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

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.