Germination, its defination, types germination, steps of germination and factor affecting in germination

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Seed Germination
Govinda Raj Sedai
Madan Bhandai Memorial Academy Nepal (MBMAN)
Urlabari, Morang

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Seed germination
Germination is the emergence and development
of a seedling from the seed embryo, which is able
to produce a normal plant under favorable
condition. All the viable seeds which have
overcome dormancy (if any) either naturally or
artificially will readily germinate under suitable
environmental conditions necessary for seed
germination i.e. water, oxygen, temperature and in
some case light.

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Seed Germination …….
Germination is the process by which a plant
grows from a seed. The most common example of
germination is the sprouting a seedling from
a seed of an angiosperm or gymnosperm. In
addition, the growth of a sporling from a spore,
such as the growth of hyphae from fungus spores,
is also germination. Thus, in a general sense,
germination can be thought of as anything
expanding into greater being from a small
existence or germ.

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Emergence of seedling
• The piercing of coleoptyle or plumule through
the soil to come up during seed germination is
known as seedling emergence.

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Pattern of seed germination
1. Imbibition of water
2. Enzyme activation
3. Initiation of embryo growth
4. Rupture of seed coat and emergence of seedling
5. Seedling establishment

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Pattern of seed germination
The major events occurring in seed germination are briefly
explained as under.
1. Imbibition of water:
If the seed is kept in moist or humid medium water is
absorbed through natural openings in the seed coat and
diffuse through the seed tissues, which is known as
imbibition. The water causes the cells to become turgid and
the entire seed grows in volume and the seed coat become
more permeable to O2 and CO2. When swelling occurs, the
seed coat often ruptures, facilitating both water and gas
uptake and emergence of the growing points.

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2. Enzyme activation:
Enzyme activation starts when the seed is hydrated at 30-50%
water content. After absorption of water, hydrolytic enzymes
formation takes place. These hydrolytic enzymes are protease,
-amylase, -amylase, lipase etc, which helps:
• To breakdown the stored tissues
• To aid in the transfer of nutrients from storage areas of
cotyledon and endosperm to the growing points.
• To trigger chemical reactions, which use breakdown
products in the synthesis of new materials.
After enzyme activation the macromolecules of protein,
carbohydrate, lipids are converted to micro molecules and
transported to growing points (root or shoot apex) and utilized
as a source of energy of embryo.

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3. Initiation of embryo growth:
Embryo increase in size by two ways i.e cell elongation and cell
division or in combination of both, but cell elongation is first.
Growth of the root shoot axis occurs at the expense of the storage
tissues, which gradually decrease as food reserves are depleted. By
the time the young seedling is able to synthesize its own food,
most of the storage tissues have been exhausted.
4. Rupture of seed coat and emergence of seedling:
Imbibition results in swelling of seed, which causes a pressure
called imbibition pressure. When imbibition pressure is as high as
200 atmosphere, there is rupturing of seed coat and first part
coming out is radicle then plumule. Ordinarily, the primary root
(radicle) is the first structure to emerge but in some species shoot
(plumule) emerge first.

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5. Seedling establishment
Initially radicle and plumule are growing by using energy of
storage tissues. Initially it undergoes a transition stage
during which it begins to produce some of its own food but it
still dependent on food breakdown from reserve storage
tissues. As the seedling becomes firmly established in the
soil, begins water uptake and manufactures most of its own
food (photosynthate), this is called seedling establishment. It
is entirely heterotophic in initial stage then transitional and
become autotrophic eventually at the last. Then the
germination process is complete.

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Types of Seed germination
There are two kinds of seed germination, which are:
• Epigeal germination
• Hypogeal germination

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1. Epigeal germination
Seed germination in which the seed leaves
(cotyledons) emerge from the ground and function as
true leaves. During root establishment the hypocotyls
begins to elongate by quick cell division and push the
seed cotyledon up which breaks the soil crust and
projecting them into the air through the soil.
Afterward the cotyledons open, plumule growth
continues and the exhausted cotyledons wither and
fall to the ground. It is found in bean, soybean, black
gram, green gram, groundnut, pigeon pea, sunflower,
pine seeds etc.

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1. Seed coat 2.Micropyle 3. Hilum 4. Raphe 5. Chelaza 6. Plumule 7. Hypocotyl 8. Radicle
9. Cotyledon (scutellum) 10. Primary root 11. Secondary root 12. Leaf
Fig. Epigeal germination of Bean seed

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2. Hypogeal germination
It is a kind of seed germination in which the seed
leaves (cotyledons) remain below the ground. The
cells of epicotyl divide faster and increase the
length of epicotyl, which pushes the seed leaves
below the ground. Regardless of their above ground
or below ground, the cotyledons continue to provide
nutritive support to the growing points throughout
germination. Most cereals or monocots except onion
are hypogeal type of germination. Some winter
pulses have also hypogeal germination eg. pea, gram,
lentil etc.

