Contrasting anatomical and biochemical controls on mesophyll conductance across plant functional types

Mesophyll conductance (g_m ) limits photosynthesis by restricting CO₂ diffusion between the substomatal cavities and chloroplasts. Although it is known that g_m is determined by both leaf anatomical and biochemical traits, their relative contribution across plant functional types (PFTs) is still unclear. We compiled a dataset of g_m measurements and concomitant leaf traits in unstressed plants comprising 563 studies and 617 species from all major PFTs. We investigated to what extent g_m limits photosynthesis across PFTs, how g_m relates to structural, anatomical, biochemical, and physiological leaf properties, and whether these relationships differ among PFTs. We found that g_m imposes a significant limitation to photosynthesis in all C₃ PFTs, ranging from 10-30% in most herbaceous annuals to 25-50% in woody evergreens. Anatomical leaf traits explained a significant proportion of the variation in g_m (R²  > 0.3) in all PFTs except annual herbs, in which g_m is more strongly related to biochemical factors associated with leaf nitrogen and potassium content. Our results underline the need to elucidate mechanisms underlying the global variability of g_m . We emphasise the underestimated potential of g_m for improving photosynthesis in crops and identify modifications in leaf biochemistry as the most promising pathway for increasing g_m in these species.