During photosynthesis, CO₂ moves from the atmosphere (C_a) surrounding the leaf to the sub‐stomatal internal cavities (C_i) through stomata, and from there to the site of carboxylation inside the chloroplast stroma (C_c) through the leaf mesophyll. The latter CO₂ diffusion component is called mesophyll conductance (g_m), and can be divided in at least three components, that is, conductance through intercellular air spaces (g_ias), through cell wall (g_w) and through the liquid phase inside cells (g_liq). A large body of evidence has accumulated in the past two decades indicating that g_m is sufficiently small as to significantly decrease C_c relative to C_i, therefore limiting photosynthesis. Moreover, g_m is not constant, and it changes among species and in response to environmental factors. In addition, there is now evidence that g_liq and, in some cases, g_w, are the main determinants of g_m. Mesophyll conductance is very dynamic, changing in response to environmental variables as rapid or even faster than stomatal conductance (i.e. within seconds to minutes). A revision of current knowledge on g_m is presented. Firstly, a historical perspective is given, highlighting the founding works and methods, followed by a re‐examination of the range of variation of g_m among plant species and functional groups, and a revision of the responses of g_m to different external (biotic and abiotic) and internal (developmental, structural and metabolic) factors. The possible physiological bases for g_m, including aquaporins and carbonic anhydrases, are discussed. Possible ecological implications for variable g_m are indicated, and the errors induced by neglecting g_m when interpreting photosynthesis and carbon isotope discrimination models are highlighted. Finally, a series of research priorities for the near future are proposed.

Mesophyll diffusion conductance to CO(2) is a key photosynthetic trait that has been studied intensively in the past years. The intention of the present review is to update knowledge of g(m), and highlight the important unknown and controversial aspects that require future work. The photosynthetic limitation imposed by mesophyll conductance is large, and under certain conditions can be the most significant photosynthetic limitation. New evidence shows that anatomical traits, such as cell wall thickness and chloroplast distribution are amongst the stronger determinants of mesophyll conductance, although rapid variations in response to environmental changes might be regulated by other factors such as aquaporin conductance. Gaps in knowledge that should be research priorities for the near future include: how different is mesophyll conductance among phylogenetically distant groups and how has it evolved? Can mesophyll conductance be uncoupled from regulation of the water path? What are the main drivers of mesophyll conductance? The need for mechanistic and phenomenological models of mesophyll conductance and its incorporation in process-based photosynthesis models is also highlighted.

The effects of short‐term (minutes) variations of CO₂ concentration on mesophyll conductance to CO₂ (g_m) were evaluated in six different C₃ species by simultaneous measurements of gas exchange, chlorophyll fluorescence, online carbon isotope discrimination and a novel curve‐fitting method. Depending on the species, g_m varied from five‐ to ninefold, along the range of sub‐stomatal CO₂ concentrations typically used in photosynthesis CO₂‐response curves (A_N–C_i curves; where A_N is the net photosynthetic flux and C_i is the CO₂ concentrations in the sub‐stomatal cavity), that is, 50 to 1500 µmol CO₂ mol⁻¹ air. Although the pattern was species‐dependent, g_m strongly declined at high C_i, where photosynthesis was not limited by CO₂, but by regeneration of ribulose‐1,5‐bisphosphate or triose phosphate utilization. Moreover, these changes on g_m were found to be totally independent of the velocity and direction of the C_i changes. The response of g_m to C_i resembled that of stomatal conductance (g_s), but kinetic experiments suggested that the response of g_m was actually faster than that of g_s. Transgenic tobacco plants differing in the amounts of aquaporin NtAQP1 showed different slopes of the g_m–C_i response, suggesting a possible role for aquaporins in mediating CO₂ responsiveness of g_m. The importance of these findings is discussed in terms of their effects on parameterization of A_N–C_i curves.

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.