Estimation of Mesophyll Conductance to CO(2) Flux by Three Different Methods

The resistance to diffusion of CO(2) from the intercellular airspaces within the leaf through the mesophyll to the sites of carboxylation during photosynthesis was measured using three different techniques. The three techniques include a method based on discrimination against the heavy stable isotope of carbon, (13)C, and two modeling methods. The methods rely upon different assumptions, but the estimates of mesophyll conductance were similar with all three methods. The mesophyll conductance of leaves from a number of species was about 1.4 times the stomatal conductance for CO(2) diffusion determined in unstressed plants at high light. The relatively low CO(2) partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O(2) sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO(2) partial pressure upon switching to low O(2). We found no correlation between mesophyll conductance and the ratio of internal leaf area to leaf surface area and only a weak correlation between mesophyll conductance and the proportion of leaf volume occupied by air. Mesophyll conductance was independent of CO(2) and O(2) partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO(2)-accumulating mechanisms was found. Some plants, notably Citrus aurantium and Simmondsia chinensis, had very low conductances that limit the rate of photosynthesis these plants can attain at atmospheric CO(2) level.