Studies on the Leaf Orientation-Adjusting Movement in Soybean Plants : I. The leaf orientation-adjusting movement and light intensity on leaf surface

It is well known for long that a soybean leaf slters its inclination in response to the intensity or derction of light on the leaf surface. This study has for its object to find quantitatively the actual state of leaf orientation-adjusting movement and to clarify the relationship of the adjusting movement with the high-yielding ability of soybeans, thus attempting to apply the findings to the high-yielding soylbean variety breeding project. In the present report, the author makes report on the results of the study which was conducted in 1965 to obtain the general information on the leaf orientation-adjusting movement in soybean plants and the changes in its effect on the light intensity on the leaf surface. 1. In early September, 1965, the author made the measurement on the entire leaves of one hill of the soybean plant comunity of Azeminori variety which was planted on May 19 and grown on a one-piece-per-hill basis, 60 cm between rows and 20 cm between hills; i. e., the azimuth of leaves and the angle of leaf inclination were measured under different climatic conditions, and the ligh intensity on the leaf surface was measured as well. In the present report, the author should like to make the report on each of the typical cases observed under both of cloudy and fine weather. 2. About the direction of leaf, the leaves having an angle of 45°with the direction of row were predominant in number (fig. 2), but abut the angle of leaf inclination, a marked difference was observed according to the climatic conditions (fig. 3), i. e., under the cloudy weather, the leaf surface which inclined upward at the angle of 0°to 30°showed an increase, while under the fine weather, horizontal leaves decreased and such leaves which inclined downward at the angle of more than 15°increased. A marked difference was observed in the adjusting movement between the two coditions. 3. Regarding the distributtion percentage of leaf number and leaf area according to the ligh intensity on the leaf surface, the distribution percentag was even to a fairly great extent under the cloudy weather, while under the fine weather, two types quite different in the distribution percentage were observed, i.e., one was of unsaturated type with low light intensity on the leaf surface, and anothor was of saturated type with high light intensity on the leaf surface (fig. 4). This fact seems to endose the light saturation theory concerning the photosynthesis of soybean plant oommunity that "even if the light becomes stronger, the light intensity on the surface of the leaves of light insaturated type shows no increase, causing an increase only in the light intensity on the surface of the leaves already saturated with light. As a result, the light becomes unavailable even though it becomes stronger". However, when applied a different measuring method, it would give rise to a different relationship between the increase in the external light and the increase in the light intensity on the leaf surface (as shown in fig. 4, the distribution percentage of leaf number and leaf area differs under the fine weather). Moreover, there is no gainsaying that, with the changes in the direction of the sunlight, the light intensity on the surface of the leaves insaturated with light in the lower layer of soybean plant community will show an increase to a fairly great extent owing to the sunbeams that may come through. Accordingly, it is considered that the light saturation phenomenon in the photosynthesis of soybean plant community of fairly luxuriant growth can be observed only in the specific cases. For this reason, the established light saturation theory concerning the photosynthesis of soybean plant community is required to be re-examined. 4. Respecting the relationship of the direction and inclination of leaves with light intensity on the leaf surface, no specific trend was observed under the cloudy weather. the rest omitted