A two-dimensional microscale model of gas exchange during photosynthesis in maize (Zea mays L.) leaves

CO₂ exchange in leaves of maize (Zea mays L.) was examined using a microscale model of combined gas diffusion and C₄ photosynthesis kinetics at the leaf tissue level. Based on a generalized scheme of photosynthesis in NADP-malic enzyme type C₄ plants, the model accounted for CO₂ diffusion in a leaf tissue, CO₂ hydration and assimilation in mesophyll cells, CO₂ release from decarboxylation of C₄ acids, CO₂ fixation in bundle sheath cells and CO₂ retro-diffusion from bundle sheath cells. The transport equations were solved over a realistic 2-D geometry of the Kranz anatomy obtained from light microscopy images. The predicted responses of photosynthesis rate to changes in ambient CO₂ and irradiance compared well with those obtained from gas exchange measurements. A sensitivity analysis showed that the CO₂ permeability of the mesophyll-bundle sheath and airspace-mesophyll interfaces strongly affected the rate of photosynthesis and bundle sheath conductance. Carbonic anhydrase influenced the rate of photosynthesis, especially at low intercellular CO₂ levels. In addition, the suberin layer at the exposed surface of the bundle sheath cells was found beneficial in reducing the retro-diffusion. The model may serve as a tool to investigate CO₂ diffusion further in relation to the Kranz anatomy in C₄ plants.