Physiological Responses In Alfalfa To Exogenous Melatonin Under Drought Stress

Alfalfa?Medicago Sativa L.?is known as the“King of Forages”.It plays a key role in the development of global animal husbandry with advantages of high crude protein content,good palatability,and high yield.In recent years,due to high speed advancement of human society's industrialization,it is a great impact on global ecosystems.The frequent extreme weather occurred and greatly affected agricultural production in China and even in the whole world.Drought is increasingly prominent in agricultural production.It is becoming increasingly important to solve the problem of crop growth in an arid environment.Melatonin,as an antioxidant,was first found in animals.Recent years,it has been widely studied on its function for stress resistance in plants,especially in the horticultural crops.In this study,Bara 310SC and Longdong cultivar were used as materials.Polyethylene glycol-6000?PEG?solution?w/v?was used to simulate drought stress and the optimal concentration of 15%PEG was selected.Under drought stress?15%PEG?,0,50,100,200and 400?mol L^-1 melatonin were applied to investigate the effect of melatonin on morphology,fresh weight,dry weight,photosynthetic pigment,antioxidant system and nitrogen metabolism.The main results are as follows:1.With drought stress simulated by PEG:the germination rate,germination potential,germination index and fresh weight decreased with the increase of PEG concentration;the activities of SOD,POD and CAT were enhanced under high concentration.2.Exogenous melatonin inhibited the accumulation of MDA,H₂O₂ and O₂·?under the drought stress induced by 15%PEG,enhanced the antioxidant enzyme activity significantly,and increase the fresh weight and dry weight.However,when the concentration of melatonin was relatively higher(200 and 400?mol L^-1),the promotion was poor and even showed a toxic effect.In addition,the optimal concentration of melatonin for Bara 310SC and Longdong was 100?mol L^-1.3.The drought stress simulated by 15%PEG inhibited the content of chlorophyll a,chlorophyll b,carotenoids and chlorophyll,while exogenous melatonin could increase chlorophyll a,chlorophyll b,carotenoids and chlorophyll under drought stress.However,the higher concentrations of melatonin had less effect on the increase of photosynthetic pigment content,and even had an inhibitory effect.4.Under drought stress of 15%PEG,total nitrogen and nitrate nitrogen content decreased,whereas the ammonia nitrogen content increased.The activities of NR,GS,GOGAT,and GDH decreased,and the nitrogen metabolism process of alfalfa was inhibited.Exogenous addition of appropriate melatonin increased total nitrogen and nitrate nitrogen content,reduced the accumulation of ammonia nitrogen,and enhanced the activities of NR,GS,GOGAT,and GDH.In addition,total phosphorus content of alfalfa decreased under drought stress,while melatonin which concentration was 100?mol L^-1 had no significant effect on total phosphorus.In summary,under drought stress,appropriate concentration of melatonin(100?mol L^-1)can enhance the antioxidant enzyme activity of alfalfa,inhibit oxidative damage,thereby improving its drought resistance.Thus it enhances the activities of nitrogen metabolism-related enzymes,and promotes the process of nitrogen metabolism.Hence,the quality of alfalfa will be improved because of the increased contents of total nitrogen.