Study On The Concentration Effect Of Melatonin On Salt Tolerance Of Tomato Seedlings And Its Physiological Mechanism

Tomatoes are often negatively affected by soil salinization,affecting their yield and quality.Melatonin(MT)is a kind of tryptophan derivative widely found in living organisms.Recent studies have found that MT can regulate many important processes of plant growth and development and plant response to MT under abiotic or biotic stress.For instance,MT improves the salt tolerance of plants such as melon,grape,Malus hupehensi,rice and Pennisetum.However,little research has been done about the effects of MT on salt tolerance of tomato(Solanum lycopersicum)seedlings.In this study,seedlings were soaked with exogenous MT at different concentrations and exogenously applied to the seedlings.In this study,the effects of MT treatment on seed germination and physiological characteristics of tomato seedlings under salt stress were investigated.Furthermore,the concentration effect of melatonin on salt tolerance of tomato seedlings and its physiological mechanism were also explored,which would provide an important theoretical basis for improving the salt tolerance of tomato plants.The main results were as follows:Under 150 mM NaCl stress,the effects of 24 h soaking with 12.5-400?M MT on tomato seed emergence rate,root length,hypocotyl length and dry/fresh weight were studied.The results showed that exogenous MT treatment had a dose-alleviated effect on NaCl inhibition of tomato seed germination,which had the best effect under the treatment of 100 ?M MT and significantly increased the seed emergence rate,hypocotyl length,root length and dry/fresh weight of tomato.In the fifth-leaf period,the tomato seedlings were treated with different concentrations of salt stress,and the growth of tomato seedlings was inhibited to varying degrees.Exogenous application of MT significantly increased the growth of tomato seedlings under low salt(75 mM NaCl)and high salt(150 mM NaCl)stress,indicating that MT treatment could effectively alleviate the damage of tomato seedlings under salt stress.With the increase of exogenous MT concentration,the improvement of biomass and chlorophyll content of tomato seedlings under salt stress was obvious.With the treatment of 100 ?M MT,the promotion effects of MT were the most significant.The accumulation of Na in roots,stems and leaves of tomato seedlings was significantly reduced,and the K/Na ratio increased significantly.Furthermore,MT treatment significantly increased the content of N,P and K in tomato seedlings under salt stress,which is of great significance for maintaining the nutrient balance in tomato,ensuring its normal physiological metabolism and promoting the growth of tomato plants.In order to further explore the physiological mechanism of exogenous MT improving the salt tolerance of tomato seedlings,MT treatment under 150 mM NaCl stress in tomato seedlings at the fourth-leaf stage were conducted and the results indicated that 100 ?M MT treatment could significantly increase tomato seedling biomass,leaf chlorophyll content and the net photosynthetic rate(Pn),and decreased the stomatal limit value(Ls).In addition,the soluble sugar content and K content in roots,stems and leaves were improved in tomato seedlings under salt stress after 100?M MT treatment,and the roots to absorption of elements such as N,P,K,Ca,and Mg,and accumulation of N,P,and K in the upper leaf were also promoted.Spraying MT could regulate the selective absorption of K and Na in tomato seedlings,while reduce the Na content and increase the K/Na ratio,especially promote K transport to the shoot.In addition,MT treatment significantly increased the activities of APX,SOD,GS,GOGAT,GR,Rubisco and GSH content,decreased GSSG and MDA content,which increased the GSH/GSSG ratio,then reduced the level of reactive oxygen species(ROS),and increased the oxidation-reduction ability,protection of photosynthetic apparatus,promotion of photosynthesis of leaves,etc.,finally enhanced tolerance of tomato seedlings to salt stress.The results showed that exogenous MT could improve the salt tolerance of plants by reducing ion poisoning,improving the balance of ionic nutrients in plant,promoting the photosynthesis of leaves,and enhancing the activity of protective enzymes in vivo.