The Dependence of the Relation between Light Quality and Photosynthesis on Optical and Anatomical Characteristics of Leaves

Abstract

The relationship was investigated between stomatal resistance and CO2 absorption rate under different light-qualities. The action spectrum of photosynthesis of leaves was discussed from the view point of CO2 diffusion resistance. CO2 absorption rate and stomatal resistance were measured at the upper and the lower surfaces of cabbage, cucumber and lettuce leaves under an irradiation of three different lights (blue-green, yellow-red and red) from either the upper or the lower surface side of the leaf.Results were as follows:(1) Stomatal resistance of lettuce-leaf was smaller at the surface receiving the light than that at the opposite surface in every light. The resistances of cabbage and cucumber-leaves were smaller at the lower surfaces than those at the upper surfaces regardless of the direction of irradiation. The stomatal resistance at the light-receiving-surface was the smallest under blue-green light in all plants. At the nonirradiated surface, the light quality caused some effect on stomatal resistances in lettuce but not in cabbage and cucumber.(2) CO2 absorption rate was larger at the surface with a smaller stomatal resistance than that with a larger resistance for all plants. But, CO2 absorption at each surface was not always large under the light situation from which a smaller stomatal resistance was derived. At the lower surface the stomatal resistance did not so remarkably affect CO2 absorption rate. At the upper surface, the stomatal resistance was the limiting factor of CO2 absorption for cabbage and cucumber, but not for lettuce.(3) The ratio of CO2 absorption rate at the upper surface to that at the lower surface varied with the light-quality and the irradiation direction for all plants. In the case of irradiation directed to the upper surface, the ratio for lettuce was the largest and those for cucumber and then cabbage followed in every light. It is considered that the differences in photosynthetic action among plants were caused by the differences in the distribution, the activity of stomata, and in the anatomical characteristics of leaves.(4) The relative value of photosynthesis in blue region in the action spectrum of photosynthesis increased with increasing ratio of the upper surface to the lower one in CO2 absorption rate in the case of irradiation directed to the upper leaf surface. This phenomena was explained by the difference in the extinction coefficient for light within a leaf. The blue light has a large extinction coefficient within a leaf and is mostly absorbed near the surface layer. Therefore, higher the ratio, the higher photosynthetic activity occurred in blue region.