Notes on keeping seed cool, clean, dry, and pest-free

This document was writen for Local Seed Network wanting to strore seeds for mid term.

Vegetable seeds are at their peak for storage when they reach maximum dry weight on the mother plant. The most vigorous seeds at harvest time will keep the longest in storage. Vigour is the seed's ability to germinate and grow rapidly with good disease-resistance. Second to breeding, the most important factor in determining the vigour of a seed is the way it is stored.

DRYING

High seed moisture is the greatest single cause of loss of viability and vigour in storage. Exceptions are fleshy seeds of certain tropical fruits, palms and most citrus (termed "recalcitrant seeds"). Harvest and store these in a cool, moist situation without drying. Fresh harvested seed of high moisture content will generate heat if left in bulk. The time-honoured way to dry seed is spread out in thin layers in the sun. This is not satisfactory in humid and hot environments.

The higher the seed moisture, the lower the drying temperature ought to be. Rapid drying using excessive heated air can be harmful. Rapid shrinkage of outer parts of the seed can shorten seed life and impair germination. In general drying temperature should not exceed 42¡C. Be clear that PROPERLY DRIED SEED IN MOISTURE-PROOF CONTAINERS CAN BE STORED IN AREAS OF HIGH HUMIDITY AND THAT STORAGE TEMPERATURES ARE LESS CRITICAL.

Silica gel crystals can absorb moisture from seeds in a sealed container. The colour of the crystals indicates how much moisture has been absorbed - blue for dry, and pink for moist.

Use 1 kg silica gel for 10 kg of seed and packets. Re-activate silica gel at 120¡ C in an oven until the beads are deep blue. They can be dried an indefinite number of times.

TEMPERATURE AND RELATIVE HUMIDITY OF STORAGE ENVIRONMENT

Temperature and Relative Humidity are the most important factors affecting seed quality during storage. Relative humidity is the more important of these. Seeds are hygroscopic i.e., they absorb moisture from wet amospheres and dry out in dry atmospheres.

Seed storage life is doubled for each 1% decrease in moisture content.

The problems of maintaining seed germination increase with seed moisture content. This can be generalised as follows:

Seed moisture above:

- 40-60% - germination occurs

- 18-20% - heating may occur

- 12-14% - fungi grown on and in seed

- 8- 9% - insects become active and reproduce

Heating is caused by the respiration of seeds, of fungi and bacteria in and on the seed, and of insect populations, which may build up rapidly in a moist environment. High temperature and high moisture will kill seeds rapidly as will invasion of microorganisms and insects.

Seed storage life is doubled for each 5¡ C reduction in storage temperature.

Refrigeration to at least 5¡ C is desirable (this is slightly warmer than domestic fridges).

The cooler the temperature the more slowly seed vitality declines. This rule apparently continues to apply even at temperatures below freezing. At 5¡ C and below, insects become inactive.

PEST MANAGEMENT

Before storing any seeds, it is worth making sure that weevils are not already in residence. The eggs of weevils and other insects hide under the seed coat of beans and corn and emerge when the temperature and moisture are right. Two days of freezing inside a sealed container kills most weevils and eggs - although some species of weevils would need much longer at lower temperatures.

African farmers thinly coat beans and other large seeds with edible oil, renewing the oil if weevils are seen. If seed moisture content is maintained below 8% and the storage is partially or completely sealed so that respiration reduces the oxygen content below 14%, insects cannot survive in the seed. Sachets of oxygen absorbers contain, basically, iron filings which absorbs the oxygen; they will go hard when spent.

GENETIC DRIFT in stored seeds during long term storage

Genetic drift is ascribed to at least two causes:-

1. As a stored lot ages, many seeds die and the few that survive are not representative of the same average genetic composition of the original lot.

The answer to this is to store so well that little or no viability is lost, or grow out seeds while they still have good viability, rather than allowing the sample to decline and then regenerating it.

2. Drift, manifested as mutations, occurs as storage time increases. Mutation rates are minimised by best conditions of low seed moisture and cool temperatures.

Classification

Subject  Seed Training
Regional relation  Global
Audience  farmers - general - localseed
Physical format  Web pages