Carbon Dioxide Diffusion inside Leaves

Leaves are beautifully specialized organs that enable plants to intercept light necessary for photosynthesis. The light is dispersed among a large array of chloroplasts that are in close proximity to air and yet not too far from vascular tissue, which supplies water and exports sugars and other metabolites. To control water loss from the leaf, gas exchange occurs through pores in the leaf surface, stomata, which are able to rapidly change their aperture. Once inside the leaf, CO, has to diffuse from the intercellular air spaces to the sites of carboxylation in the chloroplast (for C, species) (Fig. 1) or the cytosol (for C, species). These internal diffusion paths are the topic of this article. There are several reasons why internal diffusion is of interest. First, Rubisco has a poor affinity for CO, and operates at only a fraction of its catalytic capacity in C, leaves. The CO, gradient within the leaf thus affects the efficiency of Rubisco and the overall nitrogen use efficiency of the leaf. Second, prediction of photosynthetic rates of leaves from their biochemical properties requires a good estimate of the partial pressure of CO, at the sites of carboxylation, p c . Third, internal resistance to CO, diffusion results in a lower p c and reduces carbon gain relative to water loss during photosynthesis (water-use efficiency). Considerable effort is being invested selecting and identifying plants with improved water-use efficiency using the surrogate measure of carbon isotope discrimination, A, of plant dry matter (Ehleringer et al., 1993). The ratio of intercellular to ambient CO, partial pressure, pi/p,, and A are both linearly related to water-use efficiency if pc/pi is constant. To date, we have little knowledge of genetic variation in p c / p i . Until recently, it was not possible to directly measure the gradient in CO, partial pressure to the sites of carboxylation. The gradient could be inferred from a theoretical analysis of the diffusion pathway, but because several steps have unknown permeability constants, the values are uncertain. Opinion has oscillated from the existence of large to small gradients over the last 30 years. There are now two techniques that enable the gradient to be measured in C, leaves. After describing these techniques, we will consider diffusion through intercellular air spaces and diffusion across cell walls and liquid phase to sites of carboxylation. Finally, we will examine CO, diffusion into mesophyll cells and across the bundle sheath in C, leaves.