PHENOTYPIC PLASTICITY IN POLYGONUM-PERSICARIA .3. THE EVOLUTION OF ECOLOGICAL BREADTH FOR NUTRIENT ENVIRONMENT

Norms of reaction for a number of growth and reproductive characters w ere determined for 15 randomly sampled Polygonum persicaria genotypes, from two natural populations originating in sites with very different nutrient availabilities. Under severely limiting nutrient conditions, these genotypes shared not only plastic responses such as increased r oot-to-shoot ratio, but a surprising constancy in such functionally es sential characters as leaf area ratio, leaf nitrogen concentration, an d propagule nitrogen content. Because functional homeostasis depends o n flexibility in underlying characters, similar homeostatic results ca n be achieved through different combinations of underlying plastic and fixed responses in genetically different entities. For example, plant s in each population maintained a relatively constant propagule nitrog en content under extreme low-nitrogen conditions by varying either the size or the tissue nitrogen concentration of propagules. These genoty pes also tolerated excessive nutrient levels toxic to many plants, evi dently by storing excess nutrients in shoots. Although development was altered under such circumstances, reproductive fitness was maintained . Genotypes of both populations thus were universally able to tolerate very limited as well as excessive nutrient supplies and to exploit fa vorable nutrient conditions. This capacity of individual genotypes to accommodate diverse nutrient environments reflects the specific nature of mineral resources and of plant physiology: because nutrient availa bility can be manipulated via root-system adjustments and facultative uptake mechanisms, and ions can be differentially allocated and transl ocated among plant parts, nutrient supply may be to a considerable ext ent mediated by the plant individual. The results further suggest that the response mechanisms conferring ecological breadth for nutrient en vironment may entail neither physiological costs nor fitness trade-off s, conditions favoring the evolution of plasticity rather than genetic specialization. The evolution of such plasticity also reflects the hi ghly variable nutrient environment plants experience, because of fluct uations not only in soil minerals but in complex interacting factors s uch as moisture. General conclusions based on the entire, three-part s tudy follow the discussion.