Exploiting differences in the energy budget among C 4 subtypes to improve crop productivity

Summary C 4 crops of agricultural importance all belong to the NADP‐malic enzyme (ME) subtype, and this subtype has been the template for C 4 introductions into C 3 crops, like rice, to improve their productivity. However, the ATP cost for the C 4 cycle in both NADP‐ME and NAD‐ME subtypes accounts for > 40% of the total ATP requirement for CO 2 assimilation. These high ATP costs, and the associated need for intense cyclic electron transport and low intrinsic quantum yield Φ CO 2 , are major constraints in realizing strong improvements of canopy photosynthesis and crop productivity. Based on mathematical modelling, we propose a C 4 ideotype that utilizes low chloroplastic ATP requirements present in the nondomesticated phospho enol pyruvate carboxykinase (PEP‐CK) subtype. The ideotype is a mixed form of NAD(P)‐ME and PEP‐CK types, requires no cyclic electron transport under low irradiances, and its theoretical Φ CO 2is c . 25% higher than that of a C 4 crop type. Its cell‐type‐specific ATP and NADPH requirements can be fulfilled by local energy production. The ideotype is projected to have c . 10% yield advantage over NADP‐ME‐type crops and > 50% advantage over C 3 counterparts. The ideotype provides a unique (theoretical) case where Φ CO 2could be improved, thereby paving a new avenue for improving photosynthesis in both C 3 and C 4 crops.