Influences of local adaptation and genome size on Panicum virgatum (switchgrass) responses to variable precipitation timing

Background/Question/Methods

Variable precipitation patterns associated with climate change will likely expose prairie ecosystems to increasingly severe drought conditions throughout the growing season. This can impact the physiology and productivity of dominant C₄ grasses, driving changes in community structure and ecosystem functioning.  However, it is unknown if plastic responses to drought will vary within individual species, and if these responses will be driven by differences among natural populations locally adapted to geographically separated sites or by ecotypic variation in genome size.  Therefore, we investigated the influence of population origin and genome size on the physiological responses of Panicum virgatum L. (switchgrass) to precipitation variability.  P. virgatum rhizomes were collected from three locations across the Great Plains (Kansas, Oklahoma, and Texas) and planted in the Rainfall Mesocosm Facility at the Konza Prairie Biological Station.  Plants received either frequent, small precipitation events (21mm every 6 days, ‘ambient’) or infrequent, large precipitation events (42mm every 12 days, ‘altered’). Leaf-level physiology was evaluated in July and September 2011, before and after watering events, and aboveground biomass was quantified following senescence.  Flow cytometry was used to evaluate the approximate DNA content of each individual and responses to water treatment were analyzed by both population and genome size.

Results/Conclusions

P. virgatum physiology differed by population and precipitation treatment at the conclusion of the growing season when measured before a rainfall event.  Photosynthesis, stomatal conductance, and transpiration were significantly greater in Texas plants compared to Kansas and Oklahoma plants.  Additionally, these parameters were significantly lower in plants that had received the ‘altered’ precipitation treatment. Watering decreased physiological differences between treatment groups, but did not reduce differences between populations.  Aboveground biomass was significantly greater in Texas plants compared to Kansas and Oklahoma plants; however, there was no difference in biomass between plants of either precipitation treatment.  Non-significant population x precipitation treatment interactions indicated that P. virgatum responses to precipitation variability are not influenced by local adaptation.  Likewise, non-significant ANCOVA and least-squares linear regression analyses indicated that P. virgatum physiology and productivity, as well as P. virgatum responses to precipitation variability, cannot be predicted by genome size.  These results suggest that localized adaptation and ecotypic variation in genome size need not be considered when predicting responses of P. virgatum to future climatic conditions.