Phenotypic plasticity in response to light availability in Amphicarpaea bracteata L. (Fabaceae): Comparisons of varieties, ecotypes, and habitats.

摘要

The genus Amphicarpaea encompasses three species, including the North American A. bracteata and its varieties bracteata and comosa. Phenotypic differences between these two varieties and between sun- and shade-native populations of bracteata are maintained under collateral cultivation. Canonical discriminant analyses indicate that leaf morphology traits effectively distinguish among groups. These results indicate that these taxa are genetically distinct, but isozymes or other molecular markers could further support such conclusions.;Greenhouse experiments, interpreted with analysis of covariance, indicate that effects of genotype (comosa, sun-native bracteata, or shade-native bracteata) and light treatments (high or low) combine additively for two traits. High light availability increases the proportion of branching nodes and allocation to potential reproduction (total flower number corrected for plant size). In both light treatments, comosa has consistently more branching and fewer flowers. Genotype-environment interactions occur for two other traits. In high light, comosa and sun-native bracteata increase vegetative biomass per node, but shade-native bracteata show no shift in allocation pattern. Allocation to realized reproduction (fruit number corrected for size) increases in high light; this plasticity is moderate in comosa and sun-native bracteata, and more pronounced in shade-native bracteata.;Complementary to these examinations of reaction norms, a modified path analysis relates components of size and fecundity in the same greenhouse data. Relationships between size and fecundity are complex and may involve environmental sensitivity, genotypic differentiation, or both.;Interpreting the relevance of plasticity requires information about environmental heterogeneity. By conducting spatially fine-scaled sampling of light availability, soil, and vegetation data in sunny and shady habitats, it is demonstrated that sunny sites are significantly more variable. Autocorrelation analyses of this variation reveal considerably more spatial structure (i.e., patchiness) in sunny habitats. Thus, selection on A. bracteata populations may differ between habitats in ways that contribute to different degrees of specialization. These field studies, and the differentiation demonstrated in the greenhouse experiments, support the possibility that infraspecific variation for life history traits and trait plasticities is adaptive.