In order to explore the effects of spraying brassinosteroid （BR） on the growth and physiological characteristics of alfalfa （Medicago sativa） seedlings under low temperature stress， the new cold-resistant alfalfa line （P4） and Longdong alfalfa （Medicago sativa ‘Longdong’） were used as the research objects in this study. After 13 days of low temperature stress （4℃）， different concentrations of exogenous BR （0.00， 0.25， 0.50， 0.75， and 1.00 mg·L^-1， representing T₀， T₁， T₂， T₃， T₄ treatment， respectively） were sprayed on the leaves. After 5 days， the morphological and physiological indexes of the seedlings were measured. The results showed that low temperature stress significantly inhibited the growth of alfalfa seedlings. However， after applying different concentrations of BR， the plant height， leaf length， leaf width， leaf area， aboveground and underground biomass， root shoot ratio， total root length， total root surface area， root diameter， root volume， root tip number， root activity， phenylalanine ammonia lyase， cinnamic acid-4-hydroxylase， 4-coumaric acid coenzyme A ligase and total flavonoid content of alfalfa seedlings were significantly higher than those of T₀ （P<0.05）. The content of malondialdehyde decreased significantly （P<0.05）. The comprehensive evaluation of membership function showed that different concentrations of BR treatments were in the order of T₂>T₁>T₃>T₄>T₀， indicating that spraying 0.25-1.00 mg·L^-1 BR could alleviate the effects of low temperature on the growth of alfalfa seedlings， while the 0.5 mg·L^-1 （T₂） BR treatment was the best. The results provided a theoretical basis for the rational use of BR to solve the problem of chilling injury in alfalfa cultivation.

Alfalfa  /  Low temperature stress  /  Brassinosteroid  /  Physiological Characteristics  /  Membership Function