Water stress rather than N addition mitigates impacts of elevated O3 on foliar chemical profiles in poplar saplings.

Tropospheric ozone (O3) pollution can alter tree chemical profiles, and in turn, affect forest ecosystem function. However, the magnitude of these effects may be modified by variations in soil water and nutrient availability, which makes it difficult to predict the impacts of O3 in reality. Here we assessed the effects of elevated O3 alone, and in combination with soil water deficit and N addition, on the phytochemical composition of hybrid poplar (Populus deltoides cv. '55/56' × P. deltoides cv. 'Imperial'). Potted trees were grown in open-top chambers (OTCs) under either charcoal-filtered air or elevated O3 (non-filtered air +40 ppb of O3), and trees within each OTC were grown with four combinations of water (well-watered or water deficit) and nitrogen (with or without N addition) levels. We found that elevated O3 alone stimulated the accumulation of foliar nitrogen, soluble sugar, and lignin while inhibiting the accumulation of starch, but had limited impacts on condensed tannins and salicinoids in poplar saplings. Graphical vector analysis revealed that these changes in concentrations of nitrogen, starch and lignin were due largely to altered metabolic processes, while increased soluble sugar concentration related mainly to decreased leaf biomass in most cases. The effects of O3 on poplar foliar chemical profiles depended on soil water, but not soil N, availability. Specifically, O3-mediated changes in carbohydrates and lignin were mitigated by decreased soil water content. Taken together, these results suggested that nitrogen acquisition, carbohydrates mobilization and lignification play a role in poplar tolerance to O3. Moreover, the impacts of elevated O3 on phytochemistry of poplar leaves can be context-dependent, with potential consequences for ecosystem processes under future global change scenarios. Our results highlight the needs to consider multi-factors environments to optimize the management of plantations under changing environments.