The energy budget in C₄ photosynthesis: insights from a cell-type-specific electron transport model

Extra ATP required in C₄ photosynthesis for the CO₂ -concentrating mechanism probably comes from cyclic electron transport (CET). As metabolic ATP : NADPH requirements in mesophyll (M) and bundle-sheath (BS) cells differ among C₄ subtypes, the subtypes may differ in the extent to which CET operates in these cells. We present an analytical model for cell-type-specific CET and linear electron transport. Modelled NADPH and ATP production were compared with requirements. For malic-enzyme (ME) subtypes, c. 50% of electron flux is CET, occurring predominantly in BS cells for standard NADP-ME species, but in a ratio of c. 6 : 4 in BS : M cells for NAD-ME species. Some C₄ acids follow a secondary decarboxylation route, which is obligatory, in the form of 'aspartate-malate', for the NADP-ME subtype, but facultative, in the form of phosphoenolpyruvate-carboxykinase (PEP-CK), for the NAD-ME subtype. The percentage for secondary decarboxylation is c. 25% and that for 3-phosphoglycerate reduction in BS cells is c. 40%; but these values vary with species. The 'pure' PEP-CK type is unrealistic because its is impossible to fulfil ATP : NADPH requirements in BS cells. The standard PEP-CK subtype requires negligible CET, and thus has the highest intrinsic quantum yields and deserves further studies in the context of improving canopy productivity.