Abstract
The effect of changes in temperature and radiation at the heading and flowering stages on the percentage of perfect grains of rice plants (cv. Fujiminori) was examined under various combinations of constant temperatures (22.5, 20.0, 17.5, 15.0, 12.5 and 10.0°C and alternating temperatures (daytime(12 h.)-nighttime(12 h.): 22.5-17.5, 20.0-15.0, 17.5-12.5, 15.0-l0.0, 12.5-7.5 and 10.0-5.0°C), and radiations (15.4 and 5.5MJ/m2/day) in growth cabinets in plants of various repening stages from heading day. The cool damage at the heading and flowering stages was closely related to the decrease of the radiation, and was ascribed to the decrease of the mean temperature rather than to that of the maximum and minimum temperatures. The critical low temperature from heading of which the rice plants could ripen with 80% of perfect grains was higher under low radiation (5.5MJ) than under normal radiation (15.4MJ). The cool temperature damage at the heading and flowering stages was higher of constant temperatures than of alternating temperatures. The effect of growth duration of rice plants on the critical temperature for ripening from heading was examined under low radiation conditions and at different levels of constant low temperatures. It was revealed that the critical low temperature from heading was not affected by the growth duration of the rice plants. In this experiment, rice plants with different growth durations were grown under controlled growth conditions (10h. short day and 25°C) for panicle initiation seedling at different leaf ages.
