Seed dormancy is a state where seeds are prevented from germinating under normally favorable conditions. There are two main categories of dormancy - exogenous caused by external factors like a hard seed coat, and endogenous caused by internal factors in the embryo. Dormancy allows seeds to delay germination until conditions are suitable to increase the likelihood of seedling survival. Various natural and artificial methods can be used to break dormancy. Dormancy provides biological benefits like allowing seeds to survive unfavorable periods and disperse to new areas.

1. SEED DORMANCY

2. DEFINATIONS
• Seed dormancy is defined as a state in which seeds are prevented
from germinating even under environmental conditions normally
favorable for germination.
• OR
• Seed dormancy is the internal or innate inhibition of germination of
otherwise normal or viable seed even when present under most
favorable conditions required for its germination.

3. OVERVIEW
• Dormancy is a mechanism to prevent germination during unsuitable
ecological conditions, when the probability of seedling survival is low.
• True dormancy or innate dormancy is caused by conditions within the
seed that prevent germination under normally ideal conditions. Often
seed dormancy is divided into two major categories based on what
part of the seed produces dormancy: exogenous and endogenous.
• There are three types of dormancy based on their mode of action:
physical, physiological and morphological.
• One important function of most seeds is delayed germination, which
allows time for dispersal and prevents germination of all the seeds at
the same time.

4. • Factors that cause seed dormancy includes genetical factor / cause(
physiological, embryo dormancy, undeveloped cotyledon and
Immature embryo); coat imposed dormancy; embryo factor and
inhibitor factor.
• Dormancy is a mechanism to prevent germination during unsuitable
ecological conditions, when the probability of seedling survival is low.

5. CATEGORIES OF SEED DORMANCY
1) Exogenous dormancy
• Exogenous dormancy is caused by conditions outside the embryo and
is often broken down into three subgroups:
a) Physical dormancy- Dormancy that is caused by an impermeable
seed coat is known as physical dormancy. Physical dormancy is the
result of impermeable layer(s) that develops during maturation and
drying of the seed or fruit. This impermeable layer prevents the seed
from taking up water or gases. As a result, the seed is prevented from
germinating until dormancy is broken. In natural systems, physical
dormancy is broken by several factors including high temperatures,
fluctuating temperatures, fire, freezing/thawing, drying or passage
through the digestive tracts of animals. Generally, physical dormancy is
the result of one or more palisade layers in the fruit or seed coat. These
layers are lignified with malphigian cells tightly packed together and
impregnated with water-repellent

6. b) Mechanical dormancy- Mechanical dormancy occurs when seed
coats or other coverings are too hard to allow the embryo to expand
during germination. In the past this mechanism of dormancy was
ascribed to a number of species that have been found to have
endogenous factors for their dormancy instead. These endogenous
factors include low embryo growth potential.
c) Chemical dormancy- Includes growth regulators etc., that are
present in the coverings around the embryo. They may be leached out
of the tissues by washing or soaking the seed, or deactivated by other
means. Other chemicals that prevent germination are washed out of
the seeds by rainwater or snow melt.

7. 2)Endogenous dormancy
• Endogenous dormancy is caused by conditions within the embryo
itself, and it is also often broken down into three subgroups:
physiological dormancy, morphological dormancy and combined
dormancy, each of these groups may also have subgroups.
a) Physiological dormancy- Physiological dormancy prevents embryo
growth and seed germination until chemical changes occur. These
chemicals include inhibitors that often retard embryo growth to the
point where it is not strong enough to break through the seed coat or
other tissues. Physiological dormancy is indicated when an increase in
germination rate occurs after an application of gibberellic acid (GA3) or
after Dry after-ripening or dry storage. Physiological dormancy is
broken when inhibiting chemicals are broken down or are no longer
produced by the seed; often by a period of cool moist conditions,

8. • Conditions that affect physiological dormancy of seeds include:
• Drying-some plants including a number of grasses and those from
seasonally arid regions need a period of drying before they will
germinate, the seeds are released but need to have a lower moisture
content before germination can begin. If the seeds remain moist after
dispersal, germination can be delayed for many months or even years.
• Photodormancy or light sensitivity affects germination of some seeds.
These photoblastic seeds need a period of darkness or light to
germinate. In species with thin seed coats, light may be able to
penetrate into the dormant embryo. The presence of light or the
absence of light may trigger the germination process, inhibiting
germination in some seeds buried too deeply or in others not buried
in the soil.
• Thermodormancy is seed sensitivity to heat or cold. Some seeds
including cocklebur and amaranth germinate only at high
temperatures . Many plants that have seeds that germinate in early to
mid summer have thermodormancy and germinate only when the soil
temperature is warm.

9. b) Morphological dormancy- In morphological dormancy, the embryo
is underdeveloped or undifferentiated. Some seeds have fully
differentiated embryos that need to grow more before seed
germination, or the embryos are not differentiated into different
tissues at the time of fruit ripening.
Immature embryos – some plants release their seeds before the tissues
of the embryos have fully differentiated, and the seeds ripen after they
take in water while on the ground, germination can be delayed from a
few weeks to a few months.
c) Combined dormancy- Seeds have both morphological and
physiological dormancy. Morpho-physiological or morphophysiological
dormancy occurs when seeds with underdeveloped embryos, also have
physiological components to dormancy. These seeds therefore require
dormancy-breaking treatments as well as a period of time to develop
fully grown embryos.

