抄録
In 2nd report authors have discussed that the rice plant cultured in pot, i. e. in the isolated conditions, have got different demands for light in order to have its maximum photosynthetic capacity according with its different growing stage, and this differences of demands for light was due to the differences in conditions of receiving the light, which was influenced by the mutual shading rate of leaves within the individual plant. It is quite clear that the ecological conditions of the pot-cultured plant is remarkably different from that of the plant in community. In general, the rice plants are cultured in community conditions at the paddy field, therefore, in order to approach the practical problems of the rice cultivation, the studies of photosynthesis of rice plants must be done in community conditions. In this paper authors have tried to clear the relationships between the spacing of plants and the demands for light by the plants. The effect of light intensity on the photosynthetic rate of plant, which was cultured in community conditions in different spacing, was measured by the "Infrared gas analyser" described in the previous paper. The results obtained were as follows: 1. In community conditions, at the early stage of growfh the demands for light by the rice plant for its maximum photosynthetic rate are comparatively low, and the saturation point lies under about 50 K Lux, and then the demands for light by the plant increases gradually according with the growing stage, after middle stage of growth about full tiller stage) the light satutation phenomenon is extinguished, and the transition from light-saturation to -unsaturation is influenced by the spacing of the plants. The thicker the spacing of the plants, the earlier the extinction of the light-saturation phenomenon. After the extinction of light-saturation phenomenon, the relationships of photosynthesis to light intensity becomes linear gradually, and this is due to the differences in conditions of receiving the light, which is greatly influenced by the mutual shade among the plants. 2. Changes in photosynthetic rate of rice plant in community are remarkably influenced by the growing stage and also by the spacing of the plants. In the thin spacing (36 × 36 cm) and intermediate spacing (24 × 24 cm), the maximum photosynthetic rate is observed at the stage when the most active tillering takes place, then it decreases gradually untill the booting stage, and after heading time it decreases remarkably. On the other hand, the thick spacing (12 × 12 cm) shows its maximuin photosynthetic rate from full tiller stage to booting stage, and after heading time it decreases gradually. The differences of the changes in photosynthetic rate duing to the spacing of the plants may be seemed to be due to the differences of the interrelationships between the top-growth and the uptake abilities of fertilizer duing to the differences in conditions of receiving the light, which is greatly influenced by the spacing of the plants.
