| Description |
For the shade-tolerant tree species of tropical forests, the heterogeneity of light environments within such forests generates sizable gradients of light availability and, in turn, abiotic and biotic stressors. Such gradients occur horizontally, between understory and treefall gaps, as well as vertically between the forest floor and the canopy. This means that shade-tolerant tropical trees must be able to acclimatize to shifts in light availability and stressor load extemporaneously as well as across plant ontogeny. Tropical trees have adapted to respond to simultaneous shifts in light availability, abiotic stressor load, and herbivore density through plasticity in primary and secondary physiology. However, the extent to which secondary physiological plasticity, i.e. shifts in secondary metabolite production and allocation, is involved in responses to abiotic as well as biotic stressors remains to be elucidated. In addition to discerning tropical trees' chemical adaptations to naturally occurring abiotic and biotic stressors, parsing out the focal activities of the secondary metabolites that shift in response to stressor gradients may provide insights into the ecological ramifications of anthropogenic stressors such as tropospheric ozone pollution. Thus, using recently developed techniques in metabolomics and molecular networking, the research encompassed in this dissertation addressed the following question: how are responses of tropical trees to biotic and abiotic stressors, naturally occurring and anthropogenic, integrated within their suites of secondary metabolites? The results of this research indicate that such chemical responses are integrated mainly through the diversity and versatility of the phenolic class of secondary metabolites. At intermediate canopy openness, a subset of phenolics was associated with supplemental protection against abiotic stressors. However, at high canopy openness, primary physiological mechanisms of abiotic protection were upregulated rather than phenolics associated with supplemental abiotic protection. Instead, phenolics that were most abundant at high canopy openness were associated with activity against pests. As a more reliable indicator of canopy openness than visible light, UV may cue antipest defense at high canopy openness. This connection between abiotic stressors and biotic defense may also help explain the antipathogen activity that was found to result from ozone exposure. |
