterious to plants in that the availability of nutrients will
change. A lower soil pH will allow micronutrients to be more
readily leached from the soil profile, eventually resulting in
deficiencies of nutrients such as Cu and Zn. Additionally, when
the pH of the soil drops much below pH 5, the solubility of Al
and Mn can increase to such an extent as to become toxic to most
plant growth.
Plants are often thought of as passive in relation to the
environment. However this is not always a valid assumption; for
 54 Concepts in Plant Physiology

there are many plants that clearly manipulate their environment

in a fashion that tends to makes certain nutrients more readily
available.
For example, iron is a limiting nutrient in many agricultural
areas, but it comprises about 3% of the average soil which, if
available, would be far in excess of the needs of the average plant.
Some plants actively excrete protons, and the resulting decrease
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in pH increases the solubility of iron in their environment. In

addition, other plants excrete phytosiderophores that chelate the
soil iron rendering it a more available form for the plants.

Pathways of Symptom Development

At first glance, it would appear that the distinction of deficiency
symptoms for the 13 known essential mineral nutrients should be
relatively simple. But such an assumption is incorrect. In fact, the
deficiency symptoms are quite complex because each nutrient has
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a number of different biological functions and each function may
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have an independent set of interactions with a wide range of

environmental parameters.
In addition, the expression of these symptoms varies for acute
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or chronic deficiency conditions. Acute deficiency occurs when a

nutrient is suddenly no longer available to a rapidly growing
plant. Chronic deficiency occurs when there is a limited but
continuous supply of a nutrient, at a rate that is insufficient to
meet the growth demands of the plant. Most of the classic
deficiency symptoms described in textbooks are characteristic of
acute deficiencies. The most common symptoms of low-grade,
chronic deficiencies are a tendency towards darker green leaves
and stunted or slow growth.
Typically most published descriptions of deficiency symptoms
arise from experiments conducted in greenhouses or growth
chambers where the plants are grown in hydroponics or in media
where the nutrients are fully available. In these conditions, nutrients
are readily available while present, but when a nutrient is depleted,
the plant suddenly faces an acute deficiency. Thus, hydroponic
studies favour the development of acute deficiencies.
In experiments designed to study micronutrient deficiency
symptoms, micronutrients are usually omitted from the nutrient
solution. Micronutrients are often present in the seed or as
 Plant Cells 55

contaminants in the environment, so a plant of adequate size will

exhaust these trace amounts of micronutrient and develop
characteristic acute deficiency systems.
When deficiency symptoms of macronutrients are sought, the
macronutrient is removed suddenly from a suitable sized rapidly
growing plant. Alternatively the plant can initially be given a one-
time supply of the nutrient that is sufficient for a limited amount
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of growth. Because macronutrients are continuously required in

relatively large amounts by rapidly growing plants, the available
nutrients will be rapidly depleted, resulting in an acute deficiency.
In natural systems, the plant encounters many degrees and
types of stresses that result in different types of symptoms occurring
over time. Perhaps the most common nutrient deficiency in natural
environments is the case of a limited nutrient supply that is
continuously renewed at a low rate from soil weathering processes.
In such cases, the limited nutrient availability results in chronic
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nutrient deficiency symptoms.
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Effect of Nutrient Mobility on Symptom Development

The interaction between nutrient mobility in the plant, and
plant growth rate can be a major factor influencing the type and
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location of deficiency symptoms that develop.

