Unit 8 Seed Dormancy.pptx

MBMAN (Affiliate to CTEVT)
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Seed Dormancy
(3rd Year)
Govinda Raj Sedai
Madan Bhandari Memorial Academy
Urlabari, Morang

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SEED DORMANCY
Seeds of most agricultural plants usually germinate promptly if given access to
moisture and air, if provided with a suitable range of temperature and in some
instances if exposed to a proper sequence of light and dark. However, in some
plants seeds do not germinate even though they are placed under favorable
condition of temperature, air, moisture and light. Germination may delay for
days, weeks or even month. The seeds of such plants are said to be in a dormant
condition. It is not unique to seeds but is also found in other plant organ such as
buds of woody and herbaceous plants as well as buds of tuber, rhizome & bulbs.
So dormancy is defined as an arrest in the development of rudimentary
embryo, bud or spores on a condition otherwise suitable for growth.
The dormant condition appear to serve a common purpose as
 It enables to endure periods of unfavorable environmental condition thereby
providing the mechanism of survive.
 It protects seeds from sprouting on panicles before harvesting.
 It offers provision for dispersal afforded by various modifications of seed covering.
However it offers frustration to plant breeders who would like to grow plant
germination in quick succession. It also impedes the seed testing work, as the result of
germination can’t be finalized quickly in case of such cultivars.

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Types of dormancy
1.Physiological/Innate dormancy: It may be due to
presence of immature embryo, need for after ripening,
specific light and temperature requirement or the
presence of germination inhibitors.
2.Physical/induced dormancy: It may be due to presence of
impermeable or mechanically resist seed coat. It can also
be classified.
3.Special type of dormancy: Sometimes seeds germinate
but the growth of the sprouts is found to be restricted
because of a very poor development of roots and
coleoptile. This kind of dormancy is known as special type
of dormancy.

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2. Physical dormancy
1. Innate dormancy: seed born dormant
It is present immediately the new embryo ceases to grow when it is
still on parent plant. It prevents the seed from germinating
viviparously and also useful sometimes after the ripe seed is shed or
harvested. It is genetically controlled character and it is a feature of
specific seed species.
2. Induced dormancy: seed achieve dormancy
It may be introduced in the seed after it has lost its innate dormancy. It
results from sudden physiological change in otherwise non-dormant
seeds under the impact of factors. The high temperature and limited O2
supply can induce dormancy.
3. Enforced dormancy: dormancy thrust upon them
It is concerned with the dormancy of seed buried beneath the soil
surface which is removed immediately when the seed are exposed. It is
attributed to high CO2 level, darkness and lack of fluctuating
temperature.

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Factors influencing or inducing dormancy
The dormancy usually develops as a result of the action of two
kinds of processes: one within embryo and one in seed coat and
other tissues external to the embryo.
1. Dormancy due to condition of embryo
A. Immaturity of embryo
B. Need for after ripening in dry storage
2. Dormancy due to seed coat
A. Water impermeability
B. Gas impermeability
C. Mechanical resistance
3. Chemicals in seed
4. Chilling (or low temperature) requirements
5. Light sensitive seeds

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1. Dormancy due to condition of embryo:
A. Immaturity of embryo:
Embryo is rudimentary and poorly developed at seed maturity. Embryo fails to develop fully by
the time seed are shed. It is necessary for such embryo to continue their development during
the dormant period before they can germinate successfully. Examples: certain orchids,
Anemone nemorosa, Ginkgo biloba.
B. Need for after ripening in dry storage: Seed contains fully developed embryos
but unable to resume growth promptly when provided with a suitable environment. These
seeds can induced to germinate if stored moist but well aerated under low temperature
condition, a treatment referred to as stratification and sometime called after ripening. It is
possible to remove and culture embryos from seed that require after ripening. If placed at
room temperature (≥200C), the embryos germinate readily, but seedling development is often
abnormal. If cultured embryo kept at low temperature (2-50C) grow more slowly but
subsequent seedling development is normal. It appears that early germination processes of
these embryos require the formation of some promoting substances or degradation of
inhibitory molecule. In some seeds gibberlins appear to function as germination promoters
while ABA acts as an inhibitor. A proper balance between these two kinds of regulatory
substances achieve during after ripening process. Examples: Barley, Oat, Wheat, Apple, Pear,
Blackbery, etc.

