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.

Two highly contrasting variables summarizing the efficiency of transport of materials within the leaf are recognized as playing central roles in determining gas exchange and plant performance. This paper summarizes current approaches for the measurement of mesophyll conductance to CO₂ (g_m) and leaf hydraulic conductance (K_leaf) and addresses the physiological integration of these parameters. First, the most common methods to determine g_m and K_leaf are summarized. Next, novel data compilation is analysed, which indicates that, across diverse species, g_m is strongly linked with gas exchange parameters such as net CO₂ assimilation (A_area) and stomatal conductance (g_s), and with K_leaf, independently of leaf vein length per leaf area. Based on their parallel responses to a number of environmental variables, this review proposes that g_m is linked to the outside-xylem but not to the xylem component of K_leaf. Further, a mechanistic hypothesis is proposed to explain the interactions among all these and other physiological parameters. Finally, the possibility of estimating g_m based on this hypothesis was tested using a regression analysis and a neurofuzzy logic approach. These approaches enabled the estimation of g_m of given species from K_leaf and leaf mass per area, providing a higher predictive power than from either parameter alone. The possibility of estimating g_m from measured K_leaf or vice-versa would result in a rapid increase in available data. Studies in which g_m, K_leaf, and leaf mass per area are simultaneously determined are needed in order to confirm and strengthen predictive and explanatory models for these parameters and importantly improve resolution of the integrated hydraulic-stomatal-photosynthetic system.