Abstract
The author reported in a previous paper(Hosol 1989a, b and 1990) that the degree of cool tolerance at the heading and flowering stages in a rice variety should be expressed by the levels of CLT and the duration of PFP. The levels of critical low temperature(CLT) can be determined by exposing the rice plants to constant low temperatures(17.5 to 22.5°C) at 1°C intervals with a low radiation intensity for 20 days from heading. The CLT for rice varieties is the lowest temperature, at which more than 80% of total spikelets repen under a low radiation intensity from heading. The duration of the period during which fertility is preserved(PFP) can be determined by exposing the rice plants to constant low temperatures at 15'C with a low radiation intensity for 2, 4, 6, 8, 10 and 12 days. The PFP is the lon-gest duration of the period during, which more than 80% of the total spikelets are able to ripen from heading at 15'C under low radiation intensity.
