Exogenous Salicylic Acid Supplementation Enhances Cadmium-Induced Oxidative Stress Tolerance by Regulating Nutrient Metabolism, Photosynthetic Traits, and Growth in Zinnia elegans

Cadmium (Cd) toxicity is a major constraint in ornamental plant production, as it induces oxidative stress and disrupts essential physiological processes, requiring effective mitigation strategies. Salicylic acid (SA) is a key signaling molecule that activates the defense system, providing abiotic stress tolerance to ornamental plants. This study aimed to assess the potential of foliar application of salicylic acid to alleviate Cd stress in Zinnia elegans grown under Cd exposure in a lath house. The experiment followed a completely randomized design with ten treatments, including control, SA (0.5, 1.0, 1.5, and 2.0 mM/L), Cd (50 µM), and combinations of Cd with four levels of SA, each with four replicates (40 plants per treatment). Under controlled conditions, SA supplementation significantly improved growth and plant performance, while Cd exposure led to a notable reduction in plant growth (29–52.8%), photosynthetic pigments, gas exchange traits, and relative water content, along with increased oxidative damage, as evidenced by elevated levels of MDA (181.4%) and H2O2 (116.8%). Cd also hindered nutrient uptake and increased Cd accumulation in plant parts. In contrast, the application of SA, particularly at 1.5 mM/L, substantially enhanced growth (up to 107.4%), photosynthetic performance (80.8%), antioxidant enzyme activities, and osmolyte accumulation, while reducing oxidative damage and Cd accumulation (up to 51.2%). Multivariate analysis further supported these results. Overall, SA effectively mitigated Cd toxicity and enhanced plant growth. Future research should delve into the molecular mechanisms underlying these effects and validate these findings at the field level to promote sustainable ornamental plant production.