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2. Dormancy due to seed coat
Seed coat is composed of several layers of cells derived from
the integumentary tissue of ovule. Some seeds have
additional coat layers derived from the endosperm or fruit
tissues. From chemical sandpoint, seed coat consists of
complex mixture of polysaccharides, hemicellulose, fats,
waxes and proteins. During seed ripening chemical
components of seed coat become dehydrated and form a
hard, protective layer around the embryo. The seed coat has
a strong influence on the resumption of growth of embryo.
Different kinds of seed coat effect have been noted:

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A. Water impermeability:
Plants belonging to families Leguminosae, Malvaceae,
Chenopodiaceae, Solanaceae, have very hard seed coat
which are impermeable to water. If seed coat cracked or
scarified so that water can gain entrance, the seed usually
germinate promptly. Under natural condition in soil, the
fungi and bacteria acted upon the seed coat and hydrolyze
the polysaccharide and other coat components, thereby
softening them so that water can penetrate embryo. It takes
several weeks or even month for seed coat to be degraded by
biological activity.

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B. Gas impermeability:
Seeds coat, while permeable to water, appear to be
impermeable to dissolved gases like O2 and CO2 . For
example: Xanthium, some member of composite family, etc.
Early respiratory activity is characteristic of germination of
many seeds, if O2 is prevented from reaching the embryo,
prompt germination may not be able to take place.
Respiration also involves the release of CO2 and some seed
coats, while permeable to O2 , may be impermeable to CO2 .
Accumulated CO2 in the vicinity of embryo inhibits further
germination process. So, broken of seed coat or scarification
is required for setting prompt germination.

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C. Mechanical resistance:
Some seed coat permeable to water and dissolved gases, but
the coats have such mechanical strength that can’t broken by
the growing embryo. For example, Pigweed (Amaranthus),
Shepherd’s purse (Capsella), etc. The dormancy may persist
upto periods as long as 30 years in the case of pigweed if the
seed remain saturated with water. If the seed coats soften and
allows some embryos swelling and dries again, however,
further growth of the embryo may be prevented. If the seed
coat ruptured or fractured through saturation with water
again, prompt germination can occur. In some seeds, during
germination, enzymes that hydrolyze seed coat are secreted
thereby weakening it so that the growing embryo can
continue its growth.

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3. Chemicals in seed
Different kinds of chemicals affecting plant growth are found in
seeds. Some compounds inhibit plant growth whereas other
promotes growth. The inhibitors not only do inhibit seed
germination but they also inhibit the growth of seedling. The
germination inhibitors presence either in some parts of seeds such
as testa, endosperm, embryo or in structures surrounding them
such as the juice or the pulp of fruit (Example: in tomato) and
glumes (Eg. in oats). Germination in some seeds can be promoted
by the application of growth substances such as gibberellins and
cytokinins. Germination of non-dormant seeds can inhibited by
exogenous abscisic acid. So that the germination and dormancy
are controlled by interactions between these growth promoting and
growth inhibiting substances.

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5. Light sensitive seeds:
Germination of seed in many species is affected by light resulting in
seed dormancy such light sensitive seed are called photoblastic.
Response of seeds to sunlight (white light) falls into the three
category:
a. Positive photoblastic seed: Seeds are induced to germinate by
exposure to a single irradiation. Depending on the intensity of the
radiation source, the single exposure may be as brief as a few second
or as long as several hours. Example: lettuce, tobacco, shepherd’s
purse, etc.
b. Negative photoblastic seed: Seeds are prevented from
germinating by exposure to light, such seeds require total darkness
for optimal germination. Example: Allium, Helleborus niger, etc.
c. Non-photoblastic seed: Seeds germinate in either light or dark.
Example: tomato, cucumber, etc.

