Nonlinear responses of foliar phenylpropanoids to increasing O3 exposure: Ecological implications in a Populus model system.

Abstract Plant phenolic compounds (phenylpropanoids) act as defense chemicals against herbivores and can mediate ecosystem processes. Tropospheric ozone (O3) pollution alters concentrations of plant phenolics; however, little is known about how these phytochemicals respond to different levels of O3 exposure. Here, we investigated the effects of five different O3 exposure levels on foliar concentrations of phenylpropanoids (53 compounds in total) and antioxidative capacity in hybrid Populus (Populus euramericana cv. ‘74/76’) saplings grown in the presence of high or low soil nitrogen (N) load. Increasing O3 exposure initially increased and then decreased total concentrations of phenolic compounds, revealing a biphasic exposure-response profile (hormetic zone: 1.1–36.3 ppm h AOT40). This biphasic response pattern was driven by changes in a subset of phenylpropanoids with high antioxidative capacity (e.g. condensed tannins) but not in phenolics with low antioxidative capacity (e.g. salicinoids). The O3 exposure-response relationships of some phenylpropanoids (e.g. flavonoids and chlorogenic acids) varied in response to soil N, with hormesis occurring in high N soil but not in low N soil. Collectively, our findings indicated that plant phenolic compounds exhibit nonlinear responses to increasing O3 exposure, and that the responses vary in relation to phenolic compound class, antioxidative capacity, and soil nitrogen conditions. Our findings further suggest that the impact of O3 on ecological processes mediated by phenolics will be concentration-dependent, highlighting the complexity of the ecological effects of ground-level O3 pollution.