Optimal coordination and reorganization of photosynthetic properties in C 4 grasses.

Each of >20 independent evolutions of C ₄ photosynthesis in grasses required reorganization of the Calvin-Benson-cycle (CB-cycle) within the leaf, along with coordination of C ₄ -cycle enzymes with the CB-cycle to maximize CO ₂ assimilation. Considering the vast amount of time over which C ₄ evolved, we hypothesized (i) trait divergences exist within and across lineages with both C ₄ and closely related C ₃ grasses, (ii) trends in traits after C ₄ evolution yield the optimization of C ₄ through time, and (iii) the presence/absence of trends in coordination between the CB-cycle and C ₄ -cycle provides information on the strength of selection. To address these hypotheses, we used a combination of optimality modelling, physiological measurements and phylogenetic-comparative-analysis. Photosynthesis was optimized after the evolution of C ₄ causing diversification in maximal assimilation, electron transport, Rubisco carboxylation, phosphoenolpyruvate carboxylase and chlorophyll within C ₄ lineages. Both theory and measurements indicated a higher light-reaction to CB-cycle ratio (J _atpmax /V _cmax ) in C ₄ than C ₃ . There were no evolutionary trends with photosynthetic coordination between the CB-cycle, light reactions and the C ₄ -cycle, suggesting strong initial selection for coordination. The coordination of CB-C ₄ -cycles (V _pmax /V _cmax ) was optimal for CO ₂ of 200 ppm, not to current conditions. Our model indicated that a higher than optimal V _pmax /V _cmax affects assimilation minimally, thus lessening recent selection to decrease V _pmax /V _cmax .
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