Abstract
The following methods were used for obtaining the relation between rice yield and photosynthetically active solar radiation during seed ripening stage, in different areas in Japan. The yield was prefectural average yield, and the seed ripening stage was the period from average heading time to average harvesting time in each prefecture. Photosynthetically active solar radiation was cited from the data measured by the Group I-1 in the Green Energy Project of the Ministry of Agriculture, Forestry and Fisheries. Among these more than 20 measurement sites, 12 in 1979 and 11 in 1980 were selected on the basis of the availability of the data during the seed ripening stage and of normal crop situation index (91-109)(Tables 1 and 2). The accumulated photosynthetically active solar radiation during seed ripening stage was calculated by the summation of photosynthetically active radiation on every day at each measurement site, for the seed ripening stage in each prefecture in which each site locates (Table 2). Figs. 1 to 3 show the correlation between the rice yield and the photosynthetically active solar radiation during the seed ripening stage, for combined and individual data in 1979 and 1980, respectively. The correlations were significant at 0.1, 1 and 0.1 percent level, respectively. On the other hand, no correlation was found between the yield and average temperature during the ripening stage (Fig. 4). In other words, it was shown that rice yield was proportional to the photosynthetically active solar radiation during seed ripening period. This conclusion should be noted to have several conditions which were discussed in this paper : For example, there is no big difference among farmers in different prefectures in Japan for the level of rice cultivation techniques. The implications of this conclusion, however, is rather wide and important. As an example of these implications, the so-called "heat tolerance" problem in the southwestern region in Japan was discussed.
