Exploring anatomical controls of C₄ leaf photosynthesis using a 3D reaction-diffusion model

C₄ plants such as maize (Zea mays L.), sugarcane (Saccharum officinarum L.) and sorghum [Sorghum bicolor (L.) Moench.] photosynthesize at a high rate due to a CO₂ concentration mechanism (CCM) that accumulates CO₂ to saturating concentrations around the carboxylation site of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Transport of CO₂ inside a leaf and, therefore, the CCM, is affected by leaf microstructure. Gas transport models with a realistic leaf microstructure help to assess the significance of anatomical features of C₄ plants quantitatively for effective CCM. One- and two-dimensional gas transport models, when applied to analyze the gas diffusion in leaves, understate the three-dimensional nature. Here, we present the first 3D reaction-diffusion model for photosynthesis of a C₄ leaf. Equations of CO₂ transport and bicarbonate diffusion combined with a biochemical model of C₄ photosynthesis were discretized over the 3D geometry of a maize leaf tissue. The model could describe the trends in responses of photosynthesis to light and CO₂. The CO₂ profile in the leaf microstructure was highly heterogeneous. The model suggests that rapid diffusion of CO₂ to mesophyll cytosol is essential to achieve a high rate of photosynthesis.