Leaf anatomy does not explain the large variability of mesophyll conductance across C 3 crop species.

Increasing mesophyll conductance of CO 2 (g m ) is a strategy to improve photosynthesis in C 3 crops. However, the relative importance of different anatomical traits in determining g m in crops is unclear. Mesophyll conductance measurements were performed on 10 crops using the online carbon isotope discrimination method and the 'variable J' method in parallel. The influences of crucial leaf anatomical traits on g m were evaluated using a one-dimensional anatomical CO 2 diffusion model. The g m values measured using two independent methods were compatible, although significant differences were observed in their absolute values. Quantitative analysis showed that cell wall thickness and chloroplast stroma thickness are the most important elements along the diffusion pathway. Unexpectedly, the large variability of g m across crops was not associated with any investigated leaf anatomical traits except chloroplast thickness. The g m values estimated using the anatomical model differed remarkably from the values measured in vivo in most species. However, when the species-specific effective porosity of the cell wall and the species-specific facilitation effect of CO 2 diffusion across the membrane and chloroplast stoma were taken into account, the model could output g m values very similar to those measured in vivo. These results indicate that g m variation across crops is probably also driven by the effective porosity of the cell wall and effects of facilitation of CO 2 transport across the membrane and chloroplast stroma in addition to the thicknesses of the elements.