Components of mesophyll resistance and their environmental responses: A theoretical modelling analysis

Mesophyll resistance (r_m ), stomatal resistance, and biochemical limitations are recognized as three critical factors limiting leaf photosynthesis. Contrary to the expectation of being a constant, r_m not only varies with light and CO₂ conditions but also shows different responses among species. To elucidate the mechanistic basis of these responses, we derived an analytical model of r_m , which incorporates various anatomical and biochemical factors including permeabilities of cell wall and chloroplast envelope to CO₂ and HCO₃^- , carbonic anhydrase activities in cytosol and stroma, Rubisco activities, and relative location of mitochondria and chloroplast. The robustness of this model was confirmed by comparing the predicted r_m and its components to numerical models developed at cell and leaf levels, which incorporate detailed 3-dimensional cell and leaf anatomies, CO₂ hydration and diffusion processes from intercellular air space to stroma, and CO₂ fixation by Rubisco. A combination of these model analyses shows that the varying r_m is influenced by four biochemical factors: (a) nonuniform photosynthesis status across the leaf, (b) photorespiration and respiration, (c) bicarbonate leakage on the chloroplast envelope, and (d) hydration activity in cytosol and stroma. This study provides a theoretical framework to study components of r_m and their responses to environmental perturbations.