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

Increasing the mesophyll conductance to CO 2 (g m ) is considered 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 ten 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 then 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 the 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 anatomical model estimated g m values 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 membrane and chloroplast stoma are taken into account, the model can 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 cell wall effective porosity and effects of facilitation of CO 2 transport across membrane and chloroplast stroma in addition to the thicknesses of the elements.