Intercellular Diffusion Limits to CO(2) Uptake in Leaves : Studies in Air and Helox

We studied plants of five species with hypostomatous leaves, and six with amphistomatous leaves, to determine the extent to which gaseous diffusion of CO(2) among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO(2) and water vapor are 2.3 times higher than in air. For fixed estimated CO(2) pressure at the evaporating surfaces of the leaf (p(i)), assimilation rates in helox ranged up to 27% higher than in air for the hypostomatous leaves, and up to 7% higher in the amphistomatous ones. Thus, intercellular diffusion must be considered as one of the processes limiting photosynthesis, especially for hypostomatous leaves. A corollary is that CO(2) pressure should not be treated as uniform through the mesophyll in many leaves. To analyze our helox data, we had to reformulate the usual gas-exchange equation used to estimate CO(2) pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO(2) and H(2)O are known. Finally, we describe a diffusion-biochemistry model for CO(2) assimilation that demonstrates the plausibility of our experimental results.