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Selective pressures on C4 photosynthesis evolution in grasses through the lens of optimality

CO2, temperature, water availability and light intensity were potential selective pressures to propel the initial evolution and global expansion of C4 photosynthesis in grasses. To tease apart the primary selective pressures along the evolutionary trajectory, we coupled photosynthesis and hydraulics models and optimized photosynthesis over stomatal resistance and leaf/fine-root allocation. We also examined the importance of nitrogen reallocation from the dark to the light reactions. Our results show that the higher stomatal resistance and leaf/root allocation ratio conferred by the C4 photosynthesis led to C4 advantage without any change in hydraulic conductance. For the initial evolution of C4 25-32 MYA, water limitation was the primary driver, and nitrogen reallocation was necessary. Low CO2, together with light intensity, were the primary drivers during the global radiation of C4 5-10 MYA, during this period nitrogen reallocation would not have been strongly selected.