PHENOTYPIC PLASTICITY IN POLYGONUM-PERSICARIA .2. NORMS OF REACTION TO SOIL-MOISTURE AND THE MAINTENANCE OF GENETIC DIVERSITY

Adaptive phenotypic plasticity is the predicted evolutionary response to fine-grained fluctuation in major environmental factors, such as soil moisture in plant habitats. This study examines genotypes from two natural populations of Polygonum persicaria, one from a relatively homogeneous, moderately moist site, and one from a site in which severe drought and root flooding occur within single growth seasons. Norms of reaction (phenotypic response curves) were determined for a random sample of eight and ten cloned genotypes, respectively, from each of the populations over a controlled moisture gradient ranging from drought to flooding. Genotypes of both populations exhibited marked allocational and morphological plasticity in characters relating to plant function at both low moisture availability and flooded soil, including root characters that directly affect plant water status. Associated with this plasticity was the ability of all genotypes to survive and reproduce across the entire range of moisture conditions: individual genotypes possess a remarkable degree of ecological tolerance. This tolerance was equally broad in genotypes from both populations, although only one population encounters soil flooding in its native site. Among the Polygonum genotypes studied, certain individuals exhibited relatively poor growth and reproductive output at several moisture levels. Although these genotypes thus showed significantly low average fitness, the magnitude and/or direction of the fitness differentials varied from one moisture treatment to another. The precise arrays of norm of reaction diversity among these genotypes, together with patterns of moisture variability at the field sites, indicate how genotype by environment interaction and environmental variability may lead to the maintenance of genetic diversity.