Studies on Dry Matter Production of Soybean Plant : 3. Photosynthetic rate of soybean plant population as affected by proportion of diffuse light.

The relation of photosynthesis to light condition was investigated with soybean plant population. Diurnal changes of photosynthesis, horizontal light intensity both total and diffuse were recorded on 3 or 4 successive days of various weather conditions. From these data, light-photosynthesis relation was investigated with refernce to the proportion of diffuse light component to the total light. On the other hand, the manner of light distribution on photosynthetic system was investigated under various light conditions by measuring the illumination intensity on individual leaves. The results obtained were as follows: (1) Photosynthetic rate of soybean plant population depended not only on total amount of light but also on its proportion of diffuse light. When the total light intensity was same, the photosynthetic rate varied with the proportion of diffuse light, that is, the photosynthetic rate increased with increase in the proportion of diffuse light. (2) Under fine or clear condition the proportion of diffuse light was very low and many sun flecks were formed on photosynthetic system of the population. A major part of light was concentrated to these limited regions, resulting in a uneven distribution of light on photosynthetic system. On the contrary, under hazy or clowdy condition, the proportion of diffuse light was higher and distribution of light on photosynthetic system was relatively even horizontally as well as vertically. (3) Rates of the photosynthesis under various light conditions were calculated from illumination intensity at surface of leaves and light-photosynthesis curve of single leaf. Then, the photosynthetic rates so obtained were compared with each other. Under same total light intensity, the calculated rate was consistently higher when the proportion of diffuse light was higher, Further, the ratio of the two rates, calculated under higher as well as lower proportion of diffuse light, was fairly coincided with the same kind ratio of the two rates obtained by direct measurment of photosynthesis of population. From this, it was concluded that a higher photosynthetic rate under a higher proportion of diffuse light based mainly on more favorable distribution of light on photosynthetic system-even distribution of light.