Effects Of Exogenous NO On Growth And Membrane Lipid Peroxidation Of Alfalfa Under Salt Stress

Alfalfa?Meidicago sativa L.?is a perennial legume plant with an honor of?King of Forage?.Alfalfa is the most widely planted forage in the world,and is also the most widely distributed leguminous forage species with the highest economic value in China.Alfalfa is a good source of protein for livestock because of its good palatability,high yield and abundant proteins.The main producing areas of alfalfa in China are located in the northwest,northernand northeast areas,where soil salinization is serious,and the salinization area is expanding.The salinization can cause salt stress and affect alfalfa.Hence,reducing the negative effects of salt stress on the growth and development is urgently needed,and it is particularly important to improve alfalfa production.Nitric oxide?NO?is a multifunctional active molecule,which widely exists in organisms and plays an important role in plant growth and stress responses.In the present study,three common cultivars varieties of alfalfa with different salt tolerance,Bara 310SC,Zhongmu No.1 and Golden Queen were used as experimental materials.Three differentsalt solutions,low(24 mmol·L^-1),medium(72 mmol·L^-1)and high(120 mmol·L^-1)concentrations NaCl and Na₂SO₄ were used to simulated salt stress,then the exogenous NO donor sodium nitroprusside?SNP?was usedto treat alfalfa with five different concentrations of 0,0.05,0.1,0.2 and 0.3 mmol·L^-1.The effects of NO on physiological and biochemical characteristics of alfalfaunder salt stress were investigated.The main results are as follows:1.Under salt stress,the fresh weight,dry weight and chlorophyll contents of alfalfa decreased.The relative electrical conductivity,malondialdehyde?MDA?,hydrogen peroxide?H₂O₂?and peroxidase?POD?remarkably increased?P<0.05?,while ascorbic acid?AsA?contents,superoxide dismutase?SOD?and catalase?CAT?activities decreased significantly?P<0.05?,suggesting the toxic effects of salt stress on alfalfa.2.ExogenousNOsignificantly increased fresh weight,dry weight,AsA and chlorophyll levels,as well as chlorophyll a/b ratio?P<0.05?.What's more,it improved SOD,CAT and POD activities,and decreased relative electrical conductivity,MDA and H₂O₂ contents,revealing that exogenous NO could enhance the resistance of alfalfa to salt stress via increasing antioxidant enzyme activities and reducing lipid peroxidation toscavenge reactive oxygen species?ROS?.3.The effects of NO on alfalfa are dual,and the promoting effects on the growthincreased first and decreasedlater with the increase of NO concentration.Low concentration of NO alleviated oxidative damagecaused by salt stress and promoted plant growth.When NO concentration exceededa threshold,the promoting effects of NO on plant growth were inhibited.Furthermore,when NO concentration was too high,it would produce toxic effects on alfalfa and aggravate the damage of salt stress.In addition,the optimum NO concentration of each cultivar of alfalfa changed with the increase of salt concentration.When salt concentration increased,the sensitivity of alfalfa to NO decreased.Compared with low concentration of salt treatment,alfalfa needed higher concentrations of NO to achieve the maximum promoting effects.That is,the optimum concentration of NO increases with the increase of stress intensity.To sum up,the application of optimum exogenous NO can effectively alleviate the effects of salt stress on alfalfa,and promote growth and development,thereby improving production.