The energy budget in C(4) photosynthesis: insights from a cell‐type‐specific electron transport model

Extra ATP required in C(4) photosynthesis for the CO(2)‐concentrating mechanism probably comes from cyclic electron transport (CET). As metabolic ATP : NADPH requirements in mesophyll (M) and bundle‐sheath (BS) cells differ among C(4) 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 C(4) 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.