C₄ photosynthesis and CO₂ diffusion

The advantages of C₄ photosynthesis for plant productivity, particularly in warmer climates, are well characterized. High rates of biomass accumulation and high water-use efficiency and N-use efficiency make the installation of the C₄ pathway (or some other form of CO₂-concentrating mechanism) into C₃ plants an attractive proposition for biotechnologists. Here, we compare anatomical properties of leaves of C₃ and C₄ species to compare characteristics of CO₂ diffusion. We show that leaves of a wide variety of C₃ species are characterized by high exposed mesophyll and chloroplast surface area to leaf area ratios (S_mand S_c). Combining measurements of the internal conductance to CO₂ diffusion (derived from measurements of carbon isotope discrimination) with measurements of S_cshows that the CO₂ conductance across the cell wall, plasma membranes, and chloroplast membrane interface is on average 0.02 mol m^-2chloroplast area s^-1bar^-1for C₃ annual species (including rice) and 0.01 to 0.02 mol m^-2chloroplast area s^-1bar^-1for deciduous and evergreen trees. Measurements of anatomical properties of a number of C₄ species show that Sm is less in C₄ species than in C₃ species, but that high photosynthetic rates require higher conductances for CO₂ diffusion across the C₄ mesophyll cytosol interface. There is little variation in bundle sheath surface area to leaf area ratio (S_b), with average values of 1.77 ± 0.11, such that S_mis from 6 to 10 times greater than S_b. Bundle sheath conductance to CO₂ diffusion cannot be measured directly; however, the efficiency of the C₄ photosynthetic pathway can be assessed through measurements of carbon isotope discrimination. Using a mathematical model of C₄ photosynthesis, we examine the relationship between bundle sheath conductance (or its inverse, resistance) to CO₂ diffusion and the biochemical capacity of the C₄ photosynthetic pathway and conclude that bundle sheath resistance to CO₂ diffusion must vary with biochemical capacity if the efficiency of the C₄ pump is to be maintained. Finally, we use a mathematical model of single-cell C₄ photosynthesis in a C₃ mesophyll cell and examine the importance of CO₂ diffusion on such a C₄ photosynthetic CO₂ pump.