What are the Factors That Affect Seed Germination?
These seeds germinate equally well under dark as well as lighted conditions. Example – Cultivated plants like cucumber, tomato. B. Mechanism of germination when all factors necessary for germination (water, oxygen, and temperature, light) are available, then germination takes place.
Take for example, cereal grains such as grain of maize. Water penetrates the permeable seed coat and get into the seed, even into the embryo within the seed. The embryo, then, becomes active; synthesis of mRXA for different processes begins. Also, gibberellic acid is discharged into the aleurone.
In the aleurone, gibberellic acid induces synthesis of a number of hydrolases which mobilize reserve materials. For example, a-amylasc is one of such enzymes which degrade starch. They also comprise nucleases and proteases which break down nuclcic acids and proteins, respectively. As a result of the activity of nucleases, cytokinins contained in the nucleic acids are set free. The proteases release amino acids including tryptophan from which IAA can be formed.
Cytokinins and IAA now act on the embryo. The cytokinins induce cell division and IAA cell elongation. The embryo so induced starts to grow and bursts s seed coat. It is helped also by pectinases and cellulases. First the radicles appear out of the seed, then, the coleoplite.
As a result of gravity, IAA migrates underside of both the organs. This, ir turn, leads to reactions which are geotropically different owing to the different sensitivity of ooleopile and radicle.
The root turns downwards and is positively geotropic, the coleoptile turns i pwards and is negatively geotropic. As soon as the coleoptile has broken through the soil, its photosynthetic apparatus differentiates. This marks the completion of the seedling stage C. Seed dormancy and its release
Many angiospermic^seeds cannot germinate immediately after their formation. They have to undergo a period of dormancy or resting period.
The period of dormancy is not constant in all the plants. Most cereals are capable of germination immediately after harvesting but some other seeds do not germinate until one year. Some plants do not need any resting period while, in some plants, seeds fail to germinate even under suitable conditions of germination.
This state of inactivation of seeds may be due to some internal factors that inhibit the process of germination. This is known as seed dermancy. The causes of seed dormancy and its release mechanism vary in different seeds, being species specific.
(a) Dormancy due to seed coat
The seed coat testa of many seeds is extremely hard and tough. It offers mechanical resistance to germination. It is impermeable to water and oxygen. Such seeds can germinate only if the seed coat is artificially removed either by chemical treatment or by mechanical means. The mechanical method of removal of seed coat is called scarification.
(b) Dormancy due to the condition of the embryo
Dispersal of some seeds takes place when their embryos are not mature. It may take sometime to mature and this is the cause of dormancy. In some seeds, like those of rose, the embryo though fully developed is physiologically immature at dispersal.
The seeds germinate after a period of rest. Such seeds can be induced to germinate early if they are stored in moist, well aerated and low temperature conditions – a process called stratification.
(c) Dormancy due to chemicals and growth regulators
Presence of some chemical substances such as coumarin, phenolic acid, para ascorbic acid etc and hormones like abscissic acid in the embryo, endosperm and seed coat induce dormancy and seeds do not germinate as long as their concentation is high.
Concentration of such inhibitors can be reduced by various treatments such as exposure to fluctuating temperature, chilling, light, keeping under running water, treament of hydrated seeds with hormones like gibberellin and cytokinin or with certain chemicals like potassium nitrate, thiourea etc.