Beneficial Effects of 24-Epibrassinolide on Paeonia lactiflora Pall. under High-Temperature Stress

High-temperature stress poses a significant threat to the growth and development of Paeonia lactiflora Pall., an important ornamental and medicinal plant. This study investigated the protective effects of exogenous 24-epibrassinolide (24-EBR) on P. lactiflora under high-temperature stress. Through integrated physiological and molecular analyses, it was found that the application of exogenous 0.1 μM 24-EBR exhibited optimal efficacy in enhancing thermotolerance of P. lactiflora. Notably, the application of exogenous 24-EBR alleviated the leaf chlorosis induced by high-temperature and significantly reduced water loss. Furthermore, 24-EBR mitigated membrane lipid peroxidation, as evidenced by the decrease in relative electrical conductivity (REC) and malondialdehyde (MDA) content. Concurrently, this treatment regulated reactive oxygen species (ROS) accumulation by inhibiting the accumulation of superoxide anion ( $${\text{O}}_{2}^{{\bullet - }}$$ ) and hydrogen peroxide (H2O2), while enhancing the activity of antioxidant enzymes, including superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT). Additionally, improvements in chlorophyll fluorescence parameters indicated that 24-EBR effectively maintained high photosynthetic capacity under high-temperature stress. Moreover, qRT-PCR analysis demonstrated that 24-EBR upregulated the expression of chlorophyll biosynthesis genes and down-regulated the expression of chlorophyll catabolic genes, thereby preserving chlorophyll content. Taken together, these findings indicate that the application of exogenous 24-EBR mitigated high-temperature-induced damage through multi-level regulatory mechanisms, thereby providing a theoretical foundation for its potential application in horticultural stress management.