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Phenotypic Plasticity in Polygonum persicaria. III. The Evolution of Ecological Breadth for Nutrient Environment

S. E. Sultan and F. A. Bazzaz
Evolution
Vol. 47, No. 4 (Aug., 1993), pp. 1050-1071
DOI: 10.2307/2409974
Stable URL: http://www.jstor.org/stable/2409974
Page Count: 22
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Phenotypic Plasticity in Polygonum persicaria. III. The Evolution of Ecological Breadth for Nutrient Environment
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Abstract

Norms of reaction for a number of growth and reproductive characters were 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 root-to-shoot ratio, but a surprising constancy in such functionally essential characters as leaf area ratio, leaf nitrogen concentration, and propagule nitrogen content. Because functional homeostasis depends on flexibility in underlying characters, similar homeostatic results can be achieved through different combinations of underlying plastic and fixed responses in genetically different entities. For example, plants in each population maintained a relatively constant propagule nitrogen content under extreme low-nitrogen conditions by varying either the size or the tissue nitrogen concentration of propagules. These genotypes also tolerated excessive nutrient levels toxic to many plants, evidently 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 favorable 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 availability can be manipulated via root-system adjustments and facultative uptake mechanisms, and ions can be differentially allocated and translocated among plant parts, nutrient supply may be to a considerable extent mediated by the plant individual. The results further suggest that the response mechanisms conferring ecological breadth for nutrient environment may entail neither physiological costs nor fitness trade-offs, conditions favoring the evolution of plasticity rather than genetic specialization. The evolution of such plasticity also reflects the highly variable nutrient environment plants experience, because of fluctuations not only in soil minerals but in complex interacting factors such as moisture. General conclusions based on the entire, three-part study follow the discussion.

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