PHENOTYPIC PLASTICITY IN POLYGONUM-PERSICARIA .1. DIVERSITY AND UNIFORMITY IN GENOTYPIC NORMS OF REACTION TO LIGHT

Several aspects of genotype-environment interaction may act to modulat e natural selection in populations that encounter variable environment s. In this study the norms of reaction (phenotypic responses) of 20 cl oned genotypes from two natural populations of the annual plant Polygo num persicaria were determined over a broad range of controlled light environments (8%- 100% full sun). These data reveal both the extent of functionally adaptive phenotypic plasticity expressed by individual g enotypes, and the patterns of diversity among genotypes for characters relevant to fitness, in response to an environmental factor that is b oth highly variable within populations and critical to growth and repr oduction. Each Polygonum genotype expressed a set of physiologically, allocationally, and morphologically diverse phenotypes in response to contrasting light conditions. These phenotypic adjustments were consis tent with ecophysiological expectations for maximizing light intercept ion under low light intensities, and resulted in the maintenance of re lative photosynthetic efficiency as well as successful reproduction ev en under severe light limitation. Within light levels, the different g enotypes exhibited uniform responses in several characters related to light capture. Genotypes differed significantly in other traits, but t he differences were offset by negatively correlated differences in fun ctionally related characters. As a result of the functional similarity of genotypes conferred by both phenotypic plasticity and interaction among characters, morphologically diverse genotypes within each popula tion shared equivalent reproductive fitnesses across the full range of flight environments. Enormous fitness differentials did result from l ight treatment itself, however. Such environmentally evoked fitness di fferences would act to promote the maintenance of genetic diversity wi thin Polygonum populations.