Abstract:

In order to explore the effects of melatonin on the growth and development of adzuki bean seedlings under salt stress,a phytotron experiment was conducted with Baohong 876 as the material and four treatments:CK(Clear water),S(60 mmol/L NaCl),MT(50 μmol/L melatonin)and S-MT(60 mmol/L NaCl and 50 μmol/L melatonin).The growth indexes,photosynthetic indexes,mineral ion contents and antioxidant enzyme activities of adzuki seedlings were analyzed using classical physicochemical determination.The results showed as follows:under salt stress,compared with CK treatment,the growth of adzuki bean seedlings was significantly inhibited,chlorophyll content,net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductivity(Gs)and intercellular CO₂ concentration(Ci)were significantly decreased;the contents of Na⁺ in leaves,stems were significantly increased.Meantime,the content of Ca²⁺ in leaves was significantly increased while the content of Mg²⁺ was significantly decreased,and the ratio of Na⁺/K⁺ was significantly increased.The contents of MDA and H₂O₂ increased significantly,which damaged the selective permeability of membrane lipid.Compared with S treatment,the plant height,leaf area,biomass,and total root length of adzuki bean seedlings were significantly increased under S-MT condition.Meantime,chlorophyll content,leaf Pn,Tr,and Ci were increased.The contents of Na⁺ and K⁺ in leaves,stems were significantly decreased,the content of Mg²⁺ in leaves was significantly increased and Ca²⁺ was significantly decreased,and the Na⁺/K⁺ ratio was decreased.The contents of MDA, {\mathrm{O}}_2^{-} and H₂O₂ decreased.The levels of SOD,POD and CAT activity in leaves and roots were increased.Under normal conditions,the application of melatonin could promote the growth of adzuki bean seedlings,improve the photosynthetic capacity of seedlings,and increase the content of soluble protein.Spearman correlation analysis was performed on 15 representative indicators,and it was found that total root length,chlorophyll content,net photosynthetic rate,and antioxidant enzyme activities were significantly or extremely significantly negatively correlated with Na⁺/K⁺ ratio and ROS.In summary,under salt stress,melatonin can directly and indirectly reduce H₂O₂ content and maintain the stability of membrane lipid permeability by activating the antioxidant enzyme activity of adzuki bean seedlings,thus maintaining the Na⁺/K⁺ balance in plants.Meanwhile,H₂O₂ and Ca²⁺,as signaling molecules,have a synergistic effect in the regulation process of melatonin regulating intracellular ion balance,thereby improving the salt tolerance of plants.

Key words: Adzuki bean, Melatonin, Salt stress, Na⁺/K⁺ equilibrium