For very mobile nutrients such as nitrogen and potassium,
deficiency symptoms develop predominantly in the older and
mature leaves. This is a result of these nutrients being preferentially
mobilized during times of nutrient stress from the older leaves to
the newer leaves near the growing regions of the plant. Additionally,
mobile nutrients newly acquired by the roots are also preferentially
translocated to new leaves and the growing regions.
Thus old and mature leaves are depleted of mobile nutrients
during times of stress while the new leaves are maintained at a
more favourable nutrient status.
The typical localization of deficiency symptoms of very weakly
mobile nutrients such as calcium, boron, and iron is the opposite
to that of the mobile nutrients; these deficiency symptoms are first
displayed in the growing regions and new leaves while the old
leaves remain in a favourable nutrient status. (This assumes that
these plants started with sufficient nutrient, but ran out of nutrient
as they developed).
 56 Concepts in Plant Physiology

In plants growing very slowly for reasons other than nutrition

(such as low light) a normally limiting supply of a nutrient could,
under these conditions, be sufficient for the plant to slowly develop,
maybe even without symptoms. This type of development is likely
to occur in the case of weakly mobile nutrients because excess
nutrients in the older leaves will eventually be mobilized to supply
newly developing tissues.
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In contrast, a plant with a similar supply that is growing

rapidly will develop severe deficiencies in the actively growing
tissue such as leaf edges and the growing region of the plant. A
classic example of this is calcium deficiency in vegetables such as
lettuce where symptoms develop on the leaf margins (tip burn)
and the growing region near the meristems.
The maximal growth rate of lettuce is often limited by the
internal translocation rate of calcium to the growing tissue rather
than from a limited nutrient supply in the soil.
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When moderately mobile nutrients such as sulfur and
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magnesium are the limiting nutrients of the system, deficiency

symptoms are normally seen over the entire plant. However the
growth rate and rate of nutrient availability can make a considerable
difference on the locations at which the symptoms develop. If the
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nutrient supply is marginal compared to the growth rate, symptoms

will appear on the older tissue, but if the nutrient supply is very
low compared to the growth rate, or the nutrient is totally depleted,
the younger tissue will become deficient first.
Plant Competition and Induced Deficiencies
When the observed symptoms are the direct result of a nutrient
deficiency, the actions needed for correction are relatively straight-
forward. However symptoms are often the result of interactions
with other environmental factors limiting the availability of the
nutrient whose symptoms are expressed. The classic instance is
that of iron deficiency induced by an excess of heavy metals in
the environment. Transition metals such as Cu, Zn Cr and Ni
compete with Fe and each other for plant uptake. Competition for
uptake is not specific to Fe and heavy metals but is true for all
mineral nutrients that are chemically similar and have similar
uptake mechanisms. For example if the availability of Cu or Zn
is relatively less than that of Fe, then excessive concentrations of
some other metal such as Ni or Cr will induce a deficiency of one
 Plant Cells 57

of these nutrients rather than Fe. In the case of the macronutrients,

excessive amounts of Mg will compete with K for uptake and can
possibly induce a K deficiency. The barrenness of serpentine soils
is the result of such competition, with the high Mg of these soils
inducing a Ca deficiency. The toxicity of a low pH soil is another
example of a basic nutrient deficiency. Low pH has a two-fold
effect on soil nutrients: It enhances the leaching of cations, reducing
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their availability in the soil, and the relatively abundant protons

in the soil compete with Ca and other cations for uptake. Thus,
nutrient deficiencies can be induced by a number of different
mechanisms often working in concert to limit the availability of
a nutrient.
Nutrient Demand and Use Efficiency
Although all plants of the same species respond similarly to
nutrient stress, plants of similar species will often show significant
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differences in their nutrient use efficiency. This results from
differences in growth rate, root distribution, phase of development,
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and efficiency of nutrient uptake and utilization. This implies that

in any given location, plants from one species may become nutrient-
deficient, while those from another species growing in the same
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environment right next to them, may not show any deficiency symptoms.
Growth rate also affects nutrient status. When the nutrient
supply is barely inadequate for growth under existing
environmental conditions, many plants adjust their growth rate
to match that supported by the available nutrient supply without
displaying typical visual deficiency symptoms. Agricultural
systems differ from natural systems in that crop plants have been
selected primarily for rapid growth under low stress conditions.
This rapid growth rate results in a high nutrient demand by
these plants and a higher incidence of nutrient deficiency unless
supplemental fertilizers are supplied. It is not uncommon to find
agricultural crops showing severe signs of nutrient stress, with
native plants growing in the same area showing little or no
indication of nutrient stress. In agriculture systems chronic
deficiency symptoms develop mostly in crops with little or limited
fertilization. Acute nutrient deficiency symptoms most often occur
when new crops with a higher nutrient demand are introduced,
or less productive lands are brought under cultivation for the
production of rapidly growing crop plants.
 58 Concepts in Plant Physiology

