Brassinosteroids improve drought resistance in Zinnia by regulating antioxidant activity and hormonal interactions with ABA and salicylic acid

Abstract Zinnia, owing to its striking floral colors and high ornamental value, is widely cultivated in landscape designs during the warm seasons. However, increasing water scarcity and intensified drought stress have become major constraints limiting its growth and aesthetic performance in urban green spaces. This study investigated the effects of foliar application of a brassinosteroid, specifically 24-epibrassinolide (EBL), at concentrations of 0, 1, 2, and 4 µM on morphological and biochemical responses of Zinnia elegans L. var. ‘Purple Prince’ under four levels of soil moisture: 100% FC as non-stress control, 80% FC as weak drought stress, 60% FC as moderate drought stress, and 40% FC as severe drought stress. The experiment was conducted in a factorial design with a completely randomized layout and four replications. Results showed that drought stress decreased plant height, biomass accumulation, flower quality, and relative water content, with the most pronounced effect observed at 40% FC. Application of 1 µM EBL effectively alleviated these impacts, improving morphological traits and increasing plant height by ≈ 15–49%, shoot fresh weight by ≈ 9–27%, shoot dry weight by ≈ 6–26%, flower visual quality index by ≈ 3–133%, and relative water content by ≈ 1–7% across FC levels compared to the control. EBL treatment also enhanced total chlorophyll, carotenoid, and anthocyanin contents under all FC levels. In addition, total protein, total phenolic contents, and antioxidant capacity increased significantly in response to BR. Drought stress reduced the activities of antioxidant enzymes (catalase, peroxidase, and superoxide dismutase), which were maintained or elevated by BR application. Conversely, levels of hydrogen peroxide, malondialdehyde, and proline, as well as electrolyte leakage, and polyphenol oxidase activity were enhanced by drought, but markedly reduced by EBL. Moreover, EBL increased endogenous levels of salicylic acid and abscisic acid, indicating its role in reinforcing antioxidant defense and modulating phytohormonal signaling. These findings suggest that foliar application of 1 µM EBL effectively enhances drought tolerance in Z. elegans by improving morphological and biochemical traits, reducing oxidative stress, and strengthening stress-response mechanisms.