La Achira en el Sector Agroindustrial

A seed is a living system, and is therefore subject to degenerative or deterioration processes which culminate in death. These processes include loss in cell integrity which affects the biochemical mechanisms controlling the physiological processes of growth. During this deterioration there is a progressive reduction in the rapidity, uniformity and intensity of growth, and a decreasing tolerance to adverse environmental conditions. Vigour therefore decreases as the level of deterioration increases. Factors affecting this physiological response include:

- the environment during seed development, i.e. high temperatures, which induce rapid drying of immature pea seed and produce hollow heart (Halligan, 1986);

- seed maturity at harvest: either premature harvesting of immature seed, or conversely, over maturity through delays in harvesting (Matthews and Powell, 1986);

- seed size: often a larger embryo is correlated with increased seed and seedling vigour (Delouche, 1980; Powell, 1988).

A decline in seed vigour may occur during four stages of seed production; while the seed is still on the mother plant, during harvest, during or in storage. Major factors influencing the extent of ageing are seed moisture content and temperature; an increase in either or both accelerates ageing. Seed ageing on the plant is frequently referred to as "weathering ", reflecting the influence of the weather on these factors (Matthews and Powell, 1986). The degree of seed ageing that occurs in the "weathered " field clearly cannot be controlled, but it is important to recognise that delaying harvest after maturation may result in deterioration due to weather conditions. These losses of vigour occur as a result of membrane damage while the seed i s undergoing development, as influenced by environmental factors (Matthews and Powell, 1986).

The environment plays a vital role in the development of the seed from the flower to seed maturity. It is important that the characteristic changes occurring in the developing seed are well understood in a normally growing legume (Gane et al. , 1984). These physical changes in the seed are coupled with physiological activities. The climatic components of the environment are probably the most important determinants in the location of seed production (Delouche, 1980). Low humidity, minimal rainfall, and favourable temperatures reduce the spread of seed borne diseases as well as the risk associated with inclement weather during the late maturation and harvest periods. The advantages of producing seed in areas specially adapted to seed production are: seed set, seed yield, and recovery in harvesting are high and relatively stable; seed germination and seed vigour are consistently high; and seed borne diseases can often be avoided or are more easily controlled (Delouche, 1980).

Unfavourable conditions may produce drastic effects on the yield and quality of seeds. Browning and George (1981) found that 450 mg N plus 255 mg P, and 450 mg N plus 422 mg P per plant produced pea seeds with a high incidence of hollow heart compared to treatments with lower amounts of P. High nitrogen levels (800 and 1 050 mg N per plant) produced an increase in the number of bleached seeds.

Seeds attain physiological maturity at moisture contents ranging from 32- 35% (e.g. com, sorghum, rice) to 50-55% (e.g. soybeans, peanut, beans, cotton) . . Following maturation the seeds continue to dry down until they reach harvest maturity. Climatic conditions during this post-maturation preharvest period have a great influence on the quality of seed harvested (Delouche, 1980). Adverse weather conditions during the preharvest period cause seed quality problems (Delouche, 1974). Delayed harvest of soybean seed caused by inclement weather resulted in a reduction in viability and an increase in mechanical damage during harvest (Green e t al. , 1 966). Conversely, immature seeds at harvest lower the percentage viability of the lot and have a greater mortality in soil (Matthews, 1973). Immature seeds have high solute leaching into steep water because the high moisture content at harvest results in more damage during harvest and seed processing compared with mature seeds (Matthews, 1973). Heavy rainfall just prior to harvest reduces the percentage viability of dried seed and increases the leaching of solutes from the dried seed into steep water.

2 . 2 . 3. THE IMPORTANC E OF SEED INTACTNESS

To obtain sound and healthy seed, the crop must be harvested under good conditions and handled with care (Gane et ai., 1984). There is plenty of evidence which shows that however good the initial quality of the seeds, they deteriorate more rapidly if they have been injured in some way (Roos, 1 980). Freedom from mechanical damage is an 'accessory' factor, unrelated to the inherited or original physiological potential of the seeds;

but it is nevertheless thought by many seed technologists that its absence is frequently the key to the maintenance of the vigour of seeds (Hey decker,

1972).

One of the most important causes of low vigour in pea seed is seed coat cracking incurred during harvesting very dry seeds (Gane et al. , 1984). Mechanical damage, quite apart from curtailing the food supply available during germination and the early stages of growth, or even injuring the embryonic organs, provides a focus for colonisation by saprophytes (Heydecker, 1972). The effect is insidious and probably widespread. Colonised necrotic areas in seeds almost always increase in size during storage, and damaged and colonised seeds become agronomically useless well ahead of uninjured seeds even if they are still, strictly speaking, viable, because the attack,-once started, is almost impossible to arrest (Christensen and Suer, 1982).

Damage may be caused during maturation even without any interference by man (Delouche, 1980; Perry, 1980; Matthews and Powell, 1986). More frequently it is, however, due to faulty threshing or processing methods (Gane et al. , 1984) or careless handling of sacks of seeds in transit (Gleeson, 1987).