A) Internal Factor:
- Physical dormancy (Due to impermeable seed coat):
- The seeds of certain plants (families of papilionaceae, malvaceae, liliaceae, chenopodiaceae and solanaceae) have very hard seed coats, which are impermeable to water.
- The seed remain dormant in the soil until the impermeable layer of testas decay by the action of soil microorganism.
- Sometimes the hard seed coat is ruptured or weakened by the use some machines, acids or other means before sowing, the process is known as scarification.
- The degree of hardness depends on the degree of maturity, the ripening conditions and the storage time.
2) Physiological Dormancy
- Sometimes, the dormancy of seeds results due to the presence of certain germination inhibitors (abscisic acid, ferulic acid, dormin etc.) either in some parts of the seeds such as testa, endosperm, embryo or in embryo surrounding structures like the juice or the pulp of fruit (tomato) and glumes (oats).
- Abscisic acid acts as the germination inhibitor in dormant groundnut (Arachis hypogea), cotton etc.
- Dormancy due to immaturity of the embryo which fails to develop fully by the time the seeds are shed. In such case the seeds germinate only after a period of rest during which the development of embryo inside the seeds is completed.
3) Genetical dormancy
- The structural and physiological factors controlling dormancy are under control of specific genes.
- In this case dormancy is primarily determined by genetic makeup of seeds and varies widely among species and even within species.
- The intensity of dormancy in rice varieties is controlled by a varying number of partially dominant genes that have cumulative unequal effect
B) External factor
i) Low temperature or chilling requirement
- In certain plants such as apple, rose, peach etc the seeds remain dormant after harvest in the autumn because they have a low temperature of chilling requirement for germination.
- In nature this requirement is fulfilled by the winter temperature.
- In such case the seeds remain dormant throughout the winter season and germinate only in the following spring.
- Such a chilling treatment is known as stratification.
- The moistened seeds are usually preconditioned at temperature between 3-10oC.
ii) Light sensitive seeds
- In many species the germination of the seeds is affected by light resulting in seed dormancy.
- Such light sensitive seeds are called as photoblastic.
- The seeds of certain plants eg lettuce (Lactua sativa), tobacco (Nicotiana tabacum), tomato (Lycopersicum esculentum) etc are positively photoblastic and germinate only after they have been exposed to light.
- On the other hand, the seeds of certain plants eg. Nigella damascena, Silene armeria, Allium sp are negatively photoblastic and their germination is inhibited by light.
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