Abstract:

With global climate warming， drought stress has become a major environmental factor restricting crop growth and yield. Alfalfa （Medicago sativa） is an important forage crop significantly impacted by drought， and shows reduced growth and yield under drought conditions. 2，4-epibrassinolide （EBR） is a synthetic plant hormone that can enhance the stress resistance of plants， but its effects on the drought tolerance of alfalfa and the concentration dependence of such effects remain unclear. In this study， seedlings of alfalfa ‘Zhongmu No.1’ were treated with EBR （0， 0.02， 0.03， 0.05， 0.10， 0.20 mg·L^-1） by foliar spraying and then subjected to a drought stress treatment. The effects of EBR on seedling growth， photosynthetic pigment content， antioxidant enzyme activities， and reactive oxygen species （ROS） metabolism under drought conditions were evaluated. The results show that the drought treatment negatively affected plant height， stem diameter， leaf area， aboveground fresh weight， and relative water content， with these parameters being 36.7%， 35.4%， 43.6%， 66.5%， and 40.7% lower， respectively， in the drought-treated plants than in the control plants. The contents of chlorophyll a， chlorophyll b， total chlorophyll， and carotenoids were 65.6%， 73.4%， 67.7%， and 78.0% lower， respectively， in the drought-treated plants than in the control plants. The malondialdehyde （MDA） content and relative electrical conductivity were 45.1% and 65.5% higher， respectively， in the drought-treated plants than in the control plants. The superoxide anion （O₂^-·） production rate and hydrogen peroxide （H₂O₂） content were increased by 100.0% and 79.4%， respectively， and the proline （Pro） content was increased by 931.8% in the drought-treated plants compared with the control plants. Treatment with EBR， especially at 0.05 mg·L^-1， significantly alleviated the adverse effects of drought stress. Under drought stress， compared with the controls， the seedlings treated with EBR at 0.05 mg·L^-1 showed increased plant height， stem diameter， leaf area， aboveground fresh weight， and relative water content （by 31.3%， 19.9%， 62.6%， 128.6%， and 47.5%， respectively）； increased contents of chlorophyll a， chlorophyll b， total chlorophyll， and carotenoids （by 120.2%， 183.8%， 134.3%， and 221.6%， respectively）； decreased MDA content and relative electrical conductivity （by 28.8% and 26.0%， respectively）； decreased O₂^-· production rate and H₂O₂ content （by 43.9% and 41.9%， respectively）， increased activities of superoxide dismutase， catalase， peroxidase， and ascorbate peroxidase （by 62.3%， 74.6%， 83.8%， and 150.5%， respectively）； and a significant decrease of 64.5% in Pro content. The drought alleviation effect of EBR at the highest concentration （0.20 mg·L^-1） was lower than that at lower concentrations， showing a clear concentration dependence. In conclusion， treatment with EBR at a concentration （0.05 mg·L^-1） enhanced the drought tolerance of alfalfa seedlings by activating the antioxidant defense system to reduce ROS accumulation and ameliorate membrane damage. The findings of this study provide guidelines for the application of EBR in alfalfa production under drought conditions.

Key words: 2，4-epibrassinolide, alfalfa, drought stress, antioxidant enzyme system