Studies on CO<SUB>2</SUB> Exchange in Crop Plants. : III. The effect of light intensity and spacing on the photosynthetic rate of rice-seedlings.

Abstract

In the previous paper the authors have discussed that the rice plants after transplanting have got different demands for light in order to have their maximum photosynthetic capacity accompanying with their different growing stage, and this differences of demand for light by the plants was due to the differences in conditions of receiving the light, which was greatly influenced by the mutual shading rate of leaves. It is quite clear that the spacing differences of seedlings in the nursery bed is remarkably different from that of the plants after transplanting, therefore, in this paper authors have tried to clear the relationship between the spacing of seedlings and the demands for light by the seedlings. The effect of light intensity on the photosynthetic rate of seedlings in different spacing, that is thin, intermediate and thick, and also in the different growing stages of the plant was measured by the aparatus described in the previous paper. The results obtained were as follows: 1) At the early stage of growth (4.1 leaf stage) the demands for light by the plants for their maximum photosynthetic rate are comparatively low, and the saturation points lie under 30, 50 and 80 k. Lux respectively. 2) With the progress of growth, the demands for light of thick seedlings increase remarkably (Fig.2∼7), and at the 5.2 leaf stage, that is at the middle stage of growth, the saturation point does not exist within the range of below 100 K. Lux (Fig.6); on the othet hand, the demands for light by the intermediate and thin spacing seedlings increase gradually, and at the 7.5∼7.7 leaf stage, that is at final stage, the saturation point of the former lies above 100 K.Lux (Fig.12), and that of the latter lies below 80 K.Lux. 3) Relation between the changes of demands for light and the growth of seedlings in shown is Fig 16. It has been observed that from the 9th August the increasing rate of dry weight of the thick seedlings were less in comparison with that of intermediate and thin seedlings, and from 15th August the increasing rate of dry weight of the intermediate seedlings were less than that of the thin seedlings. 4) At the final stage, the photosynthetic rate of thick seedlings were less than that of intermediate under the light intensities below 80K. Lux (Fig.12). This relation is also shown in Fig.14 and 15. Fig.14 is the growth curve obtained from the seedlings grown under the unfavourable climatic conditions, and the seedlings were effected by the unfavourable climatic conditions especially at the middle and final stage of their growth, and accordind to the obtained figure, it can be said that the increasing rate of dry weight of thick spacing seedlings were less than that of intermediate seedlings. Fig.15 was obtained from the seedlings grown under the favourable climatic conditions, according this figure, it may be said that the increasing rate of dry weight of thick spacing seedlings were equal to that of intermediate seedlings. In conclusion, it can be said that at the middle stage of growth in the nursery bed the thick spacing seedlings have got no saturation point under the natural conditions, on the other hand, thin spacing seedlings have got saturation point till the final stage to come. In intermediate spacing seedlings, which is common in nursery bed in Japan, the saturation point is comparatively low at the early stage, but-with the progress of growth it increased gradually, and finally reached to the state of having no saturation point under natural conditions.