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 CO₂ among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO₂ and water vapor are 2.3 times higher than in air. For fixed estimated CO₂ pressure at the evaporating surfaces of the leaf (pⁱ), 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 CO₂ 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 CO₂ pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO₂ and H₂O are known. Finally, we describe a diffusion-biochemistry model for CO₂ assimilation that demonstrates the plausibility of our experimental results.