SEED DORMANCY

Non- germination of seeds due to absence of suitable conditions is termed as dormancy. Or A physical or physiological condition of viable seed, which prevents germination even in the presence of favorable conditions.

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Types of seed dormancy -2 types
1. Primary dormancy :- When the seed is released from the plant already in a dormant state.
2. Secondry dormancy :- When the seed becomes dormant after being released due to environmental factors.

Cause of seed dormancy/ factor responsible for seed dormancy

(A) Hard seed coat

(i) Seed coats impermeable to water

(ii) Seed coats impermeable to oxygen

(iii) Mechanically resistance seed coat

(B) Immature embryo

(C) Period of after ripening

(D) Germination inhibitors.

(E) Excessive Salts:-In Atriplex the seeds contain a high concentration of solutes which do not allow the embryo to resume its growth.

Coat imposed dormancy:
1. Seed coat factor
a. Seed coat impermeable to water – i.e. water does not entered into seed coat
b. Seed coat impermeable to oxygen (0 is not entered in seed coat)
c. Mechanically resistant seed coat.
2. Embryo factor
1. Dormant embryo
2. Immature/ Rudimentary embryo
3. Inhibitory factors :- Presence of germination Inhibitors in seeds i.e. presence of inhibitors or release of inhibitors.

(Natural Overcoming of Seed Dormancy)
1. Weakening of tough and impermeable seed coats by microbial action.

2. Rupturing or weakening of seed coats by mechanical abrasions.

3. Action of digestive enzymes present in alimentary canals of birds and other animals which happen to feed on their fruits.

4. Leaching of inhibitors present in the seed coat.

5. Inactivation or oxidation of inhibitors by heat, cold and light.

6. Production of growth hormones which can counteract the effect of inhibitors.

7. Completion of over-ripening period.

8. Attainment of maturity of embryo in case the dormancy is due to incomplete development of embryo.

9. Leaching of solutes in Atriplex where dormancy is caused- by high osmotic concentration inside the seeds.

(Artificial Overcoming of Seed Dormancy)
1. Rupturing of seed coats or scarification by abrasion through machine threshing, filing, chipping, vigorous shaking, etc.

2. Hydraulic pressure of up to 2000 kg for 5-20 minutes for weakening the tough seed coats.

3. Treatment with hot water or fat solvents for dissolution of surface inhibitors, waxes, etc.

4. Treatment with concentrated sulphuric acid for a short period followed by thorough washing to remove all traces of the mineral acid.

5. Stratification or subjecting the moist seeds in the presence of oxygen to periods of low or high temperature.

6. Counteracting the effect of growth inhibitors by soaking the seeds in potassium nitrate, ethylene chlorohydrin, thiourea, gibberellins, etc. Exposure to high concentration of oxygen has similar effect. Placing seeds in running water sometimes also removes growth inhibitors.

7. Exposure to alternate temperature, chilling or light, depending upon the type of seed dormancy.

(Biological Importance of Seed Dormancy)

1. Dormancy allows the seeds to remain in suspended animation without any harm during drought, cold or high summer temperature.

2. The dormant seeds can remain alive in the soil for several years. They provide a continuous source of new plants even when all the mature plants of the area have died down due to landslides, earth quake, floods, epidemics or continued drought.

3. It helps the seed to get dispersed over long distances through unfavourable environment or inhospitable area.

4. The small seeds with impermeable seed coat belonging to edible fruits come out of the alimentary canals of birds and other animals uninjured e.g., Guava.

5. Dormancy induced by the inhibitors present in the seed coats is highly useful to desert plants. The seeds germinate only after a good rainfall which dissolves away the inhibitors. The rainfall ensures the seed a proper supply of water during its germination.

6. It follows storage of seeds for later use by animals and man