Abstract
I. Effects of temperatures and light intensities. In 1955 and 1956, rice plants were subjected to 20 conditions differently controlled by temperatures and light intensities in different 10-day periods in growth, and the compound effects of temperature and light intensity on the ripening of rice grains were examined. 1) Under compound conditions of high temperatures and reduced light intensities, most unfavourable effects on ripening were brought about, and the effects of reduced light intensities were the more intesitive under the higher temperatures, especially on the milk-ripe stage. (cf. Figs. 1 and 2.) 2) It was considered to be worthy of attention that a high constant temperature (31-32°C) as well as a low temperature (21-20°C) in reduction division stage seemed to reduce the percentage of fertilized grains to a considerable extent. 3) An optimum temperature for ripening on the ripening stage seemed to be 22°C, approximately, when the temperatures were kept constant during the day and the night, and higher temperatures than 25°C showed unfavourable effects on ripening of rice grains. And it was made clear from the following experiment that high temperatures in nighttime are much more harmful on ripening of rice grains than those in daytime. II. Effects of the range of daily temperature. In 1956, rice plants were subjected to different ranges of daily temperature in different 10-day periods of their growth, to examine the effects of temperature ranges on the ripening of rice grains. 1) Marked effects on ripening were found in the periods from reduction division stage to yellow-ripe stage, and, in most cases, favourable effects were brought about by the ranges of 10°C or 15°C, but the optimum ranges for ripening varied with day temperatures and growth stages. 2) An important fact was found that there exists an optimum night temperature for ripening on each stage of growth independently of day temperaures, as seen in Fig.3. It was considered worthy of notice that optimum night temperatures on the ripening stages seem to be considerably low. 3) On the other hand, an optimum day temperature for ripening on each growh stage could hardly be seen, except for reduction devision stage. (An optimum day temperature on reduction devision stage seemed to be about 31°C.)
