Constitutive and induced chemical defenses as a function of leaf age in Quercus rubra (Red Oak).

Abstract

Limited resource availability results in a trade-off between growth and defense in plants. Natural selection has favored strategies in plants that efficiently allocate resources to achieve a high net benefit to cost ratio. As integral parts of their defense strategy, plants have evolved not only to have constitutive chemical defenses, but also the ability to induce defenses. Constitutive levels and inducibility may vary among leaves within a tree due to varying net benefit to cost ratio of defending different leaves. In this study, we examined the effect of leaf age on constitutive levels and inducibility of phenolics following a mechanical attack in Quercus rubra (Red Oak) . Young leaves possessed higher constitutive levels of phenolics compared to older leaves on the same trees, which were sampled eight days later. Inducibility did not differ between leaf age categories. Constitutive phenolics, but not inducibility, was negatively correlated with leaf nitrogen and positively correlated with leaf C:N ratio. Our results suggests that natural selection may have favored the evolution of spring-feeding herbivores in part because of the inability of early season leaves to induce higher levels of chemical defenses.