A leaf‐level biochemical model simulating the introduction of C 2 and C 4 photosynthesis in C 3 rice: gains, losses and metabolite fluxes

Summary This work aims at developing an adequate theoretical basis for comparing assimilation of the ancestral C 3 pathway with CO 2 concentrating mechanisms ( CCM ) that have evolved to reduce photorespiratory yield losses. We present a novel model for C 3 , C 2 , C 2  + C 4 and C 4 photosynthesis simulating assimilatory metabolism, energetics and metabolite traffic at the leaf level. It integrates a mechanistic description of light reactions to simulate ATP and NADPH production, and a variable engagement of cyclic electron flow. The analytical solutions are compact and thus suitable for larger scale simulations. Inputs were derived with a comprehensive gas‐exchange experiment. We show trade‐offs in the operation of C 4 that are in line with ecophysiological data. C 4 has the potential to increase assimilation over C 3 at high temperatures and light intensities, but this benefit is reversed under low temperatures and light. We apply the model to simulate the introduction of progressively complex levels of CCM into C 3 rice, which feeds > 3.5 billion people. Increasing assimilation will require considerable modifications such as expressing the NAD(P)H Dehydrogenase‐like complex and upregulating cyclic electron flow, enlarging the bundle sheath, and expressing suitable transporters to allow adequate metabolite traffic. The simpler C 2 rice may be a desirable alternative.