SEED GERMINATION-PROCESS & FACTORS OF GERMINATION
Germination is the fundamental process by which sprouting of seedling takes place from embryo of a seed or seed material.
Agronomically germination means the capacity of seeds to give rise to normal sprouts within a definite period fixed for each crop under proper conditions.
At the same time, metabolic machinery of the seed reactivates resulting in the emergence of radicle and plumule.
PROCESS OF GERMINATION
- The process of imbibition (uptake of water by seeds) takes place, which leads to the swelling and the breaking of the seed coat.
- When seeds are formed, most plants store a food reserve with the seed, such as starch, proteins, or oils. This food reserve provides nourishment to the growing embryo.
- The process of imbibition activates hydrolytic enzymes which break down these stored food resources into metabolically useful chemicals. After that, seedling emerges from the seed coat and starts growing roots and leaves.
- Though, seedling’s food reserves are typically exhausted, but at that time photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.
ESSENTIAL FACTORS FOR GERMINATION
The germination of seed depends on both internal and external factors.
- Water- It is essential for the process of imbibition which leads to swelling and breaking of the seed coat. Water is essential for the activation of hydrolytic enzymes (hydrolase enzyme like esterase).
- Oxygen or Air- oxygen is essential for aerobic respiration which is the main source of seedling’s energy until the green leaves grow from seedlings. Oxygen is also required by the germinating seed for metabolic process. Oxygen is found in soil pore spaces, that’s why, seed gets deprived of oxygen under Waterlogged condition or sowing seed deeply into the soil.
- Temperature- Different type of seeds have different optimum temperature range within which they germinate, otherwise not to do so. For example, many seeds germinate slightly above 14-20 C, while other germinates just above freezing. Some seed germinates when the soil is cool, while some other germinates when soil is warm. Temperature affects cellular metabolic and growth rates.
- Light or darkness- Though, most seeds do not have effect of light or darkness, nevertheless it can act as environmental trigger for germination and is considered as a type of physiological dormancy. Some seeds require adequate light for germination.
Capacity of seed to germinate and seed dormancy.
- Germination Rate- means how many seeds are likely to germinate from seed lot or from particular variety over a given period. It is expressed in percentage, e.g., an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions over the germination period given. Thus, germination rate is useful for calculating the number of seeds needed for a given area or desired number of plants. Germination rate is determined by the genetic composition, morphological features of seed and by environmental factors.
- Germination capacity- It is the number of seeds able to complete germination from a seed lot.
- Seed Index- weight of 100 seeds in case of bold seeds like maize.
- Test weight- weight of 1000 seeds in case of small seeds like rice.