A leaf–level biochemical model simulating the introduction of C₂ and C₄ photosynthesis in C₃ rice: gains, losses and metabolite fluxes

Abstract

This work aims at developing an adequate theoretical basis for comparing assimilation of the ancestral C3 pathway with CO2 concentrating mechanisms (CCM) that have evolved to reduce photorespiratory yield losses. We present a novel model for C3, C2, C2+C4 and C4 photosynthesis simulating assimilatory me-tabolism, energetics, and metabolite traffic at the leaf– level. It integrates a mechanistic descrip-tion 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 simu-lations. Inputs were derived with a comprehensive gas exchange experiment. We show trade–offs in the operation of C4 that are in line with ecophysiological data. C4 has the potential to increase assimilation over C3 at high temperatures and light intensities, but this benefit is reversed under low temperatures and light. We apply the model to simulating the introduction of progressively complex levels of CCM into C3 rice, which feeds more than 3.5 billion people. Increasing assimilation will require considera-ble modifications such as expressing the NDH complex and upregulating cyclic electron flow, enlarging the bundle sheath, and expressing suitable transporters to allow adequate metabolite traffic. The simpler C2 rice may be a desirable alternative.