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

The resistance to diffusion of CO₂ 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, ¹³C, 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 CO₂ diffusion determined in unstressed plants at high light. The relatively low CO₂ partial pressure inside chloroplasts of plants with a low mesophyll conductance did not lead to enhanced O₂ sensitivity of photosynthesis because the low conductance caused a significant drop in the chloroplast CO₂ partial pressure upon switching to low O₂. 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 CO₂ and O₂ partial pressure during the measurement, indicating that a true physical parameter, independent of biochemical effects, was being measured. No evidence for CO₂-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 CO₂ level.