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We tested the generalist annual plant Polygonum persicaria (Polygonaceae) for adaptive differentiation in drought responses. Populations from one consistently moist and two variably dry sites were grown in moist and drought conditions in a greenhouse. Physiological, morphological, and reproductive data were collected. All three populations maintained similar levels of fitness in both drought and moist conditions, indicating that these populations may be equally drought tolerant. Also, the three populations had similar water use efficiency and root biomass responses to soil moisture, indicating that substantial phenotypic plasticity is present in all the populations, including a population from a consistently moist site. The magnitude of plastic responses for specific traits varied among the populations; i.e., the physiological and morphological means by which these plants maintained reproductive homeostasis across …