Population Differentiation and Plastic Responses to Drought Stress in the Generalist Annual Polygonum Persicaria

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 moisture conditions was population dependent. In particular, the moist-site population had the greatest plasticity in water use efficiency, while the dry-site populations had greater plasticity in root biomass allocation. Selection analyses demonstrated that increased water use efficiency was adaptive in drought and that decreased root biomass allocation was adaptive in moist conditions. Overall, the maintenance of fitness in stressful conditions may involve population differentiation along different axes of functional plasticity.