Phenotypic plasticity permits individuals to express different, functionally adaptive phenotypes in response to different environmental circumstances. It has long been speculated that such plasticity increases the range of habitats that a species may occupy. I propose to determine whether phenotypic plasticity enhances ecological breadth by comparing two weedy, ecologically broad species with two ecologically restricted species in the genus Polygonum. The comparative study will consist primarily of greenhouse experiments with field-collected material, supplemented with field work to quantify the habitat differences among the species and their populations. The major experiment will be a growth study of several populations of each species in a multifactorial set of light, moisture, and nutrient treatments, designed to determine whether the four species differ in the range of environments in which they can successfully survive and reproduce. By evaluating the mechanisms of morphological and physiological response to the treatments, I can determine whether differences in environmental tolerance among the species are associated with differences in the amounts or patterns of functionally adaptive phenotypic plasticity. Finally, I will compare the environmental tolerance of populations within species to determine whether the degree of plasticity influences the degree to which conspecific populations in different habitats differentiate genetically. A second, smaller greenhouse experiment will examine in detail the specific aspects of root system plasticity that contribute to species tolerance or intolerance of variation in soil nutrients and moisture.//
