Changes in germination ability and longevity were monitored during seed development in three japonica rice cultivars and one indica rice cultivar sown on three different dates, 14 October 1993, 24 November 1993 and 5 January 1994 at Los Baños, Philippines. Germinability of the seeds varied among cultivars in the early stages of development, but it was generally similar across sowing dates. The estimates of potential longevity (determined by storage at 40°C and 15% moisture and quantified by the values of the seed lot constant K1 of the viability equation) differed among cultivars and sowing dates. While the maximum potential longevity attained across different sowing dates was similar in the japonica cultivars Ju ku and Chianan 8, it was significantly higher in the first sowing than in the second or third sowings in cv. Akihikari. In the indica cultivar IR 58, maximum potential longevity attained in the second and third sowings was significantly greater than that in the first sowing. The mean potential longevity, averaged over cultivars and sowing dates, was marginally higher in the first sowing (K1=3.81) than in the second (3.65) and third (3.63); and averaged over cultivars and harvest times it was greatest at 34.8 days after flowering (DAF) in the first sowing, 31.8 DAF in the second and 28.3 DAF in the third. These results suggest that sowing in mid-October to allow seed ripening to coincide with the cooler and drier segment of the Los Baños dry season, and harvesting 35 DAF can improve the potential longevity of some japonica cultivars.

Changes in seed dry weight, moisture content and germinability of three japonica cultivars of rice during development and maturation in the dry season (November–May) field multiplication at Los Baños, Philippines were compared with those in a cooler controlled environment (24°/18°C). Under field conditions, maximum dry weight accumulation, which indicates mass maturity, was attained 18–21 days after flowering in all cultivars. In the cooler environment however, mass maturity was achieved 21 days after flowering in Fujisaka 5 and Minehikare, and 28 days in Shuang cheng nuo. The moisture content of the seeds at mass maturity varied between 28 and 35% among cultivars in the two environments. Although the ability of seeds to germinate differed in early harvests (7 and 14 days) among cultivars and across two environments, germination of the mature seeds was similar in all. There were no significant differences in potential longevity of the seeds harvested at 28, 35 and 42 days after flowering within each cultivar, which was determined by the germination of seeds that had been hermetically stored with 15% moisture content at 35°C. Differences among cultivars within each seed production environment were also not significant. However, potential longevity of the seeds harvested from the cooler environment was higher than those from the field (P<0.05), which confirmed that japonica cultivars which evolved under temperate regions are sensitive to warmer seed production environments. However, the magnitude of differences in potential seed longevity between the two environments was not high. This indicates that the seed production environment during the dry season at Los Baños is not harsh for japonica cultivars.