CHAPTER: 14(A)
Seed
Germination
Seed
Germination:
•
The
dry seeds often remain inactive for long periods.
•
When
the seeds is placed in the soil under suitable conditions it becomes very
active and in a few days this renewal of activity results in the production of
a young seedling.
•
The
growth and development of the seed embryo into seedling is called seed
germination .
•
The
term, seedling is used for the young planter that emerges from the seed till it
develops its own photosynthetic activity.
•
The
growth and developmental changes during seed germination take place at the
expense of the stored food contain in the seed.
•
Germination
comes to an end when the young plant forms the first green leaves and
photosynthesize its own food.
Courses
of Seed Germination:
•
When
the seed is placed in warm moist and
well aerated soil it begins to imbibe water which passes in through the
micropyle.
•
The
seed swells up and presently the testa ruptures.
•
The
food contained in the two cotyledons is digested and passed on to the plumule
and the radicle tips.
•
The
latter is the first to grow and emerge out of the seed coats.
•
It
forms the first of primary root.
•
Soon
root hairs and lateral branches appear on the primary root.
•
The
latter form the secondary roots.
•
Gradually
the plumule also begins to grow in the opposite direction and forms the aerial
shoot on which the first foliage leaf appears after sometime.
•
The
cotyledons remain in the soil.
•
In
some seeds the cotyledons come out of the soil and form the first green leaves.
Changes
in respiartion/ Biochemical Changes during seed germination:
•
The
water uptake is accompanied by rapid increase in respiration rate of embryo.
Initially these may be anaerobic respiration but it is soon replaced by aerobic
one due to availability of oxygen.
•
The
essential physiological changes which take place during the germination of
seed, are of following types:
- Cell
enlargement with the help of water.
- Hydrolysis
and translocation of food with the help of water.
- Conversion
of food materials, stored in the cotyledons or the endosperm, into
structural compounds of new cells, particularly in the tips of plumule and
radicle.
- Breakdown
of organic compounds with the help of embryo.
Ø As germination
progresses there is mobilization of reserve materials.
i)
Nucleic
Acid(increase the activity of DNA, RNA)
ii)
Carbohydrates(insoluble
starch is hydrolyzed)
iii)
Lipids
iv)
Proteins
(active synthesis of protein and other enzymes)
v)
Inorganic
material (phosphate, calcium, magnesium and potassium )
Conditions
Necessary for Germination:
(Factors
Affecting Germination)
- External
Factors:
•
Air,
Water & optimum temperature is required before seeds can germinate.
- Water:
•
Water
is absorbed through the micropyle and the seed coat.
•
Water
brings about the following changes in germinating seed:
–
The
hydration of the seed coat increases its permeabiliy to oxygen and
carbondioxide.
–
Rupturing
of seed coat occur which helps in the emergence of the embryo.
–
Enzymatic
hydrolysis of organic food materials takes place.
–
Respiratory
breakdown of complex organic compounds takes place in the presence of water.
- Air:
•
Air
supplies oxygen which is essential for the respiration of germinating seeds.
•
Initially
respiration is predominantly anaerobic and changes to aerobic respiration when
the seed coats are ruptured.
- Temperature:
•
Physiological
activities of germination is affected by temperature.
•
Seeds
are not usually expected to germinate below 0oC and above 50oC
and the optimum temperature lies between 25-30oC
- Light:
•
Light
is not considered as an essential factor since germination takes place even
without light.
•
But,
recent research shown that light has pronounced effect on germination.
- Internal
Factors:
•
A
normal seed is expected to be internally capable of germination. The following
factors determines the internal capacity:
- Food
and Auxin:
•
All
normal seeds contains a supply of food which is necessary for the growing
embryo and the young seedling. It has already been seen that this food may be
contained in the cotyledon, endosperm, perisperm or testa.
•
Protein
and starch as well as Auxin is mainly required for the germination.
- Completion
of resting period (Dormancy):
•
Many
angiospermic seeds cannot germinate as soon as they are found. They have to
undergo a period of dormancy varies from plant to plant. it may be a few days
or some months.
•
Most
cereals are capable of germination immediately after harvesting while some
other seeds do not germinate till after a year.
- Viability:
•
Seeds
retain their viability (capacity of germination) for a definite period of time
after which the embryo becomes dead for all practical purpose.
•
Condition
of storage and circumstances in which the seed matured determine the viability
period.
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