Leguminous Vegetable Cultivation and Seed Production S ...

SEED PRODUCTION AND MANAGEMENT OF MUNGBEAN AND SOYBEAN 369
Under those circumstances, it is better to harvest at the onset of rain and dry the
plants in a well-ventilated area. They can be air-dried slowly to enable threshing
later.
Threshing and cleaning
High cylinder speeds of combines or threshers will result in multiple fractures and
deep-seated bruises, especially in large-seeded soybean or mungbean and in seeds
with lower moisture content (Moore 1957, 1972; Roberts 1972a). A 700 rpm cylinder
speed is better than 1155 rpm. Soybean seeds impacted at 12-16% seed moisture
germinated satisfactorily; those impacted at 8-10% and 18-20% moisture germinated
poorly (Bunch 1960).
In developing countries, soybean and mungbean usually are threshed by hand,
either by beating pods inside a bag with sticks or by trampling with animals or small
tractors. Such harsh treatment can result in mutilated seeds. Studies at IITA showed
that combine threshing is inferior to hand threshing and beating pods inside a bag
(Nangju et al 1978). Possibly the cylinder speed was not set properly or the seed
moisture content at threshing was too low.
After threshing, the seeds should be cleaned to remove plant debris, weed seeds,
dirt, and other extraneous materials that favor seed deterioration. The seeds should
be examined for purity and variants should be removed. Seeds with mottling due to
SMV should be removed.
Testing for viability and longevity
A number of factors can influence seed quality and viability during production.
Poor seed with low viability will be even poorer after storage. Therefore, seed lots
need to be examined before storage. A good-looking seed may not necessarily be
good quality. Seed viability is usually measured when the seed is planted, but it is
also measured for industrial purposes (MacKay 1978). Incubator germination, field
emergence, tetrazolium, seedling growth rates, and seed leachates are some of the
tests used to evaluate seed viability (Kueneman 1982, MacKay 1978).
Genotypic differences in seed longevity have been reported (Kueneman 1983,
Paschal and Ellis 1978). Within a variety, a number of preharvest and postharvest
factors can influence seed longevity. These tests of seed longevity have been
proposed: seed storage (normal aging of seeds), accelerated aging stress, cold stress,
hot water pregermination stress, osmotic stress, thermo stress during germination,
and methanol stress (Kueneman 1982). Accelerated aging, pregerrnination, thermo,
and methanol stress are recommended for use in breeding for seed longevity.
Seed storage
Factors to consider in storing seeds until planting are initial seed moisture content,
genotype, temperature, RH, the container in which the seed is packed, and the
method of packing. At AVRDC, seeds are sun- or air-dried to 8% moisture content
for medium-term storage at 2-5 °C and 40-45% RH. Under these conditions,
soybean seed can be stored for 20 yr (AVRDC 1985, Cromarty et al 1982). With
8.1-9.4% seed moisture content, soybean seed can be stored at 10 ° C for 10 yr; at 13%