Effects Of Exogenous Abscisic Acid And Melatonin On Physiological Characteristics Of Alfalfa Under Salt Stress

Alfalfa is a high-quality legume forage,which has high rich nutritional value and high yield.Salt stress is one of the abiotic stresses that affect the growth and development of plants and lead to the reduction of plant yield and quality.China has a large area of salinization land,which seriously limits the growth of alfalfa and ultimately affects the yield and quality of alfalfa.Hence,it is very vital to improve the salt tolerance of alfalfa.Abscisic acid(ABA)and melatonin(MT)can improve the tolerance of plants to various abiotic stresses.However,the effect of combined treatment of abscisic acid and melatonin on alfalfa under salt stress has not been systematically studied.In this study,Algonquin was used as the test material.First,the screening test of Na Cl concentration was carried out,the appropriate salt concentration was selected by measuring the plant height,chlorophyll content,relative conductivity and malondialdehyde content for subsequent tests.The salt stress treatment of subsequent tests was based on this concentration;Secondly,the screening test of the optimum concentration of ABA and MT was carried out.The alfalfa seedlings were pretreated with different concentrations of ABA and MT by foliar spraying,and then subjected to salt stress treatment.The morphological,photosynthetic parameters,chlorophyll content,MDA content,soluble protein content were measured.The measured data were analyzed using the membership function method to obtain the optimum concentration of ABA and MT,both ABA and MT used in subsequent tests are the optimal concentrations;Finally,the alfalfa seedlings were pretreated with ABA,MT,ABA+MT respectively,and then treated with salt stress to explore the effects of exogenous ABA and MT spraying on the growth and physiological characteristics of alfalfa seedlings under salt stress.The main results are as follows:1.Different concentrations of salt stress significantly inhibited the growth and development of alfalfa seedlings.When Na Cl concentration was gradually increased,the plant height and chlorophyll content gradually decreased,the phenomenon of chlorosis,yellowing and wilting of leaves gradually became serious,the plant stem gradually became soft,the growth and development status gradually became worse,the MDA content and relative conductivity in leaves gradually increased,indicating that the degree of membrane lipid peroxidation and membrane damage gradually increased.The growth of alfalfa seedlings under 250 m M treatment is the most severely inhibited,which is not conducive to the follow-up test.However,the growth of alfalfa seedlings under 200 m M treatment is obviously inhibited,and all indicators are clear.Therefore,the salt treatment concentration in the follow-up study is 200 m M.2.A certain concentration of ABA and MT can alleviate alfalfa seedlings’response to salt stress in different degrees.Foliar spraying of ABA(1,10,50,100,200μM)and MT(10,50,100,500μM).After that,compared with Na Cl treatment,when hormone concentration was increased,the plant height,leaf length,leaf width,chlorophyll content,Pn and soluble protein content of alfalfa seedlings showed a trend of increasing and then decreasing,and the MDA content showed a trend of decreasing and then increasing.The results showed that exogenous hormone treatment with appropriate concentration had a certain effect on enhancing the salt tolerance of alfalfa seedlings.The five indexes of Pn,plant height,chlorophyll content,MDA content and soluble protein content were selected for membership function analysis,and finally the optimal concentration of exogenous ABA treatment was 50μM,the optimum concentration of MT is 100μM。3.The growth and physiological characteristics of alfalfa under salt stress were studied after the pretreatment of alfalfa seedlings by spraying exogenous ABA,MT and ABA+MT.Through the measurement of morphological indicators,photosynthetic related indicators,membrane permeability indicators,antioxidant related indicators,osmoregulation substances and ion content,the study found that exogenous ABA,MT and ABA+MT treatments can significantly improve the salt tolerance of alfalfa seedlings and promote the growth and development of alfalfa seedlings,and their action pathways are similar but different,among which,exogenous ABA can enhance the ability of absorbing light energy by increasing the content of chlorophyll to promote photosynthesis,and increase the content of proline,soluble protein,and soluble sugar to enhance the resistance to osmotic stress,thus improving the salt tolerance of alfalfa.Exogenous MT can promote gas exchange by removing stomatal restriction to improve photosynthetic capacity,improve CAT,SOD,POD,APX activities and As A,GSH content to enhance the ability of scavenging ROS,and then improve the salt resistance of alfalfa.Moreover,ABA,MT and their combination treatments can inhibit the accumulation of Na~+content,increase K~+content and K~+/Na~+.According to the comprehensive morphological and physiological indexes,exogenous ABA+MT treatment has better effect and is more conducive to improving the salt tolerance of alfalfa.This study preliminarily revealed the effects of exogenous ABA,MT and ABA+MT treatments on the physiological characteristics of alfalfa under salt stress,providing a reference basis for improving the salt tolerance of alfalfa.