Seed germination is the process of active growth by which the embryo resulting in the rupture of the seed coat and emergence of new young plant under favorable condition of water, oxygen, temperature and sometimes light.
Botanically, germination may be defined as the emergence and development of the seed embryo of those essential structures which for the kind of seeds in questions are indicate of the ability to produce a normal plant under favorable condition. It is expressed in terms of percentage.
(1) Imbibition of water:
In a moist medium seed absorb water with or without any intervalling lack period. The water absorb depend upon the kinds of seed. In legumes water enter the seed through through the strophiole and other through Hilum tissue. Some species absorb water all part of the seed coat.
The water uptake is very rapid in cereal grain in which the growing embryo is specially separated from storage endosperm then the metabolic activity is enhanced. The seed protein contains hydration of water and also absorb sufficient amount of moisture. The swelling of the seed often cause the bursting of the seed coat in many species. As swelling of both water and gas and emerging of the growing point.
(2) Enzymatic and respiratory activities:
This stage is characterized by the initiation of cellular activity with the appearance of specific enzymes and increases the respiratory rate. The storage tissue of endosperm and cotyledon of germinating seeds, hydrolage whose activity promotes mobilization of reserved compound in seed and activation of respiratory enzyme.
In cereal one of the detectable feature of the metabolic activity is the activation of mRNA accomplished by an increase the capacity of embryo to synthesis protein. This follows the imbibition of water by only a few minutes. Polysome begins to increase with the rate of respiration and extension of some cell within first 12-16 hours the synthesis new DNA and RNA particularly nil.
By this time, there has been some extension growth of existing cell and the early stages meiotic cell division can be recognized. During the first 24 hours, the extension and division both depend entirely on the use of amino acid, fats and soluble carbohydrate stored in the embryo.
During this period, a considerable amount of gibberelin is secreted by the embryo. The primary mode of action of gibberelin on stimulating in RNA synthesis or the synthesis of α-amylase. It plays an important role in nucleic acid metabolism and protein synthesis.
(3) Digestion and translocation of food:
Digestion is the process of chemically breaking down of complex food to a simple one done by hydrolytic enzyme. The starch, lipid and protein is digested by diastase (α-amylase) lipase and protease into sugar, fatty acid and amino acid. Digested foods such as Glucose, Fructose, maltose, Fatty acid, Amino acid are translocated to the active growing area.
Assimilation is the process by which the digested food becomes a part of the living protoplasm. The assimilation takes place of the meristematic area to provide cellular activity growth of embryo and conservation into new cell component.
The seedling grows by the process of cell division, enlargement and differentiation of cell at the growing point. Growth and development of seedling depend upon food reserved in the seed. The emergence of Plumule above the ground brings the plant under influence of light and result in the suppression of mesocotyle, hypocotyle or epocotyle growth and stimulating the formation of chlorophyll.
They are followed a transition period during which photosynthesis gradually assume. Seedling of wheat become completely independent when the second leaf is fully emerged and third just emerging. Cucumber seedling become independent soon after the unfolding the cotyledon.