Seed longevity in two sorghum varieties with contrasting dormancy level prior to harvest

Abstract

Seed longevity was studied in two Sorghum varieties which differ markedly in their dormancy level during seed development: Redland B2, which presents high germination indices from well before physiological maturity (PM) and IS 9530 whose seeds are dormant until some weeks after crop harvest. Seeds collected at different days after pollination (DAP) were stored under accelerated ageing conditions (15% moisture content; 35°C). The study was conducted in two years with contrasting rainfall amount during the maturation period: 1993 (98.5 mm fallen between -5 to +20 days from PM) and 1995 (223.1 mm fallen between -5 to +20 days from PM). Care was taken in using seeds with no evident sign of sprouting (i.e visible radicle) for the ageing experiments. During 1993 estimates of Ki (potential longevity) gave similar values for both varieties and almost did not change significantly with time of harvest. During 1995, estimates for Ki in IS 9530 seeds resembled those obtained during 1993. Estimates in Redland B2 seeds, in contrast, were markedly lower during 1995 than those obtained during 1993. The initiation of the germination process in the mother plant in Redland B2 seeds during the 1995 season, was evidenced by an alpha-amylase activity in unsprouted grains that was, from PM onwards, 100-fold higher than that found in IS 9530 seeds. In both varieties, maximum longevity was achieved in seeds harvested 5 days (1993) to 10 days (1995) after physiological maturity, which would determine the harvest time to obtain Sorghum seeds of high quality. These results suggest that the initiation of the germination process derived from a low dormancy level prior to crop harvest combined with heavy rains during that period, leads to a decrease in potential longevity of Sorghum seeds.