0023-6152

AA00697606

22800

Unrefereed

0023-6152

AA00697606

Objective Uptil recently the ceiling in the rice yield of the south western regions of Japan was considered to be put by the high temperature, specially the high night temperature of the ripening period of these regions - in other words the small differences in the day and night temperatures at the ripening period. But the recent changes in the trend towards higher yield of the regions of northern Kyushu brought discrepancies to such concept. From the works so far been done it could not be said with certainty that prevalence of the high temperature at the ripening period actually produced ill effect on the rice yield, and one of the objectives of this experiment was to confirm this effect. The second ceiling in the rice yield in the regions of Kyushu started posing a problem and another objective was to clarify both physiologically and ecologically, the relationship of the high temperature at the ripening period with the yield and to find out some definite way to break this ceiling in yield. Experimental methods According to the objectives mentioned before, representing two rice croppings (annual) of the south western Kyushu, that is, early season and ordinary, different varieties Koshihikari and Hoyoku were used for respective experiments. In each experiment, three plots were prepared on the basis of the temperature (nigh) of the ripening period, High night temperature plot (about 2-3℃ higher than that of the average temperature), Control and Low night temperature plot (about 2-3℃ lower than that of the average temperature) and the ripening conditions of the different plots were examined and at the same time the changes in the dry weight contents, leaf area, nitrogen contents and carbohydrate contents, assimilation of dry matters, consumptions through respiration and translocation of carbohydrate were observed. Results and discussions In both the seasons the low night temperature plot showed the highest yield. And in the early season the Control plot showed the next higher yield and the high night temperature plot showed the lowest. Whereas in the ordinary cropping season the High night temperature plot showed the second highest yield and the Control plot the lowest. 1. The reasons for the Low night temperatures ‘plots’ showing highest yield in both the seasons were that, on the one hand, the decrease in the activity of the assimilatory organs with time was checked by the low temperature of the plot and on the other hand the decrease in the power of receptibillity of the organs as ‘sink’ with time was also checked by the low temperature of the plot. 2. One of the reasons of the High night temperature plot’s showing the lowest yield in the early cropping season was that though the rate of translocation increased with the high temperature of the plot the amount of ‘source’ became limited by the excessive consumption due to high temperature. Moreover, the decrease in the ability of the assimilatory organs and in the receptibility of the grains were enhanced by high. temperature of the plot. 3. In the ordinary cropping season the High night temperature plot showed a little higher yield than that of the Control plot and the reasons were as follows: One of the reasons was that in the case of the ordinary cropping, the season was autumn and the temperature of the High night temperature plot was relatively lower in comparison with the early cropping season and as a result the consumptions through the respiratory activities were comparatively lower. Moreover, as the temperature of the High night temperature plot was higher than that of the Control plot, the rate of translocation was rapid. Though the temperature of the Control plot was lower than that of the High night temperature plot and the dry matter production was comparatively large due to the low temperature the rate of translocation was slow and as the night temperature of this plot was higher than that of the Low night temperature plot, the fallling in the ability of the organs as ‘sink’ with time was enhanced and the receptibility of the grains as a consequence decreased. And for this reason it could be assumed that the panicle weight of this plot could not reach as high as that of the High night temperature plot. 4. There are many factors which determine the grain yields, and these factors are variously inter-related. Finally, if it is assumed that the stored materials of the grains are starch, then at the ripening stage specially, the synthesizing of starch, its translocation and the receptibility of the organs are considered to be very important factors. The effect of high night temperature at the ripening period is considered to be determined by the degree of its action on the interrelationships of these three main important factors. And so far as the relation of temperature (in this case night temperature) with these factors are concerned, it could be assumed that for each of these factors an optimum temperature exists.