Seed is viable and has a potential to develop into a new plant. When the seed is produced it goes into dormancy for some time. This dormancy has to be preserved till the seed is used in the next season, next year or after some years.

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Appropriate climate for storing seeds

The problem of storing seeds in an appropriate manner to ensure proper germination in the next season is as old as agriculture itself. The discovery that drying of seeds led to fairly satisfactory preservation of quality from the consumption point of view must have led to the adoption of drying as a seed storage practice, on the assumption that good table quality automatically ensures good viability and vitality. This is not surprising since even the earlier scientific theories on seed viability postulated that the loss in viability was due to embryo starvation brought about by respiratory depletion of food reserves. The fact that non-viable seeds appear as rich in food reserves as the viable ones and that seeds with abundant food reserves deteriorate more rapidly than those with lesser amounts goes against the Embryo Starvation Theory. The exact mechanism responsible for deterioration in seed quality is even today not fully understood. A decrease in the enzymatic activity of glutamic acid decarboxylase and the mutagenic effects of the end products of respiration under adverse storage conditions (leading to aberrations in embryonic tissue and normal cell division) are considered to be the possible causes.

Though respiratory depletion of food reserves is not held to be the cause of seed quality deterioration, the theories advanced for seeds becoming non-viable are related to the respiratory process as the primary cause. Again, the secondary effects of respiration appear to become operational under adverse conditions of storage and after a considerable lapse of time. The requirements for satisfactory preservation of seed quality, therefore, appear to be those that would result in a marked reduction in seed respiration and its maintenance at very low rates over long periods, say about 2 years. This is in view of the need to use the seeds in the next season and to keep a sufficient stock, in case of total failure of seed crop in the ensuing season. The respiration rate is very greatly influenced by the moisture content of seeds and the temperature conditions.

Seeds dried to a moisture content at 4% level show negligible respiratory activity while in those with over 10% moisture level seed respiration begins to increase rapidly. Unlike other plant parts where the decline in respiration may set in at about 30 0C, seed respiration continues to increase up to 500C. Under high relative humidity (90%) and high temperature (more than 320C) most of the seeds become non-viable in a short period of time. Low temperature and low humidity enhance the life of the seed.

Many of the high-yielding varieties have non-dormant seeds and hence run the risk of loss of viability by quiscent germination and absorption of atmospheric moisture in storage. For example, the vitality of the second crop varieties of paddy suffer a significant loss in viability even during the period of 9 months from harvest to next sowing season, if stored in ordinary gunny bags after air drying for a fortnight after harvest. The viability came down to 30% and the loss in viability was stepped up during rainy and winter months, since moisture absorption was facilitated by high relative humidity (R.H) and low temperature. Drying of the seeds to a moisture level of 11% is vital for maintaining viability. Thus, initial stages of seed multiplication, the need for keeping every kilo of seed in top condition of viability is apparent.

Air drying of seeds even in open does not reduce the moisture content of seeds to a low level for ever. Seeds being hygroscopic tend to attain a moisture content which is in equilibrium with the R.H. of the air to which they are exposed. Moisture equilibria values, on per cent wet weight basis, for seeds of various crops and vegetables, at different relative humidities and a constant temperature of 25 0 C given by James (1967) are set out in Tables 1,2 and 3.

Table1: Absorbed moisture content of field seed in equilibrium with air of various relative humidities at room temperature (approximately 250C)a

Table2: Approximate moisture content of vegetable seeds in equilibrium with air at different relative humdities at room temperature (approximately 250 C)a

Table3: Moisture of seed packaged

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