seed dormancy

Presented by…
Maitri. M. Thakor
☻ M.Sc. (Botany)
Department of Life Sciences,
H.N.G.U., Patan.
1

CONTENTS
 Introduction
 Types of Dormancy
 Types of Seed Dormancy
 Causes of Seed Dormancy
 Overcoming of Seed Dormancy
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d_/4/4/csm_hrach_62f4e13155.jpg)

INTRODUCTION
 Dormancy :
 During the developmental cycle of the plant, at
some phase certain structures like buds, tubers,
seeds, go though a period of temporary
suspension of growth activity or slow down for a
period of time or deep sleep, such a state is
called Dormancy.
 This minimizes metabolic activity and therefore
helps an organism to conserve to energy.
 Dormant = not active or growing but able to
become active later.
3

 Wareing (1969) defined dormancy as any phase
in the life cycle of a plant in which active growth
is temporarily suspended.
 Seed Dormancy :
 It is the incapacity of fully developed, mature,
viable seed to germinate even under favourable
conditions.
 In such cases, the completely dry ripe seed is
physiologically inactive and is said to be in a
resting stage.
 The seed is called dormant and the phenomenon
is termed dormancy. 4

 The main reason behind this condition is that
they require a period of rest before being
capable of germination.
 This Conditions may vary from days to months
and even years.
5
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TYPES OF DORMANCY
 According to wareing (1969) the dormancy
may be two types :
1) Imposed dormancy or quiescence:
 The dormancy due to unfavourable
environmental conditions is called imposed
dormancy or quiescence.
2) Innate dormancy or rest :
 The dormancy due to conditions within the
dormant plant or organ is called innate
dormancy or rest.
 It is a condition in which germination or
growth fails to occur even though the external
environmental conditions are favourable.
6

 During the entire process, there may be
following three phases of dormancy :
1) Pre-dormancy or early rest :
 During these phase, the dormant organ has
capacity to resume growth by various
treatments i.e., capacity of germination or
growth is not completely lost. It is called
predormancy.
2) Full dormancy or mid rest :
 When a seed or organ becomes completely
dormant and germination or growth cannot be
induced by changes environmental
conditions, it is called full dormancy or mid
rest.
7

3) Post dormancy or after rest :
 When a dormant seed or organ gradually
emerges from full dormancy and in it the
germination or growth can be induced by
changing enviornmental conditions, it is called
post-dormancy or after rest.
 The dormancy may be true, relative or
secondary :
1)True dormancy :
 When in a seed or organ, the germination or
growth cannot be induced under any set of
environmental conditions, it is called true
dormancy.
8

2) Relative dormancy :
 When in a seed, the germination can be
induced under specific conditions even at the
time of its deepest dormancy, it is called
relative dormancy.
3) Secondary dormancy :
 When a seed has not fully emerged from
dormancy and is again thrown back into full
dormancy by certain environmental conditions,
e.g., temperature etc., it is called secondary
dormancy. 9

TYPES OF SEED DORMANCY
 Crocker (1916) divided seed dormancy into
two types :
1) Seed coat induced dormancy :
 The dormancy of seeds due to extreme
hardness of seed coat is called seed coat
induced dormancy.
2) Embryo induced dormancy :
 The dormancy of seeds due to rudimentary or
complete dormant embryo is called embryo
induced dormancy.
10

 Other types of dormancy may be –
3) Secondary dormancy :
 When the seed become dormant again after
breaking the dormancy, it is called secondary
dormancy.
 It may be due to combination of different kinds
of dormancy in a single seed, e.g., Xanthium
pennsylvanicum.
4) Special type of dormancy :
 The failure of seedling development is not
always traceable due to dormancy of seed
itself.
11

 In many of the spring wild plants the
germination of seed takes place but the growth
is restricted due to establishment of young
roots.
 Sometimes the system of epicotyl fails to
germinate.
 In some cases, the epicotyl may be pushed
through the seed coat but remains dormant.
 This dormancy is often broken by exposure to
low temperature.
12

 According to C. Baskin and J. Baskin (1998;
2004) have proposed a comprehensive
classification system which include five
classes of seed dormancy –
(1) Physiological dormancy
(2) Morphological dormancy
(3) Morpho-physiological dormancy
(4) Physical dormancy
(5) Combinational dormancy
13

1) Physiological dormancy :
 Physiological dormancy prevents embryo
growth and seed germination until chemical
changes occur.
 It is result of seed requiring some type of
physiological condition to be met in order to
germination.
 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.
14

2) Morphological dormancy :
 This class of seed dormancy refers to seed with
under development and differentiated embryos,
this includes embryos in which the cotyledons
and hypocotyls, radical axis are differentiated,
but small in size.
 These embryo do not have physiological
dormancy and only require additional time to
grow and germinate.
 Commonly, under favourable conditions,
embryos in such seeds begin growth within a
period from a few days to several weeks, and
seeds germinate within 30 days.
15

