COS 20-10 - Selective pressures on C4 photosynthesis evolution in grasses through the lens of optimality

Monday, August 7, 2017: 4:40 PM
D138, Oregon Convention Center
Haoran Zhou, Biology, University of Pennsylvania, Department of Biology, PHILADELPHIA, PA, Brent Helliker, Biology, University of Pennsylvania, Philadelphia, PA and Erol Ak├žay, Department of Biology, University of Pennsylvania
Background/Question/Methods CO2, temperature, water availability and light intensity were potential selective pressures to propel the initial evolution (25-32 MYA) and global expansion of C4photosynthesis in grasses (5-10 MYA). We teased apart the primary selective pressures along the evolutionary trajectory through the lens of an optimality model. 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.

Results/Conclusions At CO2 concentration above 400 ppm, water limitation was the primary selective factor for C4 evolution, and even at 600 ppm there is room for C4 evolution under the driest conditions. Furthermore, we find that the carbon concentration mechanism in C4 (CCM) alone leads to enough of a reduction in water use that there would have been little selection for increased hydraulic conductance within C4 grasses. When CO2 decreased below 300 ppm, high light intensity provided an enhanced advantage for C4. Low CO2, together with light intensity, were the primary drivers during the global radiation of C4 5-10 MYA, occurring hand-in-hand with grassland expansion. Subsequent to the evolution of CCM, selection could favor the reallocation of nitrogen from dark to light reactions. the selection pressure to reallocate would have been strongest when CO2 was high, i.e., during the initial evolutionary events, when the CCM alone does not give C4 a large advantage. As CO2 decreased during the C4 radiation 5-10 MYA to Pleistocene glacial periods, the CCM alone would give C4 an advantage and selection of reallocation would lessen. However, as COincreased again from Pleistocene, our modeling results show the N reallocation may play a major role in maintaining the high productivity of C4 species in current CO2 concentration.