Tropospheric ozone pollution is a globally increasing problem, affecting carbon dynamics, ecological interactions, and agricultural productivity across continents and biomes. Through its effects on plant physiology and biochemistry, ozone pollution can alter plant defense chemistry and, in turn, influence plant-consumer interactions. While previous studies have indicated hotspots of ozone or ozone precursor pollutants in several tropical forest airsheds, the effects of ozone on vegetation have been relatively understudied in tropical regions, where plant defense chemistry and plant-consumer interaction rates are the highest of any terrestrial biome. In this study, we investigated the effects of ambient ozone levels on tropical tree seedlings in an airshed shared by forests and a large urban/commercial zone in the Republic of Panama. We grew seedlings of the regionally widespread tree Ficus insipida (Moraceae) for two months in open-top chambers supplied with filtered or ambient air. We harvested the two most distal leaves from each seedling and compared leaf chemistry between treatments and across leaf ages. The leaf chemistry analyzed included secondary metabolites and membrane lipids. Metabolite analyses were conducted using UPLC-MS and MS-MS.
Results/Conclusions
Although this airshed proved not to be an ozone hotspot, with mean ozone mixing ratios below the levels known to induce chronic ozone stress in broadleaved plants of the temperate zone, leaf chemistry differed significantly between treatments. Depending on leaf age, the secondary metabolite suite exhibited downregulation or a combination of up- and downregulation in ambient air. Membrane lipids were present at lower concentrations in older leaves grown in ambient air, suggesting accelerated senescence. Our future work will determine the impacts of these chemical changes on plant-herbivore interactions. Our results indicate that ozone pollution can affect plant defense chemistry at concentrations comparable to the global mean, well below the documented thresholds for growth inhibition or visible leaf damage. Proposed future research directions include 1) establishing dose-response relationships for ozone and leaf defense chemistry and 2) testing the sensitivity to ozone of a range of tropical species.