It includes about the seed dormancy, its types, mechanisms and overcoming measures.

1. SEED DORMANCY; TYPES,
OVERCOMING MEASURES AND
MECHANISM
Presenter
Ashmita Bajagain
MSc.Ag (HRT)
Department of Horticulture
Agriculture and Forestry University
Rampur, Chitwan

2. Outline of Presentation
• Seed Dormancy
• Biological importance of seed dormancy
• Mechanisms of seed dormancy
• Classification of seed dormancy
• Overcoming measures
• Bud dormancy
• Tuber dormancy
• Interactions of hormone and dormancy
• Conclusion

3. Seed Dormancy
• Seed is the dispersal unit of the plant, which is
able to survive the period between seed
maturation and the establishment of the next
generation as a seedling after it has germinated
• To optimize germination over time, the seed
enters a dormant state
• May occur in seeds, bulbs, tubers, buds, and
whole plants
• Seed dormancy can be defined as the state or a
condition in which seeds are prevented from
germinating even under the favorable
environmental conditions for germination

4. • Dormancy provides a strategy for seeds to spread germination
in time in order to reduce the risk of premature death in an
unfavorable environment
• When seeds are shed from the plant, they may be quiescent or
dormant
Quiescent seed are the
dry seeds that have the
ability to germinate, but
limited by their
environment (i.e.
temperature and water)
Dormant seed are the
seeds that will not
germinate even when
the environment is
suitable for
germination

5. Biological Importance of Seed Dormancy
• Seed dormancy is an evolutionary adaptation to delay germination
after the seed has been shed from the plant
• Delayed germination, which allows dispersal and prevents
simultaneous germination of all seeds
• Permitting germination only when environmental conditions favor
seedling survival
• Creation of a “seed bank.” In nature, a seed bank ensures that not
all seeds of a species germinate in a single year. Some seeds remain
dormant in a seed bank for decades
• Dormancy induced by inhibitors present in the seed coats is highly
useful to desert plants. The seeds germinate only after a good
rainfall which dissolves away inhibitors and the rainfall ensures the
seed a proper supply of water during its germination

6. MECHANISMS OF SEED DORMANCY
Mechanism
Blocks to germination within the
embryo
Blocks to germination by the covering
layers
•Undifferentiated embryo
•Immature embryo
•Chemical inhibitors
•Physiological constraints
•Interference with Water Uptake
•Interference with Gas Exchange
•Prevention of Exit of Inhibitors from
the Embryo
•Mechanical Restraint

7. Blocks to germination within the embryo
Undifferentiated embryos:
• Embryo will have to complete their developmental phase before
they can start the germination program
• Orchidaceae and Orobanchaceae falls under this group
• Orchid seeds have no endosperm and the small embryo is
enclosed only by a seed coat that is composed of a thin layer of
cell walls. These embryos will have to complete their
developmental phase before they can start the germination
program
• Seeds with undifferentiated embryos are not dormant in the strict
sense and are therefore not commonly included in current
dormancy classification

8. Chemical inhibitors:
Embryos are held in a state
of dormancy by ABA,
either generated by the
cotyledons, or synthesized
within the axis, or possibly
both.
Apple cotyledons contain
bound forms of ABA from
which free ABA is
gradually released and
transported to the radicle,
whose extension is thus
inhibited.
Physiological constraints:
ABA plays an important role in this
suppression, whereas GA may
counteract its effect. A GA–ABA
balance appears to be decisive for
the loss or maintenance of dormancy
Immature embryos:
Embryos have to grow inside the
dispersed seed prior to germination.
Radicle protrusion does not occur until
the embryo has attained a pre-defined
length and the micropylar endosperm
has been sufficiently degraded.

9. Blocks to germination by covering layers
Interference with water uptake:
• Many species have seeds with extremely hard coats which, by
preventing the entry of water, may delay germination for many
years
• Seeds of the Leguminosae, Cannaceae, Convolvulaceae,
Chenopodiaceae, and Malvaceae show such dormancy
• Lignin and water repellent components including cutin,
quinones, suberin, waxes, callose, phenolics, and hydrophobic
(lipid-like) substances play role of waterproofing in seed coat.

