List conditions essential for germination of seeds
Answers
1. Water is absorbed by a dry seed through the micro Pyle and the seed coat. Water performs a number of functions during the germination of seeds.
(a) It softens seed coat and makes it permeable. Increased permeability allows better gaseous exchange.
(b) Water activates the protoplasm of the seed cells.
(c) Insoluble food materials get solubilised in the presence of water which then diffuses from the storage region to the embryo axis.(d) Several enzymes which are essential for growth and germination develop only in the presence of water.
2. Oxygen:
Aeration of the soil is absolutely necessary for the germination of the seed because oxygen is necessary for the aerobic respiration by which the seeds get the requisite energy for the growth of the embryo.
3. Temperature:
Seeds normally germinate within a wide temperature range. However, freshly harvested seeds of several plants germinate only within a narrow temperature range which widens only when after-ripening has taken place.
4. Light:
Plants differ as to the effect of light on their germination. Seeds of many plants are light indifferent or nonphotoblastic, i.e., they are not influenced in the germination by the presence or absence of light. Most of our important crop plants belong to this category. The seeds which are affected by light are described as photoblastic.
Sensitivity to light is a specific character. The photoblastic seeds are of two types, positively photoblastic or light sensitive and negatively photoblastic or light hard. The positively photoblastic seeds require light for germination, e.g., lettuce, tobacco, many grasses and several epiphytes. The negatively photoblastic seeds cannot germinate in the presence of light e.g., Tomato, Onion, Lily, etc.
5. Other factors:
Many orchids and other plants exhibit seed germination only when an appropriate fungus partner is available. Seeds of some parasitic plants will similarly grow only in the vicinity of their host roots because the latter excrete certain growth hormones. Seeds of some aquatic plants germinate only at low or acidic pH.
Internal Factors:
1. Vitality:
The ability of a seed to germinate when provided with optimum condition is described as vitality of the seeds. It is dependent upon its stored food, size, health, etc.
2. Longevity or viability:
With the passage of time a seed looses it power to germinate. Thus each seed has longevity or a period within which it can show renewal of growth or germination. Most of the crop plants lose their viability within 2-5 years.
Legumes ordinarily retain their viability for longer periods. A number of seeds have been recorded to remain viable even after 100 years, (e.g., Trifolium, Astragalus, Mimosa species). Many species remain viable only for one season, e.g., Birch, Elm, Tea.
3. Dormancy:- It is due to the internal conditions of the seed. It is, therefore, also described as the inhibition of the germination due to the internal conditions in an otherwise viable seed. These internal restrictions must be offset before germination can occur in dormant seeds.