Abstract
How to increase the amount of photosynthetic products per unit land area in the growth period after heading under luxuriant growth conditions is the most serious problem in maximizing yield in rice cultivation. As increasing the light-receiving efficiency after heading by controlling the growth of rice plants was considered to be a new approach to the problem, the authors therefore examined the relation between morphological characteristics (the form of plant) and the photosynthetic efficiency of rice plants. The main points of the results may be summarized as follows. 1. Under sufficient light intensities no difference was found in photosynthetic effciency between a leaf-blade placed at a right angle to the sunlight and that placed horizontally, and a little difference was found between a leaf-blade at a right angle and that placed parallel to the sunlight (cf. Table 1). 2. No difference was found in photosynthetic activity between the obverse of a leaf-blade and the reverse of it (cf. Table 2). 3. Permeant sunlight which had passed through a leaf-blade was of no use on photosythesis (cf. Table 4). 4. The photosynthetic efficiency of a leaf-blade was always lower when it was curved than when it was straight, and the fact was more clearly observed when a comparison was made between plants or plant communities whose leaf-blades were curved and those whose leaf-blades were straight (cf. Table 3 & 6). 5. In case of the leaf-area being equal with each other, a plant with leaf-blades which are short in length and large in number was much higher in photosynthetic efficiency than that with leafblades which are long in length and small in number (cf. Table 7). 6. On the basis of the above mentioned facts, it was suggested that (1) leaf-blades of the ideal plant should be erect, straight and short in length and large in number, and (2) the type of the ideal plant should be heavy tillering so that the plant may increase light-receiving efficiency. 7. Judging from the photosynthetic efficiency, the percentage of ripened grains and the grain yield in actual paddy fieid, an optimum leaf-area index at heading stage was nearly 6.5 to 7.5. However, taking the resistance of plants against lodging, diseases and pests into account, 6.0 seemed to be optimum (cf. Fig. 2 & 5). 8. From the results of photosynthetic efficiency in actual fields, under luxurious growth conditions the most efficient form of rice plants in photosynthetic efficiency in the period after heading was likely to be progessively short in the length of leaf-blades from the 4 th or 3 rd leaf from above up to the upper-most leaf as the position of leaves on a stem goes up (cf. Fig. 4).
