Phenotypic Integration and the Plasticity of Integration in an Amphicarpic Annual.

In annual plants, including amphicarpic annuals, variation in light availability can evoke phenotypic plasticity in multiple traits. We examined plasticity to light availability of vegetative and reproductive performance traits in Amphicarpaea bracteata (Fabaceae) by developing and evaluating three path-analysis models using data from a greenhouse study. To assess whether light availability altered the phenotypic integration of these performance traits, we examined the models' fit to data collected within a high- or a low-light treatment. We also examined whether a single model or alternate models were required to fit data from the two contrasting light treatments. Using our path-analysis approach, we also made comparisons among three population types: shade- and sun-native populations of the widespread variety A. bracteata var. bracteata and sun-native populations of the variety A. bracteata var. comosa. Although each type of population was somewhat distinctive in patterns of integration and in the plasticity of integration, patterns did not correspond to contrasting ecological affinity, i.e., shade- and sun-native population types or to genetic relatedness of the two varieties. Counter to the prediction that selection regimes involving variation in one or more environmental factor favor intermediate levels of integration, phenotypic integration in A. bracteata was very flexible, with plasticity occurring in a trait-by-trait manner. In particular, there was an inverse relationship between aerial and subterranean reproductive modes in low but not high light. Previous studies of amphicarpic annuals have not observed this type of environment-specific life history trade-off.