Role of mesophyll diffusion conductance in constraining potential photosynthetic productivity in the field

Limited mesophyll diffusion conductance to CO2 (gm) can significantly constrain plant photosynthesis, but the extent of gm-limitation is still imperfectly known. As gm scales positively with foliage photosynthetic capacity (A), the CO2 drawdown from substomatal cavities (Ci) to chloroplasts (CC, Ci–CC¼A/gm) rather than gm alone characterizes the mesophyll diffusion limitations of photosynthesis. The dependencies of gm on A, foliage structure (leaf dry mass per unit area, MA), and the resulting drawdowns across a dataset of 81 species of contrasting foliage structure and photosynthetic potentials measured under non-stressed conditions were analysed to describe the structure-driven potential photosynthetic limitations due to gm. Further the effects of key environmental stress factors and leaf and plant developmental alterations on gm and CO2 drawdown were evaluated and the implications of varying gm on foliage photosynthesis in the field were simulated. The meta-analysis demonstrated that gm of non-stressed leaves was negatively correlated with MA, and despite the positive relationship between gm and A, the CO2 drawdown was larger in leaves with more robust structure. The correlations were stronger with mass-based gm and A, probably reflecting the circumstance that mesophyll diffusion is a complex three-dimensional process that scales better with mesophyll volume-weighted than with leaf area-weighted traits. The analysis of key environmental stress effects on gm and CO2 drawdowns demonstrated that the effect of individual stresses on CO2 drawdowns varies depending on the stress effects on foliage structure and assimilation rates. Leaf diffusion limitations are larger in nonsenescent older leaves and also in senescent leaves, again reflecting more robust leaf structure and/or nonco- ordinated alterations in leaf photosynthesis and gm. According to simulation analyses, in plants with a larger part of the overall diffusion conductance from the ambient atmosphere to the chloroplasts in the mesophyll, photosynthesis is less sensitive to changes in stomatal conductance.