Mechanism of C4 Photosynthesis

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Abstract
A theoretical model of the composition of the inorganic carbon pool generated in C14 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%. Within the calculated O2 concentration of 0.44 millimolar, the potential photorespiratory CO2 loss in bundle sheath cells would be about 35% 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.