Analysis of Yield-Determining Process and Its Application to Yield-Prediction and Culture Improvement of Lowland Rice : CIII. On factors affecting the light-curves of carbon assimilation in rice plant communities

Abstract

The light saturation point is someimes found and sometimes not found on the light-curves of carbon assimilation in rice plant communities, the authors therefore have examined various factors affecting the light-curves, except the leafiness which has already been proved to be closely related to the light-curves. The following factors have further been pointed out to affect the light-curves. 1) Light incidence angle: Even if the horizontal light intensity is identical, if the sun altitude changes, the light-receiving amount on each leaf in a rice plant community changes, and, as a result of it, the light-curve of carbon assimilation changes (cf. Fig. 1). 2) Light incidence direction: In case of the rice plants being planted in rows and their leaves being unevenly distributed in a plant community, the solar direction clearly affects the light-curves, while in case of the rice plants being evenly planted and their leaves being also evenly distributed the solar direction does not affect the light-curves at all (of. Fig. 3). 3) Plant type: Even if the leaf-area-indexes in plant communities are equal with each other, the plant type in the plant cnmmunity clearly affects the light-curves of carbon assimilation, namely the light saturation point is liable to be found on the light-curves of carbon assimilation in the plant community with long and drooped leaves, while the saturation point can hardly be found on the light-curve in the plant ocmmunity with short and errect leaves (cf. Fig. 5 and 6). 4) Nitrogen content in leaves: Eyen if the leafinesses in plant communities are identical, the lower the nitrogen content in lsaves, the more liable the light saturation point to be found on the light-curves of carbon assimilation in plant communities (cf. Fig. 7). 5) Water content in leaves: The water content in leaves decreases at mid-day after heading, and moreover, it decreases the more in leaves which receives the stronger light and also it decreases the more in plants whose root-activity is the more damaged, and the rate of carbon assimilation in plant communities progressively decreases with the decreasing water content in leaves. Accordingly, the light saturation point is liable to be found on the light curves in plant communities with decayed roots (cf. Fig. 8 and 9). 6) Amount of air sent into a plant chamber: The less the amount of air sent into a plant chamber, the more liable the light saturation point to be found on the light-curves of carbon assimilation in plant communities (cf. Fig. 10). 7) Humidity in air: Under high humidity conditions the rate of carbon assimilation does not go down and consequently the light saturation point can scarcely be found, while under low humidity conditions the water content in leaves decreases and the rate of carbon assimilation decreases as well, and consequently the light saturation point is liable to be found on the light curves of carbon assimilation. 8) From the above mentioned results it has come to be clarified that the light curve of carbon assimilation is not only affected by the leafiness in plant communities, but also it is clearly affected by several factors mentioned above, and consequently, however leafy a plant community may be, the light saturation point can frequently be found on the light-curves of carbon assimilation in the plant community.