Early ontogenetic trajectories vary among defence chemicals in seedlings of a fast-growing eucalypt

Abstract

Ontogenetic changes in leaf chemistry can affect plant–herbivore interactions profoundly. Various theoretical models predict different ontogenetic trajectories of defence chemicals. Empirical tests do not consistently support one model. In Eucalyptus nitens, a fast-growing tree, we assessed early developmental changes to seedlings, in foliage concentrations of nitrogen and the full suite of known secondary (defence) chemicals. This included the terpene, a-pinene, whose impact on marsupial herbivory is unknown. To test for the influence of abiotic conditions on the ontogenetic trajectories we overlaid a nutrient treatment. Ontogenetic trajectories varied among compounds. Sideroxylonals and cineole were barely detected in very young seedlings, but increased substantially over the first 200 days. Total phenolic concentration increased fourfold over this time. In contrast, a-pinene concentration peaked within the first 60 days and again between 150 and 200 days. Nutrients altered the degree but not the direction of change of most chemicals. A shorter trial run at a different season showed qualitatively similar patterns, although a-pinene concentration started very high. We investigated the effect of detected levels of a-pinene and cineole on food intake by two mammalian herbivores, common brushtail possums (Trichosurus vulpecula) and red-bellied pademelons (Thylogale billardierii). Under no-choice conditions neither terpene reduced intake; but with a choice, possums preferred a-pinene to cineole. The ontogenetic trajectories of most compounds were therefore consistent with models that predict an increase as plants develop. Published data from later developmental stages in E. nitens also confirm this pattern. a-Pinene, however, was the only secondary compound found at significant levels in very young seedlings; but it did not constrain feeding by marsupial herbivores. Models must allow for different roles of defensive secondary chemicals, presumably associated with different selective pressures as plants age, which result in different ontogenetic trajectories.