10. Interference with gas exchange:
• Tissues surrounding the embryo might limit the capacity for
gaseous exchange by the embryo in two ways: entry of oxygen
may be impeded or escape of carbon dioxide may be hindered
• The possibility that the seed coat imposes dormancy by affecting
gaseous exchange was suggested from the fact that in many cases
the inhibitory action of the tissues surrounding the fully imbibed
embryo is much reduced simply by scratching or puncturing them
• Stronger evidence that germination of the intact dispersal unit is
prevented by insufficient oxygen is that dormancy is frequently
overcome by oxygen-enriched atmospheres

11. Contd…
Prevention of exit of inhibitors
from the embryo:
•A range of chemical compounds
derived from seeds or dispersal
units, including phenolic acids,
tannins, and coumarins, may
inhibit germination
•Germination may be accelerated
by extensive rinsing of seeds
containing inhibitory chemicals
with water
Mechanical Restraint:
•Tissues surrounding the embryo will
almost always impose a certain
mechanical restraint to expansion of
the embryo.
• Demonstrated in the species
tomato, Datura ferox, tobacco, and
coffee.
•Removal or partial removal of these
tissues will often lead to normal
embryo growth, indicating that the
block to completion of germination
is entirely located in the embryo
coverings

12. CLASSIFICATION OF DORMANCY
Based on nature of origination
• Innate dormancy: It is the condition of seeds which is incapable of
germination even if conditions suitable for seedling growth are
supplied. This inability to germinate may be due in certain species to
the embryo being immature at the time of dispersal.
• Enforced dormancy: It is the condition of seeds which is incapable of
germination due to an environmental restraint which include, an
adequate amount of moisture, oxygen, light and a suitable
temperature
• Induced dormancy: This type of seed dormancy occurs when the seed
has imbibed water but has been placed under extremely unfavorable
conditions for germination. Finally, seed fails to germinate even
under more favorable conditions.

13. Classification based on time of
origin
Primary Dormancy Secondary Dormancy
•Dormancy condition at the end of
seed development
•Exogenous Dormancy
•Endogenous Dormancy
•Combinational Dormancy
• If for some reason seeds fail to
germinate after primary dormancy is
broken, seeds of many species can re-
enter dormancy
•Thermo-dormancy
•Conditional Dormancy

14. Primary Exogenous Dormancy
• This is imposed upon the seed from factors outside the embryo, including
the seed coat and/or fruit parts
• Physical Dormancy: Seeds with physical dormancy fail to germinate
because seeds are impermeable to water, gas exchange as the outer
integument layer of the seed coat hardens
• Chemical Dormancy: Chemicals like various phenols, coumarin, and
abscissic acid accumulate in fruit and seed-covering tissues during
development and remain with the seed after harvest may act as germination
inhibitors
• Representative genera: Baptisia, Convolvulus, Beta, Iris, Lupinus,
Gleditsia

15. Primary Endogenous Dormancy
• Fails to germinate primarily because of factors within the embryo
which factors can be either physiological or morphological
• Physiological Dormancy: the embryo lacks the growth potential
to allow the radicle to escape the restraint of the seed coverings
and include non-deep, intermediate, and deep physiological
dormancy
Non-deep: species respond to short periods of chilling stratification, that
require light or darkness to germinate (photo-dormancy), and species that
can undergo an after-ripening period for dormancy release.
Short period of dry storage can break such dormancy

16. • Seeds are considered to have morphological dormancy if they
require more than 30 days to germinate, have an embryo that fills
less than ½ size of the mature seed
• Seeds with morphophysiological dormancy have underdeveloped
embryo and also displays physiological dormancy
Intermediate: Embryo germinates if
separated from the seed coat. Often
responds to gibberellic acid.
Moderate period (upto 8 weeks)of
cold stratification is required to break
dormancy
Deep: Embryo does not
germinate when removed from
seed coat or will form a
physiological dwarf.
Long periods (>8 weeks) of
cold stratification is required to
break dormancy

17. Combinational Dormancy
• Refers to seeds that have both physical and physiological
dormancy
• One type requires an initial period of warm temperature to relieve
non-deep physiological dormancy prior to alleviation of physical
dormancy and imbibition
• Second type requires loss of physical dormancy to allow
imbibition, followed by a cold stratification period to relieve
physiological dormancy
• Sequential combinations of dormancy-releasing treatments
Example: scarification followed by cold stratification

