Local adaptation and genome size do not impact Panicum virgatum (switchgrass) responses to variable precipitation timing

Responses of the genetically and phenotypically diverse C₄ grass species, Panicum virgatum L. (switchgrass) to climate change may be driven by local adaptation to broad-scale regional climates or intra-specific variation in genome size. However, it is unknown if intra-specific variation in genome size is more important in driving population responses to climate change than local adaptation. Therefore, we investigated P. virgatum responses to water variability between natural populations collected across a latitudinal gradient and among individuals spanning a range of genomes sizes within populations. P. virgatum rhizomes collected from Kansas, Oklahoma, and Texas populations were planted in a rainfall mesocosm facility at the Konza Prairie Biological Station and received frequent, small water events (“ambient”) or infrequent, large water events (“altered”) to simulate contrasting rainfall variability. We measured genome size, leaf-level physiology, and aboveground biomass for each individual. The altered water treatment reduced soil water availability and leaf-level physiology, but not biomass yields, for all populations. We also found evidence of local adaptation in most traits measured but this had no effect on the way populations responded to variation in water timing, as indicated by the lack of a significant population x treatment interaction for all measured variables. Additionally, genome size was not a significant predictor of physiological differences among P. virgatum individuals, and had little impact on P. virgatum responses to water treatments. Our results suggest that trait variation in P. virgatum is primarily attributed to local adaptation between populations, unrelated to genome size, and that neither localized adaptation nor genome size may be important predictors of P. virgatum responses to future climatic conditions.