Exogenous substances can alleviate plant damage under adverse conditions. In order to explore whether different concentrations of salicylic acid (SA) can play a role in the resistance of Bougainvillea × buttiana ‘Miss Manila’ to nitrogen dioxide (NO₂) stress and the relevant mechanisms of their effects, different concentrations of SA were applied locally under the control experiment condition of 4.0 μL·L⁻¹ NO₂, and the role of SA in alleviating injury was studied. The findings noted a significant increase in metabolic adaptations and antioxidant enzyme activities following 0.25–0.75 mM SA application (p < 0.05), except 1 mM. Superoxide dismutase (SOD) and catalase (CAT) in particular increased by 21.88% and 59.71%, respectively. Such an increase led to effective control of the reduction in photosynthetic pigments and the photosynthetic rate and protection of the structural stability of chloroplasts and other organelles. In addition, the activity of nitrate reductase (NR) increased by 83.85%, and the content of nitrate nitrogen (NO₃⁻-N) decreased by 29.23% in nitrogen metabolism. Concurrently, a principal component analysis (PCA) and a membership function analysis further indicated that 0.75 mM SA provided the most notable improvement in NO₂ resistance among the different gradients. These findings suggest that 0.25–0.75 mM SA can relieve the stress at 4 μL·L⁻¹ NO₂ injury by effectively improving the antioxidant enzyme activity and nitrogen metabolizing enzyme activity, protecting the photosynthetic system and cell structure, but 1 mM SA had the opposite effect. In the future, the specific reasons for inhibition of SA at high concentrations and the comprehensive effects of the application of other exogenous compounds should be further studied.

The air pollutant NO₂ is one of the major constraints on plant growth, and the ecological value of the ornamental plant Bougainvillea glabra can be weakened by NO₂. In this study, an indoor 4 μL·L⁻¹ NO₂ simulated fumigation test was conducted with three treatments, CK (normal growth with clean air), T1 (4 μL·L⁻¹ NO₂ + 8 h/d), and T2 (4 μL·L⁻¹ NO₂ + 24 h/d), which were set up with considerations for time and concentration. The results demonstrated that most of the morphological parameters of B. glabra ‘Elizabeth Angus’, except for the floral organs, were decreased in the root, stem, leaf, and bract. Continuous fumigation significantly attenuated the growth rate and reduced the water and pigment contents of organs. Excessive NO₂ reduced the number and transfer rate of photoelectrons by destroying the photosynthetic reaction center, which in turn weakened photosynthesis, but the plants with intermittent fumigation recovered after fumigation. The Kaplan-Meier (K-M) survival curve displayed median survival periods of 41 and 55.5 h for T1 and T2, respectively, and the morphological structure and most of the indicators of photosynthetic reaction centers changed significantly during stress. Acute injury to B. glabra ‘Elizabeth Angus’ was caused by 4 μL·L⁻¹ NO₂, and B. glabra ‘Elizabeth Angus’ had limited ability to regulate high concentrations of NO₂ acute stress.

Urban green spaces, as crucial components, can effectively mitigate atmospheric pollutants such as NO₂. However, the heterogeneous driving forces and the underlying quantitative mechanisms of different vegetation community structures in response to NO₂ exposure remain insufficiently explored. This study utilized a laboratory-based artificial fumigation method to examine the NO₂ mitigation benefits and response mechanisms of three representative vegetation structures (tree–shrub–grass, tree-shrub, and tree-grass) as well as their monoculture communities under NO₂ stress. The objective was to elucidate the variations in NO₂ reduction capacity and the adaptive mechanisms associated with different vegetation structures. The results demonstrated that, under NO₂ exposure, the NO₂ reduction rate of the tree–shrub–grass mixed community was 56.15 mg·h⁻¹. NO₂ stress inhibited leaf morphogenesis and plant growth. However, the tree–shrub–herbaceous community enhanced its NO₂ tolerance by reallocating photosynthetic products and increasing epidermal thickness, stomatal density, and the compactness of tissue structure, thereby strengthening its mechanical barrier function. The NO₂ reduction rate was positively correlated with parameters such as net photosynthetic rate, stomatal density, leaf width, transpiration rate, and stomatal conductance, but negatively correlated with fluorescence and the leaf length-to-width ratio. A comprehensive evaluation based on fuzzy membership functions ranked the NO₂ mitigation and tolerance capacities of plant communities as follows: the tree–shrub–herbaceous community exhibited the strongest capacity for NO₂ reduction and stress tolerance. Thus, in NO₂-polluted areas, priority should be given to establishing tree–shrub–grass composite vegetation structures, whose multidimensional resistance mechanisms provide both a theoretical foundation and a technical pathway for the ecological restoration of urban green spaces.