Response of three typical varieties to high temperature at different stages of growth and on flowering day was studied in naturally lighted rooms of phytotron.
Spikelet sterility was induced by high temperature largely on flowering day. Within the flowering day, high temperature during anthesis time was the most detrimental to spikelet fertility, high temperature right before anthesis being the second most. High temperature after anthesis had little influence on spikelet fertility; when spikelets were exposed to high temperature one hour after anthesis, their fertility was normal. The major causes of high temperature-induced sterility were attributed to disturb pollen shedding and decreased viability of pollen grains, resulting in decreased number of germinated pollen grains on a stigma. The ability of pistil to be fertilized appeared to remain unaffected by high temperature up to 41°C. When spikelets were exposed to high temperature for 8 hours a day, the critical temperature to induce 20% sterility was 36.5°C for N22, heat tolerant, and 32°C for BKN6624, heat susceptible. At 38°C, the critical duration to induce 20% sterility was 4 hours for N22 and 2 hours for BKN6624. At 41°C, 2 hour treatment induced about 90% sterility in all the three varieties. Dihiscence characteristic of anther that allows good pollen shedding and early morning anthesis that avoids high temperature were considered two important characteristics for heat tolerance of rice varieties at flowering.