Seeds and Sowing- Seed Dormancy
Seed Dormancy: Failure of fully developed & mature viable seed to germinate under favorable conditions of moisture & temperature is called resting stage or dormancy and the seed is said to be dormant.
Kinds of Dormancy in Seeds:
1. Primary dormancy: The seeds which are capable of germination just after ripening even by providing all the favorable conditions are said to have primary dormancy. E.g.: Potato.
2. Secondary dormancy: Some seeds are capable of germination under favorable conditions just after ripening but when these seeds are stored under unfavorable conditions even for few days, they become incapable of germination.
3. Special type of dormancy: Sometimes seeds germinate but the growth of the sprouts is found to be restricted because of a very poor development of roots & coleoptiles.
Causes of Dormancy:
The dormancy in seeds may be due to any single or a combination of more than one of the following causes.
1. Seed coats being impermeable to water: Some seeds have a seed coat which is impermeable to water. Such seeds even when fully matured & placed in favorable conditions; fail to germinate because of failure of water to penetrate into the hard seed coats. These seeds become permeable, if they are treated with H2SO4 or dipped in boiling water for few seconds. E.g.: Cotton.
2. Hard seed coat: Seeds of mustard, amaranths, etc. contain a hard & strong seed coat which prevents any appreciable expansion of embryo. Thus, if the seed coats fail to burst the embryo will remain dormant even after providing all the favorable conditions for germination.
3. Seed coats being impermeable to O: The seed coats are impermeable to O2 & if the seed coats do not rupture the seed fails to sprout.
4. Rudimentary embryo of seeds: The seeds which are apparently ripened contain a rudimentary or imperfectly developed embryo and the germination of such seeds naturally gets delayed until the embryo develops properly.
5. Dormant embryo: The seeds of an apple, peach, pinus, etc do not germinate even though the embryos are completely developed and all the favorable conditions for germination are provided. In such seeds, physiological changes called after ripening take place during the period of dormancy which enables the seeds for germination.
6. Synthesis & accumulation of germination inhibitors in the seeds: Plant organs synthesize some chemical compounds which are accumulated in the seeds at maturity and these chemicals inhibit the germination of their seeds.