Diffusion of CO2 in the intercellular airspaces of the leaf mesophyll is one of the many processes that can limit photosynthetic carbon assimilation there. This limitation has been largely neglected in recent years, but both theoretical and empirical evidence is presented showing that it can be substantial, reducing CO2 assimilation rates by 25% or more in some leaves. Intercellular diffusion is fundamentally a three-dimensional process, because CO2 enters the leaf through discrete stomata, and not through a uniformly porous epidermis. Modelling it in one dimension can cause major underestimation of its limiting effects. Resistance and conductance models often fail to account well for the limitation, in part because they are usually one-dimensional representations, but also because they treat continuously interacting processes as if they were sequential. A three-dimensional diffusion model is used to estimate the intercellular limitation in leaves of different structural types. Intercellular gaseous diffusion probably limits CO2 assimilation by at most a few percent in many of the agricultural plants commonly studied by laboratory physiologists, as they usually have thin, amphistaomatous leaves. However, it may cause substantial limitations in the thicker, often hypostomatous leaves of the wild plants that occupy large areas of the earth. Several physiological implications of intercellular diffusion are discussed. I close by reiterating published suggestions that the gas-exchange parameter commonly termed pi should be renamed pes (es for evaporating surfaces) to avoid the confusing implication that p1, is‘the’intercellular CO2 pressure.
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