Mechanism of C4 Photosynthesis

A theoretical model of the composition of the inorganic carbon pool generated in C(4) leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO(2) and O(2) in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO(2) concentration of 70 micromolar during steady state photosynthesis in a typical C(4) plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO(2). Under these circumstances the flux of carbon through the C(4) acid cycle would have to exceed the net rate of CO(2) assimilation by 15.5%. With the calculated O(2) concentration of 0.44 millimolar, the potential photorespiratory CO(2) loss in bundle sheath cells would be about 3% of CO(2) assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO(3) (-), the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO(2) and O(2) diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C(4) plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C(4) acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate.