Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

The conductance of carbon dioxide (CO2) from the substomatal cavities to the initial sites of CO2 fixation (g(m)) can significantly reduce the availability of CO2 for photosynthesis. There have been many recent reviews on: (i) the importance of g(m) for accurately modelling net rates of CO2 assimilation, (ii) on how leaf biochemical and anatomical factors influence g(m), (iii) the technical limitation of estimating g(m), which cannot be directly measured, and (iv) how g(m) responds to long- and short-term changes in growth and measurement environmental conditions. Therefore, this review will highlight these previous publications but will attempt not to repeat what has already been published. We will instead initially focus on the recent developments on the two-resistance model of g(m) that describe the potential of photorespiratory and respiratory CO2 released within the mitochondria to diffuse directly into both the chloroplast and the cytosol. Subsequently, we summarize recent developments in the three-dimensional (3-D) reaction-diffusion models and 3-D image analysis that are providing new insights into how the complex structure and organization of the leaf influences g(m). Finally, because most of the reviews and literature on g(m) have traditionally focused on C-3 plants we review in the final sections some of the recent developments, current understanding and measurement techniques of g(m) in C-4 and crassulacean acid metabolism (CAM) plants. These plants have both specialized leaf anatomy and either a spatially or temporally separated CO2 concentrating mechanisms (C-4 and CAM, respectively) that influence how we interpret and estimate g(m) compared with a C-3 plants.