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 so il moisture in plant habitats. This study examines genotypes from two natural populations of Polygonum persicaria, one from a relatively hom ogeneous, moderately moist site, and one from a site in which severe d rought and root flooding occur within single growth seasons. Norms of reaction (phenotypic response curves) were determined for a random sam ple of eight and ten cloned genotypes, respectively, from each of the populations over a controlled moisture gradient ranging from drought t o flooding. Genotypes of both populations exhibited marked allocationa l and morphological plasticity in characters relating to plant functio n at both low moisture availability and flooded soil, including root c haracters that directly affect plant water status. Associated with thi s 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 wa s equally broad in genotypes from both populations, although only one population encounters soil flooding in its native site. Among the Poly gonum genotypes studied, certain individuals exhibited relatively poor growth and reproductive output at several moisture levels. Although t hese genotypes thus showed significantly low average fitness, the magn itude and/or direction of the fitness differentials varied from one mo isture treatment to another. The precise arrays of norm of reaction di versity among these genotypes, together with patterns of moisture vari ability at the field sites, indicate how genotype by environment inter action and environmental variability may lead to the maintenance of ge netic diversity.