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13. Pericarp 14. Point of caryopsis attachment 15. Endosperm 16. Coleoptile 17. Coleorhiza
18. Seminal root
Fig. Hypogeal germination of maize

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Factors affecting germination
• External, internal and agronomical factors affect
the germination of seeds. The process of
germination is quite complex and both factors
interact for modifying germination pattern.
• The major external factors affecting seed
germination are briefly described as under:

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A) External factors
a) Temperature
b) Moisture
c) Air (O2 and CO2)
d) Light
B) internal factors:
a. Reserved food materials
b. Resting period
c. Viability of seed
d. Presence of poisons and inhibitors
C) Agronomic factors
Factors affecting germination

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A) External factors:
a. Temperature: Different seeds germinate at different temperature
regimes. Very low or very high temperature retard germination of seeds.
Generally germination is most satisfactory around 25-30oC. The
minimum and maximum temperature just permit germination while the
optimum temperature permits highest percentage of germination in the
shortest period of time. The values of maximum, minimum and
optimum temperature are termed as cardinal temperature The
response to temperature depends on the species, variety, growing region
and duration of time from harvest.
Seed type
Temperature
Minimum Optimum Maximum
Wheat 3-5 15-31 31-43
Barley 3-5 19-27 30-40
Rye 3-5 25-31 31-40
Maize 8-10 32-35 40-44

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b. Moisture:
• The water content of stored seeds of most of the crop plant
is about 5-12%. Since this moisture content is too low to
allow rapid metabolism and the first step in the germination
must be an increase in water content.
• Swelling is due to water imbibing proteins, cellulose and
pectic compounds present in seed. Water is essential for
enzyme activation, thus permitting breakdown,
translocation and use of reserve storage material. Field
capacity moisture (at FC all the micro-pores are filled with
water) is about optimum for germination in soil and
extreme moisture may inhibit germination.

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c. Air (O2 and CO2):
O2 is needed for respiration and all physiological activities,
which proceeds at an accelerated rate at this stage. Water
acts as a vehicle to carry additional O2 in seed. Ordinary air
is composed of about 21% O2, 0.03% CO2 and about 79% N
gases. It is observed that O2 is required for germination of
most species; CO2 concentration higher than 0.03% retard
germination and N gas has no influence. If the O2
concentration is reduced substantially below that of air,
germination of most seeds is retarded.

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d. Light: Seeds of different types show different response to
light. Tobacco seeds germinate better in light than in
darkness. Freshly harvested lettuce seed would also
germinate in light but after storage germinability will be
manifested in red light only. Datura, tomato and onion
would germinate at a faster rate in dark than in light. Light
intensities of 100-200 foot candles from indirect light in the
average seed laboratory are probably adequate for
germination of most species. A bright sunny day provides
upto 10,000-foot candles and a cloudy day about 1500 foot-
candles. The greatest germination occurs in the red area
(660-700 nm) with a peak of 670 nm (nanometers) followed
by an inhibition zone in the far-red area above 700 nm.

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a. Reserved food materials:
Seed contain stored food materials like CHO, proteins and
lipids. If these are not accumulated appropriate amount in
the seed, it may not be germinate.
b. Resting period:
Many angiospermous seeds cannot germinate immediately
after maturity even if provided with all favorable environments
for germination. This condition is known as seed dormancy,
which may not be germinated immediately after maturity
because of immature embryo. After seed maturity, a rest period
is necessary to develop embryo after harvesting. This period is
known as after ripening period or resting period.
B) internal factors:

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c. Viability of seed:
Seeds retain their viability for certain period of time, after which
the embryo becomes dead. Storage conditions and circumstances
in which the seed mature often determine the period of viability.
Non-viable seeds cannot germinate.
d. Presence of poisons and inhibitors:
Many different kinds of compounds are known to affect seed
germination. Hydrogen cyanide will poison and kill growing
embryo. Extracts from fruits, leaves, twigs and root also have been
found to inhibit seed germination. For example the seeds of
tomato will not germinate as long as they are enclosed within the
fruit, but if they are removed and thoroughly washed free of fruit
tissue, they will germinate. Plant extracts consists of a variety of
complex organic compounds like alkaloids, essential oils and
abscisic acid (ABA), which may inhibit seed germination.

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C) Agronomic factors
• Defective crop husbandry during crop maturity and
biotic and abiotic stresses during seed setting and
maturity due to natural disruption in environmental
condition affects seed viability and germination
capacity.
• Cloudy days during grain filling produce chaffy
non-viable rice seeds. Inadequate plant protection
during fruit ripening may cause total loss of seed
germination.
• Mechanical injured to seed due to rough handling
during production, harvest and packaging may
cause about 20-30% loss in germination.