SEED VIABILITY AND SEED
DORMANCY
Department of Plant Physiology & Crop Production
Dr. Lawal Omoniyi Isiaq
INTRODUCTION
• What is a seed? • What is Seed Viability?
• a small embryonic plant • Viability of a seed refers to the
enclosed in a covering called ability of a seed to germinate
the seed coat, usually with and produce a “normal”
some stored food. seedling.
• It is the product of the • Seed viability is probably
ripened ovule of highest at the time of
gymnosperm and physiological maturity, though
angiosperm plants which environmental conditions on
occurs after fertilization and the parent plant may not
some growth within the permit germination.
mother plant.
Seed Viability Testing
• GERMINATION TEST: • Seed germination stages
Germination test is most • 1.Germination: This include water
commonly used method to imbibition and all the biochemical
determine seed viability. and physiological processes that
• Seed germination is the culminate in the emergence of the
consecutive number of steps radicle and the plumule.
that will cause a non-dormant • 2. Underground elongation
or active seed to produce a
normal plant. • 3. Emergence
• 4. Independent gowth
• Underground elongation
• At this stage of germination,
elongation of both the radicle and
the plumule takes place at the
expense of food reserve in the
endosperm.
• The soil depth through which the
plumule can emerge depends on the
amount of food reserve in the
endosperm.
• Emergence
• It is at this stage that the aerial parts of
the seedling emerge above the ground.
• Seedling emergence may be hypogeal
if cotyledons remain below the soil
surface e.g maize, rice, wheat, or
epigeal if the cotyledons are forced
above ground by elongation of the
epicotyls e.g. cowpea, pigeon pea,
soybean and most seeds of
leguminous species.
• Independent growth
• The period starts with the onset of
photosynthetic activity by the seedling
plant.
• Normal functioning of the seedling is
established at this stage.
• With large-seeded species and sprouts
from vegetative propagules, there is
considerable overlap in time between
parental independence and
development of photosynthetic food
source.
Seed Dormancy
• A dormant but viable seed is one,
which fails to germinate at a
favourable temperature when
supplied with water and air.
• Dormant but viable seeds eventually
germinate but only when some
special condition has been satisfied,
such as a particular treatment with
light, low temperature, hormone etc.
• Dormancy is therefore biologically
important, providing a mechanism for
dispersal of the plant.
• Dormancy and the breaking of
dormancy ensure that the seed
germinates only at a certain time of the
year.
• Dormancy therefore preserve the
seed against temporarily unsuitable
conditions such as may occur
during periods between seed
collection and storage.
• A physiologically sound seed may
remain quiescent and therefore
may germinate due to a lot of
prevailing factor.
CAUSES OF DORMANCY
• Dormancy due to seed coat
• Due to impermeability of seed coat to
water and oxygen some seeds having hard
and tough seed coat fail to absorb them
and ultimately cannot germinate.
• Sometimes the hardness of the seed coat
is the factor as found in seeds of clover
and sweet pea and some other tree and
non-tree legumes.
• In such cases germination is
delayed till the seed coat is
decayed in the soil as a result of
bacterial action.
• The same is true with the seeds
of some plants belonging to
families Leguminosae and
Malvaceae, where there is also the
mechanical barrier afforded by
the seed covering to embryo
emergence.
• Dormancy due to immature
embryos:
• Some seeds are shed before the
embryo is mature and thus are
not fit to grow.
• Such seeds require an ‘after
ripening period’ during which
certain changes occur within the
seed.
• It is believed that there is some change
in acidity, enzyme activity and
respiratory rate or
• production of some growth promoting
hormones or may be inactivation of
some germination-inhibitory
substances within seeds.
• Once embryo development is complete
the seed then germinate without any
special treatment.
• 4. Dormancy due to chemical
inhibitors:
• The seeds present inside the juicy fruits
such as Azadiracta indica, Mileci exelca,
Gmelina arborea, oranges and tomatoes do
not germinate while in the fruit where
plenty of liquid is present.
• It is due to the presence of germination-
inhibitory substances. Coumaruins and
parascorbic acids are known to cause
such inhibitions.
• 5. Dormancy requiring for after-
ripening in dry storages:
• At the time of harvesting, seeds of
many plants are dormant but they do
not require any special treatment to
overcome dormancy.
• Simply keeping them under dry
storage conditions at normal
temperature over a period ranging
from a few weeks to several months,
overcome dormancy.
• Many of common cereals such as
barley, rice, oats, wheat etc show this
type of dormancy.
• 6. Dormancy requiring chilling
treatment
• Many seeds of temperate species show
dormancy, which is overcome by
chilling. Simple examples of this
category are the freshly harvested seeds
of apple, rose and peach which will not
germinate if planted under moist
conditions at 200C
• but will germinate, if planted under moist
conditions at 0-50C for several weeks and
then transferred to warmer conditions.
• Dormancy due to light sensitivity:
• The germination of many seed is affected
by light. Such seeds are said to be
photoblastic.
• The seeds in which germination is
stimulated by light are called positively
photoblastic seeds
• e. g Alilicia excels, Amarathus reflexus,Digitalis
purpurea, lycopersicum esculentum, Nicotiana
tabacum etc.
• whereas those in which germination is
inhibited by light are negatively photoblastic
e. g Silene armeria, Nemophala insignis etc.
Breaking of seed dormancy
• Seed dormancy can be broken by
SCARIFICATION before planting to permit better
water uptake, germination and stand establishment.
• Mechanical scarification involves rubbing the seeds
against an abrasive surface. Excessive or careless
scarification may damage the seed.
Methods of scarification
• Shaking with sand;
• Cutting with knife;
• Rasping with a file;
• Rubbing with emery cloth or sand
paper;
• Soaking in concentrated sulphric acid
or alcohol.
• ACID TREATMENT
• i) Place the concentrated sulphuric acid
(95%) in a beaker.
• ii) Place 10 seeds of Leucaena leucophala
• iii) Immerse the seeds until covered.
• iv) Allow the seeds to soak for 5 minutes.
• V) Remove the seeds from the acid and wash
in cool running water for 5 to 10 minutes to
remove all traces of acid.
vii) Stir the seed carefully during rinsing.
Vii) Dry seeds
Viii) Plant in germinating bags provided
(other seeds are Centrosema spp, Stylosanthes
gracilis and cotton- Gossypium spp)
APPRECIATION
THANK YOU
