Effects Of Exogenous Proline And Salicylic Acid On Physiological Characteristics And Antioxidant Enzyme Gene Expression Of Alfalfa Under Saline-alkali Stress

Alfalfa is a perennial herb of the leguminous family Medicago.It is known as the"king of pastures".Because of its high economic benefits and ecological value,it is the most widely cultivated artificial forage in China and the world.During the production process,Alfalfa often suffers from"physiological drought"or even"burning seedlings"due to soil salinization,which seriously restricts its quality and yield as a diet.In order to find an effective way to improve the alfalfa’s salt and alkali tolerance,this study took a"golden queen"alfalfa as the material,under three saline and alkali concentrations(50 mmol·L^-1,100 mmol·L^-1,200 mmol·L^-1),and applied a single proline(30 mmol·L^-1),a single salicylic acid(0.05 mmol·L^-1)and two The effects of exogenous proline and salicylic acid on alfalfa under saline-alkali stress were studied from the three aspects of Growth index,physiological characteristics,and antioxidant enzyme gene expression.The main results are as follows:（1）50 mmol·L^-1 low-concentration saline-alkali treatment has a slight promotion effect on alfalfa’s plant height,root length,fresh weight,dry weight and leaf area,but 100 mmol·L^-1 medium concentration and 200 mmol·L^-1 high concentration salt stress are severely inhibited Growth index of alfalfa.Exogenous proline and salicylic acid can alleviate the inhibition of alfalfa’s Growth index to a certain extent,but it can’t completely eliminate the damage of alfalfa by medium and high concentration In addition,the proline and salicylic acid treatments had better ameliorative effects on alfalfa Growth index than the single treatment.（2）Saline-alkali stress caused a large accumulation of O₂^·-and H₂O₂ in alfalfa plants,SOD activity,POD activity,CAT activity,MDA content,relative conductivity,proline content,soluble sugar content,and soluble protein content increased,leaves Relative water content and root activity decreased.The total chlorophyll content and net photosynthetic rate of alfalfa were not significantly affected under 50 mmol·L^-1low-alkali stress,but both were significantly suppressed under 100 mmol·L^-1 medium and 200 mmol·L^-1 high-stress.It is consistent with the trend of Growth index.Exogenous application of proline and salicylic acid can increase the SOD,CAT and POD activities of alfalfa to varying degrees,effectively remove H₂O₂ and O₂·^-from the leaves,and significantly inhibit the MDA content and relative conductance induced by saline-alkali stress,and reduced the degree of membrane lipid peroxidation.and induced alfalfa to accumulate more organic osmotic adjustment substances such as proline,soluble sugar and soluble protein,effectively alleviating osmotic stress and increasing the root vitality of alfalfa.The chlorophyll loss of alfalfa under saline-alkali stress was improved,photosynthesis was promoted,and the oxidative damage of alfalfa seedlings was significantly alleviated by saline-alkali stress.The proline and salicylic acid treatments had better alleviating effects on alfalfa physiological indexes than the single treatment.（3）Alfalfa’s Fe-SOD,Cu/Zn-SOD,Mn-SOD,POD,and CAT gene expression increased due to salt stress,while exogenous proline and salicylic acid could further promote the up-regulation of antioxidant enzyme gene expression.The oxidative stress on alfalfa was effectively alleviated,and the combined effect of proline and salicylic acid on the expression of antioxidant enzyme genes in alfalfa was better than the single treatment.Exogenous proline and salicylic acid can effectively up-regulate CAT gene expression at 50 mmol·L^-1 low and 100 mmol·L^-1 medium saline-alkali stress,but exogenous proline and salicylic acid under 200 mmol·L^-1 high saline-alkali stress Acid has no significant effect,which is consistent with the trend of CAT activity,indicating that exogenous proline and salicylic acid have limited relief effects under high concentration of saline-alkali stress.