Crop-Scientific Studies on the Yield-Forecast of Lowland Rice (Preliminary report) : XXV. Carbon assimilation of rice plant under natural conditions and its application to the yield-forecast

As the plant chamber of the apparatus reported in the previous paper was higher in temperature and in humidity than under natural condition, the authors made an effort to improve the defects by setting up a refrigerator (2.H.p.), resulting in a good success. Using the improved apparatus, they made further experiments and tried to apply the results to the yield-forecast. 1. Relation between the rate of carbon assimilation and light intensity. The relation of light intensity to the rate of carbon assimilation in different growing stages showed as much the same relation as that of the previous paper, as shown in Fig. 1. Light saturating points were found at the intensity of approximately 0.6 cal./cm2./min. in the period before the heading time, and in subsequent period the intensity seemed to begin to fall off day by day, as seen in the former report. In spite of using 6 varieties differing in earliness and in ecotype, varietal differences of saturating light intensity could hardly be seen so far as this experiment was concerned. An there were no clear differences regarding saturating intensity among plants grown in differently fertilized conditions, as shown in Fig. 2. Moreover, even in the conditions differing in spacing and in temperature differences of saturating light intensity could also hardly be distinguished, though the experiments were not enough to ascertain. 2. Diurnal changes of the rate of carbon assimilation. Diurnal changes of the rate of carbon assimilation on fine days were closely resembling to those of the previous paper. From this experiment it was reaffirmed that the stronger light intensity than 0.6 cal./cm2./mln. is of no use for carbon assimilation. Diurnal changes on partly cloudy days or cloudy days were invesigated, as shown in Fig. 3. From these figures it can be drawn out that the assimilating activity holds almost constant in the higher light intensity than the saturating point (0.6 cal.), but it changes almost completely parallel to light intensity in the lower intensity than the saturating one. 3. Seasonal changes of the rate of carbon assimilation. Seasonal change of the carbon assimilation on middle maturing variety (Norin-No. 15) was traced up in 1953-'54, and the results in Fig. 4 were obtained. As the curve of 1953 in Fig. 4 was obtained under the higher condition by 10°C than the normal and the weather of the year was abnormally bad, the curve seemed to be less reliable than that of 1954 as the seasonal change in general. The curve in 1954 shows that maximum rate of photosynthesis as a whole plant occurs at the maximum stage in number of tillers........the stage is aiso the beginning stage of young panicle formation........and the second maximum occurs at heading time, and the former is much higher than the latter. 4. Application to the yield-forecast. From the resuts above mentioned it was made clear that relative amount of carbon assimilation in rice-plants at a given stage can be estimated by measuring light intensity to a considerable extent. Using daily data of sunlight intensity obtained from "Robich solarimeter", covering the period from transplanting time until ripening in 1951-'54 the authors tried to calculate the index of daily relative amount of cabon assimilation and arranged them in mean value for 10 days as written in Table 1. For instance, in the first decade of July in 1951 the relative amount of carbon assimilation is 69.8% as compared with that of the case of complete fine weather. The index of relative amount of carbon assimilation has to be corrected by the differences of the day-length and of the rate of photosynthesis in each decade. Moreover, the inexed at the stage of reduction division and of milk-ripening are necessary to be weighted, because the amounts of carbon assimilation at those two stages have always marked effects upon the yield of rice. [the rest omitted]