In this article, we propose to discuss the types of seed dormancy and its role in plants life.
The types are: (1) Immature Embryo (2) After-Ripening (3) Impermeability of Seed Coats (4) Tough Seed Coats (5) Inhibitors and (6) Excessive Salts.
Seed dormancy or rest is the internal or innate inhibition of germination of otherwise normal or viable seed even when present under most favourable conditions required for its germination. Quiescence is the non-germination of viable and non-dormant seeds, due to absence of favourable environmental conditions, e.g., seeds kept in a dry jar. Dormancy is of several types.
Types of Seed Dormancy:
1. Immature Embryo:
The embryo is not fully developed at the time of seed shedding. The period of seed dormancy corresponds to the period required by the embryo for its complete development, e.g., Ginkgo biloba (a gymnosperm), Eranthis hiemalis.
After their shedding, the seeds of barley, wheat, oat, etc. require an interval of ripening before they attain the power to germinate. The period of after-ripening is needed to produce the necessary growth hormones.
3. Impermeability of Seed Coats:
The seed coats are impermeable to water (e.g., Chenopodium, Trigonelta), gases (Apple, Sinapis arvensis) or chemicals (e.g., Xanthium).
4. Tough Seed Coats:
The seed coats are hard and provide mechanical resistance to the growth of embryo, e.g., Lepidium, Capsella.
They are chemical substances which do not allow the seeds to germinate. They may be present inside the fruit (e.g., Tomato juice possesses inhibitor ferulic acid), in the embryo (e.g., Xanthium), endosperm (e.g., Iris) or seed coat (e.g., Cucurbita). The chemical inhibitors are of several types like abscisic acid (ABA), phenolic acids, coumarin, short chain fatty acids, etc.
6. Excessive Salts:
In Atriplex the seeds contain a high concentration of solutes which do not allow the embryo to resume its growth.
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.
Role in Plants Life:
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.