Studies on Photosynthesis in Sugar Beet Plant : III. Photosynthesis in the community.

Abstract

The effect of light on the photosynthesis in the community of sugar beet plants was studied from the view point of intensity of light received by the individual leaves of the community. The results obtained are as follows. (1) Light response curves of photosynthesis in community obtained by calculation on the basis of light intensity on the surface of individual leaves and light response curves of photosynthesis of individual leaves, showed the same tendency as those obtained by direct measurement on photosynthesis of community. (2) These light response curves are light saturation curves, having a saturation point at the horizontal light intensity of ca. 40 Klx. The similar fact is seen in the communities of different leaf area indices (LAI) ranging from 0.9 to 5.6. Light receiving coefficient (ρ) under the incident horizontal light intensity where the photosynthesis shows saturation phenomenon, is lower than 1, and the higher the LAI of community, the lower the ρ. (3) When the horizontal light intensity of incident light becomes higher, some leaves in community receive more and more intense light. However, there remain some leaves on which light intensity does not increase, regardless of the increment of incident light intensity. Even though horizontal light inetnsity gets higher than 30 or 40 Klx, the area of leaves that receive light intensity below the saturation point, does not decrease, and their mean light intensity does not increases either. Consequently photosynthetic rates of these leaves remain at low level. On the other hand, accompanying with the increase of incident light intensity, the photo synthetic intensity of another leaves will become higher untill light intensity of these leaves reaches at saturation point of single leaves. These circumstances cause the light saturation phenomenon in photosynthesis in community. (4) The percentage of leaves whose photosynthetic intensity remains at low level, increases as the LAI of community becomes higher, resulting in the lower ρ in the community of larger LAI.