Uniformity of Nutrient Status

Not all tissues of a plant are at the same nutrient status during
times of stress. Leaves on the same plant that are exposed to
different environmental conditions, (such as light), or those of
different ages may have considerable differences in nutrient status.
Mineral nutrients are for the most part acquired by the roots and
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translocated throughout the plant. The distance of any part of the

plant to the roots will influence nutrient availability, particularly
in the case of the less mobile nutrients. In plants recovering from
nutrient deficiency, the root and conductive tissues recover first.
For example, in the case of recovery from Fe deficiency, it is
common to see the veins re-green while the interveinal tissue
remains chlorotic and Fe-deficient.
In order to maintain rapid, optimal growth, all plant tissues
must have a favourable nutrient status. Although a plant may be
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marginally low in a number of nutrients, only one nutrient at a
time will limit overall growth. However, if the supply of that
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limiting nutrient is increased even slightly, the resulting increase

in growth will increase the demand for all other nutrients and
another nutrient, the next lowest in availability, will become
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limiting.
Other Diagnostic Tools
Although visual diagnostic symptoms are an extremely
valuable tool for the rapid evaluation of the nutrient status of a
plant, they are only some of the tools available. Other major tools
include microscopic studies, spectral analysis, and tissue and soil
analysis. These methods all vary in their precision, rapidity and
their ability to predict future nutrient status. Because of the close
interaction between plant growth and the environment, all
predictions of future nutrient status must make assumptions about
how the environment will change in that time frame.
The principle advantage of visual diagnostic symptoms is that
they are readily obtained and provide an immediate evaluation
of nutrient status. Their main drawback is that the visual symptoms
do not develop until after there has been a major effect on yield,
growth and development.
Tissue analysis is nutrient-specific but relatively slow; tissues
must be sampled, processed and analysed before the nutrient
 Plant Cells 59

status can be determined. An analysis of the mineral nutrient

content of selected plants tissues, when compared against Critical
Level values, can be used to evaluate the plant nutrient status at
the time of sampling with a relatively high degree of confidence
and can be extrapolated to project nutrient status at harvest. Soil
analysis is similar to tissue analysis but evaluates the potential
supplying power of the soil instead of plant nutrient status. Plant
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analysis provides information as to what the plant needs, while soil

analysis provides information about the status of the nutrient supply.
Spectral analysis of nutrient status is still in its infancy and
is presently used primarily in the inventory of global resources
and in specialized studies. Microscopic studies are most valuable
in looking at the physiological aspects of nutrient stress rather
than the evaluation of plant nutrient status on a whole plant or
crop basis.
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Symptom Descriptions
It is unusual to find any one leaf or even one plant that
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displays the full array of symptoms that are characteristic of a

given deficiency. It is thus highly desirable to know how individual
symptoms look, for it is possible for them to occur in many possible
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combinations on a single plant. Most of the terms used below in

the description of deficiency symptoms are reasonably self evident;
a few however have a distinct meaning in the nutrient deficiency
field. For example, the term chlorotic, which is a general term for
yellowing of leaves through the loss of chlorophyll, cannot be
used without further qualification because there may be an overall
chlorosis as in nitrogen deficiency, interveinal, as in iron deficiency,
or marginal, as in calcium deficiency. Another term used frequently
in the description of deficiency symptoms is necrotic, a general
term for brown, dead tissue. This symptom can also appear in
many varied forms, as is the case with chlorotic symptoms.
Nutrient deficiency symptoms for many plants are similar,
but because of the large diversity found in plants and their
environments there is a range of expression of symptoms. Because
of their parallel veins, grasses and other monocots generally display
the affects of chlorosis as a series of stripes rather than the netted
interveinal chlorosis commonly found in dicots. The other major
difference is that the marginal necrosis or chlorosis found in dicots
is often expressed as tip burn in monocots.
 60 Concepts in Plant Physiology

