Anatomical basis of variation in mesophyll resistance in eastern Australian sclerophylls: news of a long and winding path

In sclerophylls, photosynthesis is particularly strongly limited by mesophyll diffusion resistance from substomatal cavities to chloroplasts (r _m), but the controls on diffusion limits by integral leaf variables such as leaf thickness, density, and dry mass per unit area and by the individual steps along the diffusion pathway are imperfectly understood. To gain insight into the determinants of r_m in leaves with varying structure, the full CO₂ physical diffusion pathway was analysed in 32 Australian species sampled from sites contrasting in soil nutrients and rainfall, and having leaf structures from mesophytic to strongly sclerophyllous. r_m was estimated based on combined measurements of gas exchange and chlorophyll fluorescence. In addition, r_m was modelled on the basis of detailed anatomical measurements to separate the importance of different serial resistances affecting CO₂ diffusion into chloroplasts. The strongest sources of variation in rm were Sc/S, the exposed surface area of chloroplasts per unit leaf area, and mesophyll cell wall thickness, tcw. The strong correlation of r_m with tcw could not be explained by cell wall thickness alone, and most likely arose from a further effect of cell wall porosity. The CO₂ drawdown from intercellular spaces to chloroplasts was positively correlated with tcw, suggesting enhanced diffusional limitations in leaves with thicker cell walls. Leaf thickness and density were poorly correlated with S_c/S, indicating that widely varying combinations of leaf anatomical traits occur at given values of leaf integrated traits, and suggesting that detailed anatomical studies are needed to predict r_m for any given species.