18. Thermo-dormancy
• After primary dormancy is relieved, high temperature induces
dormancy
• Seeds experiencing thermo-dormancy will not germinate when
the temperature returns to near optimum temperatures, while
thermal-inhibited seeds will germinate when temperatures are
lowered
• Growth regulators or cold stratification is the condition
required
• Representative genera: Apium, Lactuca, Viola

19. Conditional Dormancy
• As seeds come out of dormancy, or begin to enter secondary
dormancy, they go through a transition stage where they will
germinate, but only over a narrow range of temperatures. This
transition stage is termed conditional dormancy.
• Seeds of many species cycle through years of dormancy and
non-dormancy based on germination temperature

20. OVERCOMING MEASURES
• Low temperature treatment (Chilling)
• Scarification
• Light
• After-ripening
• Fluctuating temperature
• Treatment with chemicals
• Water

21. Low temperature treatment (Chilling)
• Stratification is a cold, moist period that breaks seed dormancy
Seed moisture: Dehydration stops the stratification process, and
seeds may revert to secondary dormancy
At chilling temperatures, however, the embryo’s oxygen
requirement is low and oxygen is generally adequate
Temperature is the single most important factor controlling
stratification
• Mechanism for Action Stratification The embryo can be shown to
increase in growth potential while seed coverings become weaker
• Active changes occur through gene activation and increased
enzyme activity, and the result is an embryo that can produce more
radicle force to escape the seed coverings

22. Hormone changes during stratification
• A triphasic change in endogenous
hormone is typical for many seeds
during treatment to overcome
dormancy
• Changes in hormone level during
chilling stratification have been
extensively studied in peach and
apple
• During stratification, gibberellins
are either synthesized at the
chilling temperatures or are
converted to an available (or
unbound) form Fig. Triphasic changes in hormone production
during chilling stratification

23. Scarification
• Any process of penetrating the protective seed coat of dormant
seeds
• Mechanical Scarification: Involves breaking or weakening the
seed coat and can be carried out using sand-paper or a file to
abrade the seed coat, or using a knife to nick the coat, or using
a hammer to crack the coat for allowing water to enter
• Hot water scarification: Some seed coat can be softened in
warm or hot water (75-95°C) the seeds are left to soak in the
water as it cools for 12 to 24 hours before planting
• Acid scarification: The seeds are put in a glass container and
covered with concentrated sulphuric acid at about twice the
volume of seed

24. Light
• Important factor for releasing seeds from dormancy
• Almost all light-requiring seeds have coat-imposed dormancy
• Seeds of many species are affected by exposure to white light
for just a few minutes or seconds (e.g., lettuce) whereas others
require intermittent illumination for sometimes prolonged
periods of time (e.g., Kalanchoe blossfeldiana)

25. Fig. The last quality of light the lettuce seeds are exposed to determines the
dormancy state (Source: Hartmann et al., (2014))

26. After-ripening
• After-ripening is the time required for seeds in dry storage to lose
dormancy
• Technique used historically to indicate any change that occurs in seeds
leading to release from endogenous physiological dormancy
• It is more appropriately used to describe changes that occur in seeds
during dry storage that lead to dormancy release
• In nature, temperature and seed moisture content are changing on a
continual basis, but the relationship among after-ripening time,
temperature, and seed moisture remains consistent for a particular plant
type, and time to dormancy release can be predicted using a
hydrothermal time model

27. Fluctuating temperature Treatment with chemicals
• Several characteristics of
diurnal temperature cycles
could be responsible for
stimulating germination
• Chenopodium album, where
dormancy-releasing
effectiveness improved as
the amplitude of
temperature fluctuation
increased from 2.4 to about
15℃
• Soaking in potassium nitrate
(0.2%), gibberellic acid
(200 to 500 ppm), or
thiourea (0.2%) solution
prior to sowing has been
found to stimulate
germination of different
kinds of seeds

28. Water
• Soaking seeds in water overnight softens a hard seed coat
enough to allow moisture inside so that the seed can germinate
• For quicker results, pour boiling water over the seeds and let
them soak until the water cools
• Many types of legumes benefit from this treatment, including
the garden pea (Pisum sativum)