Magnesium: The Mg-deficient leaves show advanced interveinal

chlorosis, with necrosis developing in the highly chlorotic tissue.
In its advanced form, magnesium deficiency may superficially
resemble potassium deficiency. In the case of magnesium deficiency
the symptoms generally start with mottled chlorotic areas
developing in the interveinal tissue.
The interveinal laminae tissue tends to expand proportionately
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more than the other leaf tissues, producing a raised puckered

surface, with the top of the puckers progressively going from
chlorotic to necrotic tissue. In some plants such as the Brassica
(The mustard family, which includes vegetables such as broccoli,
brussel sprouts, cabbage, cauliflower, collards, kale, kohlrabi,
mustard, rape, rutabaga and turnip.), tints of orange, yellow, and
purple may also develop.
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Figure: Magnesium deficiency symptoms in tomato.

Manganese: These leaves show a light interveinal chlorosis
developed under a limited supply of Mn. The early stages of the
chlorosis induced by manganese deficiency are somewhat similar
to iron deficiency. They begin with a light chlorosis of the young
leaves and netted veins of the mature leaves especially when they
are viewed through transmitted light. As the stress increases, the
 Plant Cells 61

leaves take on a gray metallic sheen and develop dark freckled

and necrotic areas along the veins. A purplish luster may also
develop on the upper surface of the leaves. Grains such as oats,
wheat, and barley are extremely susceptible to manganese deficiency.
They develop a light chlorosis along with gray specks which elongate
and coalesce, and eventually the entire leaf withers and dies.
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V
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M

Figure: Manganese deficiency symptoms in tomato.

Molybdenum: These leaves show some mottled spotting along
with some interveinal chlorosis. An early symptom for
molybdenum deficiency is a general overall chlorosis, similar to
the symptom for nitrogen deficiency but generally without the
reddish coloration on the undersides of the leaves. This results
from the requirement for molybdenum in the reduction of nitrate,
which needs to be reduced prior to its assimilation by the plant.
Thus, the initial symptoms of molybdenum deficiency are in
fact those of nitrogen deficiency. However, molybdenum has other
metabolic functions within the plant, and hence there are deficiency
symptoms even when reduced nitrogen is available. In the case
of cauliflower, the lamina of the new leaves fail to develop, resulting
in a characteristic whiptail appearance. In many plants there is an
upward cupping of the leaves and mottled spots developing into
large interveinal chlorotic areas under severe deficiency. At high
concentrations, molybdenum has a very distinctive toxicity
symptom in that the leaves turn a very brilliant orange.
 62 Concepts in Plant Physiology
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V
Figure: Molybdenum deficiency symptoms in tomato.
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Nitrogen: The chlorotic symptoms shown by this leaf resulted

from nitrogen deficiency. A light red cast can also be seen on the
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veins and petioles. Under nitrogen deficiency, the older mature

leaves gradually change from their normal characteristic green
appearance to a much paler green. As the deficiency progresses
these older leaves become uniformly yellow (chlorotic).
Leaves approach a yellowish white colour under extreme
deficiency. The young leaves at the top of the plant maintain a
green but paler colour and tend to become smaller in size. Branching
is reduced in nitrogen deficient plants resulting in short, spindly
plants. The yellowing in nitrogen deficiency is uniform over the
entire leaf including the veins.
However in some instances, an interveinal necrosis replaces
the chlorosis commonly found in many plants. In some plants the
underside of the leaves and/or the petioles and midribs develop
traces of a reddish or purple colour.
In some plants this coloration can be quite bright. As the
deficiency progresses, the older leaves also show more of a tendency
to wilt under mild water stress and become senescent much earlier
than usual. Recovery of deficient plants to applied nitrogen is
immediate (days) and spectacular.
 Plant Cells 63
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Figure: Nitrogen deficiency symptoms in tomato.

