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

Limited mesophyll diffusion conductance to CO2 (g(m)) can significantly constrain plant photosynthesis, but the extent of g(m)-limitation is still imperfectly known. As g(m) scales positively with foliage photosynthetic capacity (A), the CO2 drawdown from substomatal cavities (C-i) to chloroplasts (C-C, C-i-C-C=A/g(m)) rather than g(m) alone characterizes the mesophyll diffusion limitations of photosynthesis. The dependencies of g(m) on A, foliage structure (leaf dry mass per unit area, M-A), 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 g(m). Further the effects of key environmental stress factors and leaf and plant developmental alterations on g(m) and CO2 drawdown were evaluated and the implications of varying g(m) on foliage photosynthesis in the field were simulated. The meta-analysis demonstrated that g(m) of non-stressed leaves was negatively correlated with M-A, and despite the positive relationship between g(m) and A, the CO2 drawdown was larger in leaves with more robust structure. The correlations were stronger with mass-based g(m) 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 g(m) 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 non-senescent older leaves and also in senescent leaves, again reflecting more robust leaf structure and/or non-co-ordinated alterations in leaf photosynthesis and g(m). 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. Accordingly, in harsher environments that support vegetation with tougher long-living stress-tolerant leaves with lower g(m), reductions in stomatal conductance that are common during stress periods are expected to alter photosynthesis less than in species where a larger part of the total diffusion limitation is determined by stomata. While structural robustness improves plant performance under environmental stress, low g(m) and inherently large CO2 drawdown in robust leaves limits the photosynthesis of these plants more severely under favourable conditions when stomatal conductance is high. The differences in overall responsiveness to environmental modifications of plants with varying g(m) need consideration in current large-scale ecosystem productivity models.