Mathematical review of the energy transduction stoichiometries of C4 leaf photosynthesis under limiting light

A generalized model for electron (e-) transport limited C4 photosynthesis of NAD–malic enzyme and NADP–malic enzyme subtypes is presented. The model is used to review the thylakoid stoichiometries in vivo under strictly limiting light conditions, using published data on photosynthetic quantum yield and on photochemical efficiencies of photosystems (PS). Model review showed that cyclic e- transport (CET), rather than direct O2 photoreduction, most likely contributed significantly to the production of extra ATP required for the C4 cycle. Estimated CET, and non-cyclic e- transport supporting processes like nitrogen reduction, accounted for ca. 45 and 7% of total photosystem I (PSI) e- fluxes, respectively. The factor for excitation partitioning to photosystem II (PSII) was ca. 0.4. Further model analysis, in terms of the balanced NADPH : ATP ratio required for metabolism, indicated that: (1) the Q-cycle is obligatory; (2) the proton : ATP ratio is 4; and (3) the efficiency of proton pumping per e- transferred through the cytochrome b6/f complex is the same for CET and non-cyclic pathways. The analysis also gave an approach to theoretically assess CO2 leakiness from bundle-sheath cells, and projected a leakiness of 0.07–0.16. Compared with C3 photosynthesis, the most striking C4 stoichiometry is its high fraction of CE