The multivariate defences of Solanum carolinense in the field

From a plant perspective, defence against natural enemies is conventionally assessed using the relationship between single resistance characters and the abundance of, or damage by, a single herbivore species.  These pairwise correlations are often weak, and plants are rarely attacked by a single species, suggesting that effective defence requires coordination of a multivariate defence phenotype before and after damage.  Correlations among defence traits could heighten or attenuate defence expression, depending on the ecological context.  To test for these possibilities, I quantified levels of variation in, and correlations among, putative defence traits of the andromonoecious, short-lived perennial, Solanum carolinense (Carolina horsenettle).  I established a common field garden of seedlings of 30 genetic (half-sibling) families, arrayed in blocks that were either left exposed to insect herbivory, or sprayed with an insecticide. Excised leaves were analysed for levels of proteinase inhibitors, polyphenol oxidases, peroxidases, total protein, total alkaloids, phenolics and diterpene glycosides; I also assessed trichome and spine density, and leaf toughness.  I analysed these for phenotypic and genetic correlations with percent leaf damage by the predominant insect herbivore at the site, flea beetles in the  genus Epitrix, and with a first-season estimate of plant performance (aboveground biomass).  I also assessed the relationship of damage to overall defence trait variation using principle components analysis.  Defence traits showed evidence for broad-sense (total) genetic variation, damage by flea beetles varied with combinations of defence characters, and several of these quantitative traits were correlated, indicating that the ecology and evolution of defence against flea beetles is likely to involve coordination of multiple traits.