3) Morpho-physiological dormancy :
 In this class, seeds have embryos that are
underdeveloped (in size), but differentiated (e.g.,
into cotyledons and hypocotyls radical) as well
as a physiological component to their dormancy.
 Thus, to germinate these seeds require time for
embryo growth and a dormancy breaking
treatment.
 Morphological dormancy can be divided into two
types- epicotyls dormancy and double dormancy.
16
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Double dormancy

4) Physical dormancy :
 Seed coats are impermeable in water due to
macrosclereid cells, mucilaginous outer cell
layer or hardened endocarp.
 Depth of the puncture to the seed coat
increased, so did the permeability of seed
coat of water.
 E.g., Olive, Peach, Plum, Cherry etc.
(hardened endocarp of seed). 17

5) Combinational dormancy :
 This class groups seeds with simultaneous
physiological and physical dormancy.
 In this case, physiological dormancy is
generally characterized as non- deep.
 A cold stratification treatment of seed after
scarification to permit imbibitions is a
common dormancy breaking treatment in this
class of seeds. 18

CAUSES OF THE SEED DORMANCY
 Bewlay and Black (1994) have divided seed
dormancy into two categories, seed coat based
and embryo based.
 The dormancy of seeds may be either due to
single or a combination of many different
following factors.
1) Seed Coat induced dormancy / Hard seed
coat :
 The seed coat is mostly formed by the
integumentary layers of ovules.
 The seed coat consists of two layers – the
outer is testa and the inner is tegmen.
19

 The testa particularly is composed of a
complex mixture of polysaccharides,
hemicelluloses, fats, waxes and proteins.
 During the maturity of seed dehydration takes
place in the seed coat and thus it forms a tuff
protective layer.
 Hard seed coat prevents germination due to
following reasons.
(i) Water impermeability :
 Seed coats of many plants belonging to
families Leguminosae, Chenopodiaceae,
Malvaceae, Convolvulaceae, Solanaceae and
Nymphaeaceae etc. have very hard seed coats
so, it cannot permeable to entry of water in
seed.
20

(ii) Gas impermeability :
 The seed coat of certain seeds are
impermiable to gases such as oxygen(o2)
and carbon-di-oxide (co2).
 Since oxygen is required for early
respiratory activity in germinating seeds, the
seeds fail to prolong germination.
 e.g., Xanthium.
iii) Mechanical resistance :
 In certain wild plants the such hard and
tough seed coat physically prevents the
expantion of the embryo. Thus they remain
dormant.
 E.g., Alisma, Amaranthus, Capsella etc.
21

2) Embryo Induced Dormancy :
 Dormancy due to embryo condition may be of
two types-
(i) Rudimentary and poorly developed embryo
(ii) Embryo fully developed but unable to resume
growth.
(i) Rudimentary and poorly developed embryo :
 Instance the embryos are still immature and
rudimentary when the seeds are dispersed.
 This is seen in many species like Anemone
nemorosa, Ginkgo biloba, members of
Orchidaceae, Orobanchaceae etc.
 In such seeds the embryo does not develop
as rapidly as surrounding tissues.
22

(ii) Embryo fully developed but unable to resume
growth :
 In many species, e.g., Seeds of apple, peach,
Iris, cherry, tulip, poplar, pines, peas, etc.,
although the embryos are completely
developed in ripe seed but the seeds fails to
germinate even when the environmental
conditions for germinations are favourable.
 The embryo of such seeds does not germinate
even if the seed coats are removed.
 The germination in such seeds can be induced
if they are stored in moist, well aerated and low
temperature conditions. This process is called
Stratification or after- ripening.
23

3) Dormancy due to Specific Light Requirement :
 The seeds of certain plant species such as
Lactona sativa, Lythrum salicaria, Nicotiana
tabacum etc. have specific light requirement for
germination.
 Light not only qualitatively but also quanti-
tativly.
 The germination of certain seeds requires a
specific photoperiod, e.g., Bignonia requires a
photoperiod of 12 or more hours for seed
germination.
 The light sensitive seeds are called photoblastic.24

4) Dormancy due to Germination Inhibitors :
 Seeds of certain plants contain compounds
which inhibit their germination. Such natural
germination inhibitors have been found in the
pulp of the fruits, seed coat, endosperm and
embryos or structures surrounding them etc.
(e.g., in tomatoes, in glumes of Oats etc.).
 A number of chemical substunces such as
organic acids, phenolics, tannins, alkaloids,
unsaturated lactones, ammonia and cyanide
releasing substunces, indoles and gibberellins
etc. have isolated from seed which are
germination inhibitors. Besides, other inhibitors
are ABA, ammonia, phthalides, coumarin and
parascorbic acid.
25

OVERCOMING OF SEED DORMANCY
 The dormancy of seeds, though very useful to
man, is not liked by the farmers who would like
the seeds to germinate soon after they have
been harvested.
 A number of methods are, therefore, employed
for the breaking of dormancy. The methods
employed vary from species to species
depending upon the cause of the dormancy.
 There are main two types of overcoming of seed
dormancy :
1) Natural overcoming of seed dormancy
2) Artificial overcoming of seed dormancy
26

