Optimal coordination and reorganization of photosynthetic properties in C4 grasses

Abstract

C3 and C4 are major functional types in terrestrial biosphere models, with photosynthesis traits as important input parameters. The evolution of C4 required reorganizations of Calvin-Benson-cycle and coordination of C4-cycle enzymes, resulting in divergences of physiological traits between C3 and C4. In addition, photosynthesis further optimized after the evolution of C4 causing diversification within C4 lineages due to different evolutionary histories. We combined optimality modeling, physiological measurements and phylogenetic analysis to examine how various aspects of C4 photosynthetic machinery were reorganized and coordinated within C4 lineages and as compared to closely-related C3 in grasses. Optimality models and measurements indicated a higher maximal electron transport to maximal Rubisco carboxylation ratio (Jmax/Vcmax) in C4 than C3, consistent with the optimal prediction to maximize photosynthesis. The coordination between Calvin-Benson and C4 cycles (Vpmax/Vcmax), however, is in line with the optimal modeling results under 200 ppm, as opposed to current CO2. Such inconsistencies can be explained by a slowly declining assimilation rate beyond optimal Vpmax/Vcmax. Although rapid coordination occurred early in C4 evolution, C4 is still under optimizing processes and photosynthetic measures have continued to increase across time. Lastly, better understandings of Jmax/Vcmax, Vpmax/Vcmax and fluorescence-based-electron-transport proffer enhanced approaches to parameterize terrestrial biosphere models.