Salvia splendens adapted to moderate ozone concentration by growth compensatory rather than defense

Ground-level ozone (O₃) poses a serious threat to urban ornamental plants, while the underlying adaptive strategies of many common species remain poorly understood. In this study, Salvia splendens, a widely cultivated herbaceous ornamental plant, was exposed to three O₃ regimes in open-top chambers: ambient air (AA), moderate O₃ (AA + 40 ppb), and high O₃ (AA + 80 ppb). Over a 63-day fumigation period, we comprehensively evaluated plant growth, photosynthetic performance, non-structural carbohydrates (NSCs), stomatal characteristics, and antioxidant defense responses. Moderate O₃ markedly promoted vegetative growth (plant height, crown width, leaf area, and biomass) and enhanced the net photosynthetic rate, suggesting an evident compensatory growth strategy. By contrast, high O₃ triggered the antioxidant defense system, as reflected by increased total antioxidant capacity (T-AOC), superoxide dismutase activity (SOD), total flavonoid (TF) and reduced ascorbic acid (rAsA) content. However, this defensive activation occurred at the expense of biomass accumulation and ultimately failed to alleviate growth inhibition. Additionally, high O₃ increased stomatal conductance, stomatal density and pore area, which tended to facilitate O₃ uptake rather than enforcing stomatal avoidance. We conclude that S. splendens adapts to moderate O₃ stress mainly via compensatory growth, instead of relying on physiological antioxidant defense or stomatal avoidance. These findings provide mechanistic insights and practical guidance for selecting O₃-tolerant ornamentals in urban greening.