1) Natural overcoming of seed dormancy :
 Nature of dormancy stops when the embryo
gets appropriate environment such as adaptive
moisture and temperature.
 The seed coat that exists in many species
becomes permeable due to the rupturing of
smoothing action of natural agents like
microorganism, temperature and abrasion by
the digestive tracts of birds and animals that
feed on these seed.
 Completion of over-ripening period. 27

 Leaching of inhibitors present in the seed coat.
 Inactivation or oxidation of inhibitors by heat,
cold and light.
 Production of growth hormones which can
counteract the effect of inhibitors.
 Attainment of maturity of embryo in case the
dormancy is due to incomplete development of
embryo.
28

2) Artificial over coming of seed dormancy :
i) Scarification :
 The method is used for breaking dormancy of
seeds caused by hard seed coats which
become impermeable to water and gases etc.
 The method employed in softening or
weakening the seed coat is called
Scarification.
 When mechanical breaking of seed coat is
done at one or more places, it is called
Mechanical Scarification.
 Mechanical Scarification is done by shaking
the seeds with sand or by scratching or
nicking the seed coat with knife.
29

 The treatment of seed coat with strong mineral
acids or other chemicals is called Chemical
Scarification.
 Chemical scarification is usually done by
dipping the seed into strong acids like H2SO4 or
into organic solvents like acetone or alcohol.
 It can also be done by boiling the seeds in water.
30
Sources :(http://www.culture-acre.com/wp-
content/uploads/2017/07/seed-
scarification.jpg)
(Sources : http://www.usa-
gardening.com/seed-
scarification/germinating-seeds/scarification-
techniques.jpg)
(Sources:https://image.slidesharecdn.com/bo
tanyforgardeners-2014-final-140202175042-
phpapp02/95/botany-for-gardeners-2014-70-
638.jpg?cb=1391363643)
Scratching by Knife Shaking by Sandpaper By Chemical

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ii) Stratification:
 This method is used to
break the dormancy of
seeds caused due to
condition of embryo.
 In this process the seeds
are exposed to well
aerated, moist conditions
under low temperature
(0°C to 10°C) for weeks
to months. This treatment
is called Stratification or
after-ripening. 32(Sources:https://image.slidesharecdn.com/methodsfor
breakingseeddormancy-150208101506-conversion-
gate02/95/methods-for-breaking-seed-dormancy-5-
638.jpg?cb=1423390603)

 During this treatment, anatomical and
biochemical changes takes place in the seed.
 Growth promoting hormones increases and
growth retarding hormones decreases.
iii) Alternating temperature :
 In some seeds, the dormancy is broken by the
treatment of an alternating low and high
temperatures.e.g., Poa pratensis
 The difference between the alternating
temperature should not be more than 10°C-
20°C.
 This method is beneficial in those seeds in
which the dormancy is due to immature
embryos.
33

 Alternating temperatures of 15°C and 25°C is
useful in breaking the dormancy of photoblastic
seeds like Rumex crispus.
iv) Impaction :
 In some of the seeds a strophiolar plug blocks
(testa pores) the entry of water and oxygen into
the seed.
 In order to remove the plugs the seeds are
shaken vigoursly and this treatment is known
as Impaction.
 E.g., Crotolaria, Trigonella
seed. 34
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v) Light :
 Dormancy of photoblastic seeds can be over-
come by light exposure.
 In positive photoblastic plants light induces
germination. E.g., Amaranthus, Betula,
Capsella, Epilobium, Lactuca etc.
 The dormancy of positive photoblastic seeds
can be broken by exposing them to red light
(660 nm). E.g., Lactuca sativa.
 In negative photoblastic plants light infact
prevents germination.
 In such cases seeds have to be stored in dark
for some time and allow to germinate in
darkness before they are shifted to light.
35

vi) Pressure :
 Davies (1928) reported the seed germination in
certain plants like sweet clover (Melilotus alba)
and alfalfa (Medicago sativa) can be greatly
improved after being subjected to hydraulic
pressure of about 2000 atm. at 18°C for about
5-20 minutes.
 It is strongly believed that the pressure
increases the permeability of the seed coats to
water.
36

vii) Growth Regulators :
 Kinetins and gibberellins have been used to
induce germination in positively photoblastic
seeds like lettuce and tobacco etc.
 Counteracting the effect of growth inhibitory
by soaking the seeds in KNO3, ethylene
chlorohydrin, thiourea, gibberellins.
37
(Source : https://historicjamestowne.org/wp-content/uploads/tobaccoseed.jpg)
Tobacco seed

1) A textbook of Plant Physiology, Biochemistry
and Biotechnology
Author : S.K. Verma, Mohit Verma
Edition : 2007 (Sixth Edition)
2) Textbook of Plant Physiology
Author : V. Verma
Edition : 2007 (First Edition)
3) www.biologydiscussion.com
4) www.slideshare.net
38
References

seed dormancy

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