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4. Chilling (or low temperature) requirements
Some seeds such as apple, rose, peach, etc. remains in
dormant after harvest in the autumn because they have a low
temperature or chilling requirements for germination. In
nature, this requirement is fulfilled by winter temperature.
Seeds remain dormant throughout winter season and
germinate only in the following spring.

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Methods of breaking seed dormancy
1. Scarification: rupturing, piercing and pricking
2. Temperature Treatment
a. Low temperature treatment/Stratification
b.High temperature treatment
c. Alternating temperature treatment
3. Light treatment
4. Chemical treatment
5. Application of pressure to seeds

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Dormancy can be broken by any one of the following methods:
1. Scarification:
The process of rupturing or weakening the seeds coats by
mechanical or other means is called as scarification. Dormancy
can be broken by rupturing, piercing and pricking the seed coat
or pericarp near embryo. Piercing in outer layers of pericarp or
integument with sharp needles increased the germination in case
of sorghum. Concentrated sulphuric acid, hydrochloric acid
scarification for 3 minutes gave highest germination in case of
black gram. Care should be taken during scarification so that the
embryos are not damaged.

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2. Temperature treatment
a) Low temperature treatment /Stratification :
Low temperature (5-10oC) treatment for 5-30 days is found more
effective in case of cool season crops like barley, oat and hot season
crop sorghum. The dormancy due to physiological changes in embryo or
due to after ripening may be broken by low temp. treatment. For
example: in case of apple, peach and rose, to break the seed dormancy
the seeds are treated in moist medium at low temperature (5-10oC) for
sufficient period of time and this process is called stratification.
Artificial stratification is done by altering the layers of seeds with layres
of wet sphagnum (peat moss), sand or some other suitable materials and
keeping them at low temperature. This is mostly applicable when
dormancy is inherent in the embryo. The length of time required for
stratification varies depending on the species. Seeds of wild rose (Rosa
multiflora) require a two months stratification period.

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b) High temperature treatment:
Seeds of oil palm require as high temp as 50-60oC to break their
dormancy. Nut grass (Cyperus rotundus) seeds have been heated at
400C on moist media for 3-6 weeks to break dormancy that otherwise
would have lasted for 7-8 years where as low temp. stratification did
not aid germinationn.
c) Alternating temperature treatment:
An alternation of low and high temperature (the difference of the two
being not more than 10 to 20oC) greatly improves the germination of
seeds in certain plants such as Poa pratensis, Chinese red pine
(Pinus densiflora) and Japanese black pine (Pinus thimbergii).
The need for alternating temperature conditions during germination
is apparently associated with embryo dormancy and is widely
recognized for many agricultural species.

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3. Light treatment
Light intensity, light quality, and light duration are all known to
affect the germination of seeds, which have embryo dormancy.
The dormancy of positively photoblastic seeds can be broken by
exposing them to red light (most effective near 670 nm). Within
limits the germination response depends upon the quantity of light
received. The promotion of germination by red light and inhibition
by far-red light (>700 nm) probably involves the operation of a
proteioaceous pigment called as phytochrome. The germination
responses of seeds like tobacco (Niconiana tabacum) and some
varieties of Lettuce (Lactuca sativa) are promoted by light
(positively photoblastic) while others such as Phacelia
tenacetifolia and Nemophila insignis, Allum spp. are inhibited by
light (negatively photoblastic).

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4. Chemical treatment :
Chemicals like potassium nitrate, thiourea, acetone,
mercuric chloride, hydrogen peroxide, carbon monoxide,
methylene blue, gibberallic acid, kinetin and ethylene can
be used for breaking the dormancy in many crop seeds.
S.
5. Application of pressure to seeds:
In certain plants e.g. sweet clover (Melilotus alba) and
alfalfa (Medicago sativa) the germination of seeds can be
improved by 50-200% if the seeds are subjected to hydraulic
pressure of 2000 atm at 18oC for about 5-20 minutes. This
effect of pressure on germination results due to changes in the
permeability of the testas to water and oxygen.

Unit 8 Seed Dormancy.pptx

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Unit 8 Seed Dormancy.pptx