Phosphorus: These phosphorus-deficient leaves show some
necrotic spots. As a rule, phosphorus deficiency symptoms are not
very distinct and thus difficult to identify. A major visual symptom
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is that the plants are dwarfed or stunted. Phosphorus deficient
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plants develop very slowly in relation to other plants growing

under similar environmental conditions but without phosphorus
deficiency. Phosphorus deficient plants are often mistaken for
unstressed but much younger plants. Some species such as tomato,
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lettuce, corn and the brassicas develop a distinct purpling of the

stem, petiole and the under sides of the leaves. Under severe
deficiency conditions there is also a tendency for leaves to develop
a blue-gray luster. In older leaves under very severe deficiency
conditions a brown netted veining of the leaves may develop.

Figure: Phosphorus deficiency symptoms in tomato.

 64 Concepts in Plant Physiology

Sulfur: This leaf shows a general overall chlorosis while still

retaining some green colour. The veins and petioles show a very
distinct reddish colour. The visual symptoms of sulfur deficiency
are very similar to the chlorosis found in nitrogen deficiency.
However, in sulfur deficiency the yellowing is much more uniform
over the entire plant including young leaves. The reddish
colour often found on the underside of the leaves and the petioles
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has a more pinkish tone and is much less vivid than that found
in nitrogen deficiency. With advanced sulfur deficiency
brown lesions and/or necrotic spots often develop along the petiole,
and the leaves tend to become more erect and often twisted and
brittle.

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Figure: Sulfur deficiency symptoms in tomato.

Zinc: This leaf shows an advanced case of interveinal necrosis.
In the early stages of zinc deficiency the younger leaves become
yellow and pitting develops in the interveinal upper surfaces of
the mature leaves. Guttation is also prevalent. As the deficiency
progress these symptoms develop into an intense interveinal
necrosis but the main veins remain green, as in the symptoms of
recovering iron deficiency. In many plants, especially trees, the
leaves become very small and the internodes shorten, producing
a rosette like appearance.
 Plant Cells 65
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Figure: Zinc deficiency symptoms in tomato.

Boron: These boron-deficient leaves show a light general
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chlorosis. The tolerance of plants to boron varies greatly, to the
extent that the boron concentrations necessary for the growth of
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plants having a high boron requirement may be toxic to plants

sensitive to boron. Boron is poorly transported in the phloem of
most plants, with the exception of those plants that utilize complex
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sugars, such as sorbitol, as transport metabolites. In a recent study,

tobacco plants engineered to synthesize sorbitol were shown to
have increased boron mobility, and to better tolerate boron
deficiency in the soil.
In plants with poor boron mobility, boron deficiency results
in necrosis of meristematic tissues in the growing region, leading
to loss of apical dominance and the development of a rosette
condition. These deficiency symptoms are similar to those caused
by calcium deficiency. In plants in which boron is readily
transported in the phloem, the deficiency symptoms localize in
the mature tissues, similar to those of nitrogen and potassium.
Both the pith and the epidermis of stems may be affected, often
resulting in hollow or roughened stems along with necrotic spots
on the fruit. The leaf blades develop a pronounced crinkling and
there is a darkening and crackling of the petioles often with
exudation of syrupy material from the leaf blade. The leaves are
unusually brittle and tend to break easily. Also, there is often a
wilting of the younger leaves even under an adequate water supply,
pointing to a disruption of water transport caused by boron deficiency.
 66 Concepts in Plant Physiology

