Seasonal Soil Moisture Variability, Not Drought, Drives Differences in Photosynthetic Physiology of Two C4 Grass Species

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Plant Ecology

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DOI: 10.1007/s11258-022-01236-7


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 C4 grass species to drought. Here, we characterized seasonal patterns in plant physiology, including stomatal and non-stomatal limitations of photosynthesis, for two dominant C4 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 (Vcmax) and electron transport (Jmax), thereby limiting net CO2 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 C4 physiology. Our results demonstrate that photosynthesis of co-occurring, dominant C4 grasses is primarily limited by RuBP regeneration. Interestingly, Jmax 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 Jmax, 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 C4 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 C4 biochemistry.


Plant Ecology, Vol. 223, No. 6 (June 2022): 627-642. DOI.