Seed Germination

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:

1. Cell enlargement with the help of water.
2. Hydrolysis and translocation of food with the help of water.
3. 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.
4. 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)

1. External Factors:

•      Air, Water & optimum temperature is required before seeds can germinate.

1. 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.

2. 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.

2. Temperature:

•      Physiological activities of germination is affected by temperature.

•      Seeds are not usually expected to germinate below 0^oC and above 50^oC and the optimum temperature lies between 25-30^oC

4. 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.

2. Internal Factors:

•      A normal seed is expected to be internally capable of germination. The following factors determines the internal capacity:

1. 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.

2. 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.

3. 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.