29. Bud Dormancy
• Bud dormancy in woody perennials is a physiological stage that
enables plants to survive long periods of adverse conditions and is
characterized by growth cessation, arrest of cell division, and
reduced metabolic and respiratory activity
• Lang (1987) identified the dormancy states as paradormancy(A),
endodormancy (B) and (C) ecodormancy
Bud
growth
A. signal produced in other
part inhibit
B. donot grow bud even if
environment is conducive to growth
C. Environmental
conditions inhibits

30. Tuber Dormancy
• In potatoes, dormancy is the physiological state of the tuber in
which autonomous sprout growth will not occur within two weeks,
even when the tuber is kept in conditions ideal for sprout growth
• The intensity of dormancy in stored tubers declines with time and
eventually they develop the capacity to sprout
• During the development of the tubers on the plant, the buds in the
eyes of the tuber successively become dormant, starting at the stolon
end whereas the apical eye is the last to become dormant

31. Contd:
• Dormancy period also depends on soil and weather conditions
during growth, tuber maturity at harvest, storage conditions, and
whether the tuber is injured or not
• During storage, fluctuating storage temperatures shorten
dormancy more than constantly high temperatures
• Although cool temperatures during storage can prolong the
dormancy period, they generally result in an increase in reducing
sugar content, primarily glucose, which is undesirable in the
processing industry due to darkening of fried products

32. Interactions of hormone and dormancy
GA: - stimulate germination by inducing enzymes that weaken the
seed coverings (endosperm or seed coat) surrounding the radicle,
inducing mobilization of seed storage reserves, and stimulating cell
expansion in the embryo
- mode-of-action for gibberellin is the deactivation of gene-
expression repressors called DELLA proteins
ABA: - increase with maturation of the fruit and may prevent vivipary
and induce primary dormancy
- inhibits gibberellin-biosynthesis enzymes and promotes gibberellin-
degradation enzymes that impact endogenous gibberellin
accumulation

33. Contd..
Ethylene: - evident that ethylene production may be linked with aspects
of germination rate and seed vigor
- involved with dormancy release in some seeds
Cytokinin: - does not appear essential for germination
- exogenous application of cytokinin can offset ABA effects and rescue
seeds from thermodormancy
- antagonistic interaction between cytokinin and ABA may involve
cytokinin enhanced ethylene production, which, in turn, reduces the
seed’s sensitivity to ABA
Auxin: - does not appear to play a major role in seed dormancy
- important for post-germinative growth rather than in initial
germination or dormancy release

34. Conclusion
• Failure of fully developed and mature viable seed to germinate
under favorable conditions of moisture and temperature is known to
be dormancy
• Dormancy has evolved as a strategy to avoid germination under
condition where seedling survival is likely to be low
• Dormancy can be found among most forms of plant life and may
occur in seeds, bulbs, tubers, buds, and whole plants
• Embryo growth potential must increase to allow radicle extension
growth and protrusion through the covering layers
• In both embryo and coat-imposed dormancies, the embryo is unable
to overcome the constraints imposed on it

35. Bibliography
• Bewley, J.D., Bradford, K.J., Hilhorst, H.W.M., & Nonogaki, H. (2013). Seeds;
Physiology of Development, Germination and Dormancy. (3rd ed.). Springer
• Bradford, K.J. & Nonogaki, H. (2007). Seed Development, Dormancy and
Germination. UK: Blackwell Publishing Limited.
• Hartmann, H.T., Kester, D.E., & Davies, F.T. (2014). Plant Propagation; Principles
and Practices. (8th ed.). Harlow; England: Pearson education Limited.
• Horvath, D.P. (2010), Bud Dormancy and Growth. Doi: 10.1007/978-3-642-02301-
9_4. Retrived from:
https://www.researchgate.net/publication/226224957_Bud_Dormancy_and_Growth
• Lang, G.A., Early, J.D., Martin, G.C., & Darnell R.L. (1987), Endo-, para-, and
ecodormancy: physiological terminology and classification for dormancy research.
HortScience 22:371–377.
• Jane, M., Jackson, K., Hussein, S., & Rob, M. (2014), Regulation of potato tuber
dormancy: A review; Australian Journal of Crop Science; 8, 5: 754-759 ISSN:
1835-2707.
• https://homeguides.sfgate.com/methods-break-seed-dormancy-73304.html
• https://byjus.com/biology/seed-dormancy/

36. THANK YOU