Genotype and Environment as Determinants of Intraspecific Variation in Quaking Aspen Phytochemistry and Consequences for an Insect Herbivore (Populus Tremuloides)

Although genetic variability and resource availability both influence plant chemical composition, little is known about how these factors interact to mod- ulate costs of resistance, expressed as negative correlations between growth and defense. We evaluated genotype · environment effects on foliar chemistry and growth of quaking aspen (Populus tremuloides) by growing multiple aspen genotypes under variable conditions of light and soil nutrient availability in a common garden. Foliage was analyzed for levels of nitrogen, phenolic glycosides and condensed tannins. Bioassays of leaf quality were con- ducted with fourth-stadium gypsy moth (Lymantria dis- par) larvae. Results revealed strong effects of plant genotype, light availability and nutrient availability; the importance of each factor depended upon compound type. For example, tannin concentrations differed little among genotypes and across nutrient regimes under low light conditions, but markedly so under high light condi- tions. Phenolic glycoside concentrations, in contrast, were largely determined by genotype. Variation in phenolic glycoside concentrations among genotypes was the most important factor affecting gypsy moth perfor- mance. Gypsy moth biomass and development time were negatively and positively correlated, respectively, with phenolic glycoside levels. Allocation to phenolic glyco- sides appeared to be costly in terms of growth, but only under resource-limiting conditions. Context-dependent trade-offs help to explain why costs of allocation to resistance are often difficult to demonstrate