Seasonal soil moisture variability, not drought, drives differences in photosynthetic physiology of two C grass species

Seasonal changes in soil moisture drive the phenology of grassland plants during the growth period, yet we do not understand the biochemical processes underlying seasonal changes in grass photosynthesis. This lack of understanding at least partially stems from the paucity of information describing the metabolic and stomatal responses of dominant C grass species to drought. Here, we characterized seasonal patterns in plant physiology, including stomatal and non-stomatal limitations of photosynthesis, for two dominant C grass species, Bouteloua curtipendula and Schizachyrium scoparium. We also tested how rainfall reduction might modify seasonal patterns in photosynthesis for both species. Specifically, we predicted that drought would reduce carboxylation (V) and electron transport (J), thereby limiting net CO assimilation (A) and suppressing biomass for Bouteloua curtipendula and Schizachyrium scoparium. We tested these predictions using the first in situ drought experiment to measure the impact of drought on C physiology. Our results demonstrate that photosynthesis of co-occurring, dominant C grasses is primarily limited by RuBP regeneration. Interestingly, J was not reduced by drought for either B. curtipendula or S. scoparium, enabling both species to maintain constant A under drought. Seasonal changes in soil moisture did decrease J, which in turn reduced A, for S. scoparium. Photosynthesis of B. curtipendula, on the other hand, remained stable throughout the growing season. That two common C species possess such different biochemical and photosynthetic responses to soil moisture highlights the physiological variability inherent within plant functional groups, and underscores the need for more field studies of C biochemistry.