Seeds germinate as soon as they undergo maturation and are provided with suitable conditions for germination, as in bean, maize, peas etc.,. In some species, seeds germinate while they are still within the fruit or attached to the plant. This type of germination is called vivipary, as noticed in case of Carica papaya etc.
However, in many plant species, the seeds fail to germinate for some period after maturation and harvest even if suitable conditions for germination are provided to them. This state of inactivation of seeds may be due to some internal factors that inhibit the process of germination and is referred to as seed dormancy.
Thus seed dormancy is the internal or innate inhibition of germination of viable seed even when present under most favourable conditions required for its germination.
Germination may not always be prevented due to dormancy. Sometimes seeds may be quite capable of germination but they fail to germinate due to the non-availability of sufficient moisture or temperature. This state is called quiescence state of seed and the seed and the embryo are called quiescent seed and quiescent embryo.
The quiescence state of seed is usually referred to a state of suppression of embryo growth imposed by external factors outside the seeds. On the other hand, dormant state is the suppression of embryo growth due to internal factors within the seeds.
Thus,seed dormancy is defined as the condition of seed when it fails to germinate even though the environmental conditions usually considered favourable for active growth is present.
Causes of seed dormancy:
Seed dormancy may be caused by various factors. Some of the important factors causing dormancy are as follows:
Dormancy due to seed coat:
The seed coat or testa of most of the seeds are hard, tough and dehydrated forming a protective covering around the embryo. It is impermeable to water and oxygen gas and it does not allow them to reach the embryo thereby suppressing germination. The hard seed coat offers mechanical resistance for the growth of the germinating embryo e.g., Capsella, Lepidium.
Such seeds, however, readily germinate if the seed coat is artificially removed either by chemical treatment or by mechanical means. This process is called scarification. It is the rupturing of seed coats by abrasion through machine threshing, filling, chipping, vigorous shaking, etc.
Dormancy due to condition of embryo:
The embryo may be immature and rudimentary when the seeds ripe and it may take some time to get fully developed, causing a period of dormancy. In some other cases, however, the embryo may be fully developed when the seed matures but it fails to germinate immediately due to some internal factors causing physiological immaturity as in case of seeds of apple, pear, cherry etc. Such seeds can be induced to germinate early if they are stored in moist well-aerated and low temperature condition. This process is called stratification.
Dormancy due to chemical inhibition:
Presence of some chemical substances such as coumarin, phenolic acid, para-ascorbic acid etc., and hormone like abscisic acid in the embryo, endosperm or seed coat of certain seeds may induce dormancy. Such seeds fail to germinate so long as the concentration of these chemical remains high.
Concentration of such chemical substances and growth inhibitors can, however, be reduced by various treatments such as chilling exposure to fluctuating temperature, exposure to light, treatment with growth promotors like gibberellins and cytokinins and chemicals like thiourea, potassium nitrate etc.
Seeds subjected to chilling or treatment with low temperature results in the breaking down of abscisic acid and promotion of gibberellic acid synthesis. This helps in releasing the seed dormancy caused due to the presence of inhibitors.
Role of growth regulators in seed dormancy:
Growth regulators such as auxins, gibberellins, abscisic acid, coumarins, and some phenolic compounds have been isolated from seeds of different plants. The effects of gibberellic acid, abscisic acid and ethylene in seed dormancy are discussed below.
Gibberellic acid and seed dormancy: Seeds of some wild plant species require light or cold to induce germination. These changes in the conditions are usually associated with changes in gibberellin levels, therefore, the dormancy of such seeds can be overcome by application of gibberellin. It may be a natural regulator in the process of germination. In barley, application of gibberellic acid has been found to stimulate α-amylase secretion in the aleurone layer of the seed. The α-amylase consists of multiple isozymes.
Besides this, β-amylase and other starch-degrading enzymes such as proteases are involved in mobilizing the food reserve in the endosperm. Mobilization of food reserves probably facilitates seed germination.
Abscisic acid and seed dormancy: Arabidopsis a member of Brassicaceae family has been used to demonstrate the role of abscisic acid in seed dormancy. It has several ecotypes some of which are abscisic acid deficient while the wild types have abscisic acid.
Abscisic acid deficient mutants are not dormant and readily germinate at maturity. Exogenously applied abscisic acid in these seeds could not induce dormancy. Not only this, paternal or maternal abscisic acid also failed to induce dormancy when reciprocal crosses were made between wild types and abscisic acid deficient types.
Dormancy was shown by these seed only when the embryo itself produced abscisic acid. The influence of abscisic acid on seed dormancy is also contributed by the concentration of other hormones. For example, in most plants, it has been observed that as the level of indole acetic acid and gibberellic acid declines, the concentration of abscisic acid increases.
Ethylene break seed dormancy: Ethylene breaks seed dormancy and initiates germination in cereals. In several species, ethylene increases the rate of seed germination. Besides this, ethylene treatments are also known to break the dormancy of buds. It is used to promote sprouting in potato tubers and other bulbs.