Spatial photosynthesis modelling sets guidelines to constructing a viable single-cell cytoplasm-to-stroma C4 cycle

It has been proposed that introducing C 4 photosynthesis into C 3 crops would increase yield. The simplest scheme in- volves concentrating carbon originating from the cytosol in the chloroplast stroma of mesophyll cells without altering leaf or cell anatomy. Photosynthetic efficiency would then strongly depend on the chloroplast envelope permeability to CO 2 . We examine the performance of this C 4 cycle with a spatial model of carbon assimilation in C 3 mesophyll cell geometry, conducting a thorough exploration of parameter space relevant to C 4 photosynthesis. For envelope perme- abilities below 300 µm/s C 4 photosynthesis has a higher quantum efficiency than C 3 . However, even when envelope permeability is above this threshold, the C 4 pathway can provide a substantial boost to carbon assimilation with only a moderate decrease in efficiency. Depending on the available light-harvesting capacity of plastids, C 4 photosynthesis could boost carbon assimilation anywhere from 20% to 100%. Gains are even more prominent under CO 2 deprivation, and can be achieved in conjunction with lower investment in plastids if chloroplast surface coverage is also altered. A C 4 pathway operating within individual mesophyll cells of C 3 plants could hence lead to higher growth rates and better drought resistance in dry, high-sunlight climates.