Brassinolide Improves the Tolerance of Hydrilla verticillata to Low-Temperature Stress

Low temperature severely constrains the growth and ecological application of submerged macrophytes in aquatic ecosystem restoration. Although brassinolide (BR) has been shown to alleviate abiotic stress in terrestrial plants, its role in submerged macrophytes remains poorly understood. This study investigated the effects of different BR concentrations (0, 0.05, 0.1, and 0.5 mg L^-1) on growth, photosynthesis, antioxidant defense, and osmotic adjustment in Hydrilla verticillata under low-temperature stress (2 °C) over 15 days. Exogenous BR significantly alleviated low-temperature-induced damage in H. verticillata in a concentration-dependent manner. Among all treatments, 0.05 mg L^-1 BR showed the strongest overall effect, increasing fresh weight, dry weight, and plant height by 16.22%, 22.67%, and 9.52%, respectively, compared with the control. It also promoted photosynthetic performance, with Fv/Fm and Y(II) increasing by 251.93% and 262.83%, respectively, on day 10, and enhanced stress resistance, as reflected by a 32.44% increase in SOD activity and a 112.59% increase in soluble sugar content on day 15. In contrast, higher BR concentrations (0.1 and 0.5 mg L^-1) were less effective overall. Membership function analysis ranked the treatments as 0.05 mg L^-1 (0.95) > 0.1 mg L^-1 (0.47) > 0.5 mg L^-1 (0.19) > 0 mg L^-1 (0.09). These results indicate that BR enhances low-temperature tolerance in H. verticillata through coordinated regulation of photosynthetic performance, antioxidant defense, and osmotic adjustment, providing a physiological basis for its application in submerged macrophyte restoration under low-temperature conditions.