Calcium: These calcium-deficient leaves show necrosis around

the base of the leaves. The very low mobility of calcium is a major
factor determining the expression of calcium deficiency symptoms
in plants. Classic symptoms of calcium deficiency include blossom-
end rot of tomato (burning of the end part of tomato fruits), tip
burn of lettuce, blackheart of celery and death of the growing
regions in many plants. All these symptoms show soft dead necrotic
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tissue at rapidly growing areas, which is generally related to poor

translocation of calcium to the tissue rather than a low external
supply of calcium. Very slow growing plants with a deficient
supply of calcium may re-translocate sufficient calcium from older
leaves to maintain growth with only a marginal chlorosis of the
leaves. This ultimately results in the margins of the leaves growing
more slowly than the rest of the leaf, causing the leaf to cup
downward. This symptom often progresses to the point where the
petioles develop but the leaves do not, leaving only a dark bit of
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necrotic tissue at the top of each petiole. Plants under chronic
calcium deficiency have a much greater tendency to wilt than non-
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stressed plants.
Chloride: These leaves have abnormal shapes, with distinct
interveinal chlorosis. Plants require relatively high chlorine
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concentration in their tissues. Chlorine is very abundant in soils,

and reaches high concentrations in saline areas, but it can be
deficient in highly leached inland areas. The most common
symptoms of chlorine deficiency are chlorosis and wilting of the
young leaves. The chlorosis occurs on smooth flat depressions in
the interveinal area of the leaf blade. In more advanced cases there
often appears a characteristic bronzing on the upper side of the
mature leaves. Plants are generally tolerant of chloride, but some
species such as avocados, stone fruits, and grapevines are sensitive
to chlorine and can show toxicity even at low chloride
concentrations in the soil.
Copper: These copper-deficient leaves are curled, and their
petioles bend downward. Copper deficiency may be expressed as
a light overall chlorosis along with the permanent loss of turgor
in the young leaves. Recently matured leaves show netted, green
veining with areas bleaching to a whitish gray. Some leaves develop
sunken necrotic spots and have a tendency to bend downward.
Trees under chronic copper deficiency develop a rosette form of
growth. Leaves are small and chlorotic with spotty necrosis.
 Plant Cells 67

Iron: These iron-deficient leaves show strong chlorosis at the

base of the leaves with some green netting. The most common
symptom for iron deficiency starts out as an interveinal chlorosis
of the youngest leaves, evolves into an overall chlorosis, and ends
as a totally bleached leaf. The bleached areas often develop necrotic
spots. Up until the time the leaves become almost completely
white they will recover upon application of iron. In the recovery
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phase the veins are the first to recover as indicated by their bright
green colour. This distinct venial re-greening observed during
iron recovery is probably the most recognizable symptom in all
of classical plant nutrition. Because iron has a low mobility, iron
deficiency symptoms appear first on the youngest leaves. Iron
deficiency is strongly associated with calcareous soils and anaerobic
conditions, and it is often induced by an excess of heavy metals.
Potassium: Some of these leaves show marginal necrosis (tip
burn), others at a more advanced deficiency status show necrosis
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in the interveinal spaces between the main veins along with
interveinal chlorosis. This group of symptoms is very characteristic
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of K deficiency symptoms. The onset of potassium deficiency is

generally characterized by a marginal chlorosis progressing into
a dry leathery tan scorch on recently matured leaves. This is
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followed by increasing interveinal scorching and/or necrosis

progressing from the leaf edge to the midrib as the stress increases.
As the deficiency progresses, most of the interveinal area becomes
necrotic, the veins remain green and the leaves tend to curl and
crinkle. In some plant such as legumes and potato, the initial
symptom of deficiency is white speckling or freckling of the leaf
blades. In contrast to nitrogen deficiency, chlorosis is irreversible
in potassium deficiency, even if potassium is given to the plants.
Because potassium is very mobile within the plant, symptoms
only develop on young leaves in the case of extreme deficiency.
Potassium deficiency can be greatly alleviated in the presence of
sodium but the resulting sodium-rich plants are much more
succulent than a high potassium plant. In some plants over 90%
of the required potassium can be replaced with sodium without
any reduction in growth.