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

Ontogenetic changes in leaf chemistr y can affect plant�herbivore interactions profoundly. Various theoretical models predict different ontogenetic trajectories of defence chemicals. Empirical tests do not consis- tently suppor t 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 secondar y (defence) chemicals. This included the terpene, a-pinene, whose impact on marsupial herbivor y 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 ver y 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 shor ter trial run at a different season showed qualitatively similar patterns, although a-pinene concentration star ted ver y high. We investigated the effect of detected levels of a-pinene and cineole on food intake by two mammalian herbivores, common brushtail possums (Tr ichosur us vulpecula) and red-bellied pademelons (Thylogale billardier ii). 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 secondar y compound found at significant levels in ver y young seedlings; but it did not constrain feeding by marsupial herbivores. Models must allow for different roles of defensive secondar y chemicals, presumably associated with different selective pressures as plants age, which result in different ontogenetic trajectories.