Germination is initiated from the seed. The seed is a ripened ovule. It consists of an embryo, stored food material, and a protective covering. When metabolic activities are initiated there is the emergence of radicle and plumule from the seed which is termed as germination. The germination leads to the production of seedling plants.
Requirements for germination
- Viability of seed
- Availability of proper environmental conditions.
- Dormancy free seed
In the course of development, seeds lose moisture and reach maturity. However, some seeds don’t approach the maturation stage and they germinate while still attached to the plant. Those seeds are termed viviparous seeds. In another kind, there are seeds that don’t lose moisture when they reach maturity. Such seeds withstand a small degree of desiccation. These are termed recalcitrant seeds. They germinate when the innate content of moisture is high in the seed. However, in the case of orthodox seeds which undergo maturation and drying pass through a series of events in the course of germination. These events are as under: –
1. Imbibition: – It is the swelling of the seed after the intake of water. Water absorption in seeds is due to water potential. The absorption of water finally ends when the seed enters the germination’s leg phase.
2. Lag Phase: – In this phase, water absorption is less but metabolic activities are accelerated. Activities of this phase: –
i) Mitochondrial activities:- After imbibition the mitochondria become dehydrated and its membrane becomes active. As a result, respiration and ATP synthesis increase significantly.
ii) Protein Synthesis:- Seed hydration is followed by the formation of polysomes which form new proteins from mRNA within hours of completion of the imbibition.
iii) Metabolism of reserve material: – The seed contains proteins, carbohydrates, and lipids which are helpful in the early development of the embryo. As a result of metabolic activities, proteins are converted into amino acids, carbohydrates (starch-amylase) into glucose plus maltose, and lipids into glycerol plus free fatty acids. These changes bring about a change in the water potential which helps in radicle protrusion.
iv) Specific enzyme production: – Specific enzymes are produced which are responsible for loosening the cell wall. It has been proven that there is ATP-activated acidification of the cell wall which contributes to the loosening of the cell wall. The process eventually leads to the elongation of the radical.
3. Radicle protrusion: – This is the proof of germination. This occurs as a result of cell enlargement. The process of protrusion is caused by (a) negative osmotic potential due to the metabolism of the storage reserve (b) flexibility of the cell wall in the radical (c) expansion of the cell around the radical. A combination of these factors is involved in the protrusion of the radical.
4. Seedling emergence: – When the root and shoots of the embryonic axis begin to elongate, then the shoots come out. The embryo has an axis containing one or more cotyledons. The growing point of the root is called the radical. It originates from the base of the embryonic axis. The growing point of the shoot is called the plumule and it emerges from the upper end of the embryonic axis above the cotyledon. The stem of the seedling is divided into two parts:- hypocotyl and epicotyl. The hypocotyl is the underside of the cotyledon and the epicotyl is above the cotyledon. When seedling growth begins from the embryonic axis, the fresh weight and dry weight of the seedling increase, and the total weight of the storage tissue decreases. Emergence can be of two types; Epigeus and hypogeous germination. In epigeous germination, the hypocotyl elongates and forms a hypocotyl hook. The hypocotyl hook raises the cotyledons above the ground. In hypogeous germination, there is no extension of the hypocotyl and only the epicotyl emerges above the ground.