Anatomy of CO2 diffusion in leaf photosynthesis

Abstract

In some C3 species, Cc can be as low as half the ambient CO2 in the air. This clearly indicates that resistance to CO2 diffusion within the leaf is substantial. Affinity of Rubisco to CO2 is low. The presence of oxygen further lowers the carboxylation rate. Moreover, the photorespiration path consumes energy and releases CO2 . Clearly, it is advantageous for the leaf to minimize resistance to CO2 diffusion to increase photosynthesis. We will discuss problems concerning the resistance to CO2 diffusion from the mesophyll cell surface to the chloroplast stroma ('internal' resistance) from several viewpoints. 1) There is a correlation between cell wall thickness and the internal resistance. The clear correlation was also observed during the leaf development. In broad-leaved evergreen trees and some alpine plants, large internal resistance may be mainly attributed to their thick cell walls. 2) When Hg2+, an inhibitor of the water channel, was fed via the petiole of Phaseolus vulgaris, the internal resistance of the leaf increased by several, which suggests that CO2 would diffuse through the water channels. 3) Sun leaves should have larger chloroplast surface area facing the intercellular spaces per unit leaf area (Sc) than shade leaves to conduct efficient photosynthesis at high Cc level. This may be an answer to the question why sun leaves are thicker. We also show how sun and shade leaves differentiate in response to light environment. The mature leaves rather than the differentiating leaves per se sense the light environment.