Seed longevity of rice cultivars and strategies for their conservation in genebanks

Changes in seed quality during ripening were studied in sixteen cultivars of rice, representing the three ecogeographic races of Oryza sativa, and one cultivar of O. glaberrima, grown during one dry season (Nov.-May) 1992-1993 at Los Banos, Philippines. Mass maturity (defined as the end of seed filling period) among the cultivars was attained between 18.5 and 21.6 d after anthesis (DAA). The seed moisture content at mass maturity varied between 24 and 40%. Germination ability of seeds in the early stages of development varied significantly, but as mass maturity approached, germination increased to the maximum and no significant differences were found among cultivars. The seeds were stored hermetically at 35 degrees C with 15 +/- 0.2% moisture content and the resultant seed survival data were analysed by probit analysis. Potential longevity (quantified by the value of seed lot constant Ki of the seed viability equation) was greatest between 33 and 37 DAA, i.e. about 2 weeks after mass maturity. The stage during development at which seeds achieve maximum potential longevity is described by the term storage maturity. Lowland japonica cultivars, large seeded accessions (seed mass greater than or equal to 40 mg) and O. glaberrima had shorter storage longevity (standard deviation of the frequency of seed deaths in time = 1.47 weeks) while cultivars with purple pericarp survived longer than other cultivars (r = 2.33 weeks). The initial germination of the japonica cultivars at storage maturity was high (99-100%) and the estimates of maximum potential longevity (Ki) which ranged between 3.3 (Shuang cheng nuo) and 4.4 (Minehikare) were close to those of the indica cultivars. This research suggests that seed production environment between Nov. and May at Los Banos is benign for the temperate japonica cultivars. The implications of these results on management of rice genetic resources are discussed.