Mechanism of C₄ Photosynthesis: A Model Describing the Inorganic Carbon Pool in Bundle Sheath Cells Free

A theoretical model of the composition of the inorganic carbon pool generated in C₄ leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO₂ and O₂ in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO₂ concentration of 70 micromolar during steady state photosynthesis in a typical C₄ plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO₂. Under these circumstances the flux of carbon through the C₄ acid cycle would have to exceed the net rate of CO₂ assimilation by 15.5%. With the calculated O₂ concentration of 0.44 millimolar, the potential photorespiratory CO₂ loss in bundle sheath cells would be about 3% of CO₂ assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO₃⁻, the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO₂ and O₂ diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C₄ plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C₄ acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate.