Seeds are dormant forms of life of most plant. They are the products of fertilization of male and female gametes from plants. From seeds seedling and plant sprout out spontaneously under appropriate conditions. The seed is the way plants propagate their offspring. Some of them fall near the plant while some are carried over by winds, animals etc to far off places. But where ever and whatever, the method of germination of seed is always the same for a particular plant.
Seeds have an embryo which is the actual diploid cell having life. Besides the embryo, reserve food material necessary for seed germination is present. It mostly contains starch, fats and other reserve food material with least amount of moisture. Hence the seed appears very dry and can be stored for many years without any damage by infection etc.
Stages involved in plant seed Germination :
Seed germination means the growth of embryo inside the seed into seedling and plant. Most of the higher plants produce fruits which also bear the seeds. These seeds fall down from the plant and get spread to different places by wind, grazing animals etc.
There are three stages of seed germination viz.
1. Imbibition phase.
2. Latent phase.
3. Exponential growth phase.
Imbibition phase:
When a seed fall into soil and receives sufficient water and moisture, the actual process of germination starts.
The water permeates through the seed coat and the seed material inside is drenched.
The internal seed content imbibes (absorbs) water and the seed swells greatly providing eruption of outer coat for germination.
Latent phase:
The food material inside the seed now in presence of water from surroundings provide sufficient energy for embryo to sprout out of the seed as a seedling. There is internal active metabolism for the seed and it is a bit long time-consuming process. The metabolic reaction is active and there is release of heat. This stage makes the seed ready to sprout out.
Exponential growth phase:
All the biochemical reaction get activated and plant growth hormones then guide the process of germination.
By regulation of auxin a growth hormone, the root tends to grow into the soil while the stem forms and grows towards light. This process continues till the germination of seed into small plant is complete.
Till the level of formation of leaves, the growth of plant is sustained by reserve food material, water and oxygen from air.
Once leaves form, food is synthesized from light using carbon-dioxide and water.
Factors affecting germination of seeds:
Seed germination is affected by many factors like
Rains /water supply, climate, season, predators, infection to seeds, fertility of soil, temperature, sun light, Oxygen etc. Among them, rains, water supply, soil fertility, temperature at optimum levels favor germination while predator, rough climates inhibit seed germination.
Water: Seeds are most driest forms and they require water for germination. It helps seed in
- Activation of enzymes to digest reserve food
- To rupture seed coat
- To mediate all the physiological processes of seed.
Temperature:
- Soil temperature for seed germination is critical. Both low temperatures and high temperatures are not suitable for seed growth. Because enzymes are not active at low temperatures and physiology of growth is slow while at high temperatures they get denatured.
Oxygen:
- Oxygen is vital to oxidize reserve food and provide energy for seed germination. Without oxygen seed do not germinate as seen in seeds sown very deep in the soil.
Sunlight:
- Seed germination varies in plants due to light. Some require light for germination while in other germination is inhibited by light. But light seems to guide direction of roots and stem from the seed.
INTERNAL FACTORS:
Seed dormancy:
- Seeds germinate immediately after being sown. But some seeds stay inactive in the soil for some time period and then go for germination. This period of rest is called seed dormancy.
Viability:
Seeds ability to germinate varies on the method of storage, health of the parent, age of seed, maturity of seed, infections etc. So the seeds with good viability germinate faster.
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