10. CAUSES OF SEED DORMANCY
1) Hard Seed Coat
Seeds of many species possess hard seed coat. Most of these seeds
belong to the family. Leguminosae and Malvaceae. Such seeds remain
dormant. Hard seed coat prevents germination due to following
reasons:
(a) Hard seed coat prevents the entry of water into the seed.
(b) Hard coat obstruct exchange of gases, especially oxygen. Oxygen is
necessary for respiration.
(c) Hard seed coat causes mechanical resistance. Thus radicle does not
come out

11. 2) Immature Embryo
The embryo is fully developer inside the seed in most of the plants
before shedding from the parent plant. But development of the
embryo is incomplete at dispersal in few cases like Fraxinus and
Anemone.
Such seeds remain dormant and fail to germinate. Such seeds imbibe
water and the development of the embryo is completed in a few
weeks. Then the embryo is fully developed Now the seed can
germinate under favourable conditions.

12. • 3. Light Sensitive Seeds
• There two type of seeds for light sensitivity:
• (i) Positive photoblastic seeds: Seeds of most of the species
germinate equally well both in the dark and in light. Therefore light is
not necessary for germination. However, seeds of some species like
lettuce remain dormant in the dark. –They do not germinate till these
are exposed to light. These are called positive photoblastic seeds.
Therefore dark inhibits germination in these seeds.
• (ii) Negative photoblastic seeds: Seeds of some species like phlox.
Nemophila and Silene remain dormant when exposed to light. They
germinate only in the dark. These are called negative photoblastic
seeds.

13. • 4. Chilling Requirement
• Seeds of many temperate trees remain dormant and do not
germinate after harvest. Its examples are apple. walnut and pinus.
Such seeds need chilling temperature (1 -5 degrees) for a few weeks.
• This requirement is met in nature. These seeds lie buried in the soil
during winter.
• Therefore they get chilling temperature.
• But some seed become dormant in nature. They need artificial
chilling.

14. • 5. Dormancy due to growth inhibitory chemicals
• There are several naturally occurring compounds which inhibit
germination of seeds.
• Seeds of many species contain several phenolic compounds. These
phenolic compounds are ferulic acid, parasorbic acid and coumarin.
• These compounds are present in the embryo or in the seed coats.
They inhibit the germination.
• Sometimes, an inhibitory hormone abscisie acid prevents
germination.
6. Excessive Salts
• In Atriplex the seeds contain a high concentration of solutes which do
not allow the embryo to resume its growth.

15. Natural Overcoming of Seed Dormancy
• 1. Weakening of tough and impermeable seed coats by microbial
action.
• 2. Rupturing or weakening of seed coats by mechanical abrasions.
• 3. Action of digestive enzymes present in alimentary canals of birds
and other animals which happen to feed on their fruits.
• 4. Leaching of inhibitors present in the seed coat.
• 5. Inactivation or oxidation of inhibitors by heat, cold and light.
• 6. Production of growth hormones which can counteract the effect of
inhibitors.
• 7. Completion of over-ripening period.
• 8. Attainment of maturity of embryo in case the dormancy is due to
incomplete development of embryo.
• 9. Leaching of solutes in Atriplex where dormancy is caused- by high
osmotic concentration inside the seeds.

16. Artificial Overcoming of Seed Dormancy
• 1. Rupturing of seed coats or scarification by abrasion through machine
threshing, filing, chipping, vigorous shaking, etc.
• 2. Hydraulic pressure of up to 2000 kg for 5-20 minutes for weakening the tough
seed coats.
• 3. Treatment with hot water or fat solvents for dissolution of surface inhibitors,
waxes, etc.
• 4. Treatment with concentrated sulphuric acid for a short period followed by
thorough washing to remove all traces of the mineral acid.
• 5. Stratification or subjecting the moist seeds in the presence of oxygen to
periods of low or high temperature.

17. Biological Importance of Seed Dormancy
• 1. Dormancy allows the seeds to remain in suspended animation without
any harm during drought, cold or high summer temperature.
• 2. The dormant seeds can remain alive in the soil for several years. They
provide a continuous source of new plants even when all the mature plants
of the area have died down due to landslides, earth quake, floods,
epidemics or continued drought.
• 3. It helps the seed to get dispersed over long distances through
unfavourable environment or inhospitable area.
• 4. The small seeds with impermeable seed coat belonging to edible fruits
come out of the alimentary canals of birds and other animals uninjured
e.g., Guava.
• 5. Dormancy induced by the inhibitors present in the seed coats is highly
useful to desert plants. The seeds germinate only after a good rainfall
which dissolves away the inhibitors. The rainfall ensures the seed a proper
supply of water during its germination.
• 6. It follows storage of seeds for later use by animals and man.

18. By innocent manga
midlands state university
innocentmanga0@gmail.co