Mitigation Effect Of Exogenous Melatonin Application On Cotton(Gossypium Hirsutum L.) Under Salt Stress And Its Mechanisms

Melatonin(MT)is a plant growth regulator that enhances plant tolerance to a variety of environmental stresses,including salt damage.In this study,based on the comparison of salt tolerance of two cotton varieties(Gossypium hirsutum L.)Lu 7619 and Xin lu zao 51 at the germination and seedling stages,on the one hand,MT was applied exogenously to germination of cotton seeds under salt stress.The mechanism of exogenous melatonin application to alleviate germination of cotton seeds under salt stress was analyzed by germination phenotype,germination-related indicators,lipase activity and transcriptional patterns of GDSL lipase gene GhGLIP.On the other hand,cotton seedlings were respectively soaking seed with melatonin and foliar spraying of melatonin under salt stress.Suitable concentration of melatonin was selected to alleviate the growth of cotton seedling by growth phenotype and fresh weight per plant of cotton seedling stage.Based on this,cotton seedlings that treated with suitable concentration of melatonin were selected to determine relative electrolyte leakage rate,root vigor,hydrogen peroxide content,antioxidant enzyme activity,Na+,K+,and Cl-contents and to detect the transcriptional patterns of gene(GhSOS1,Gohir.D04G077700 and Gohir.Dl1G062600)that related to ion transport of cotton seedlings.The mechanism of exogenous application of melatonin to alleviate the growth of cotton seedlings under salt stress was analyzed.The main results are as follows:After treatment of Lu 7619 and Xin lu zao 51 with 50,150,250 mM NaCl solution,comprehensive identification and comparison of relative germination potential,relative germination rate and other indexes of cotton seeds and relative height per plant,relative fresh weight per plant,relative electrolyte leakage of cotton seedings showed that the inhibition of salt damage on Xin lu zao 51 was greater than that on Lu 7619.Lu 7619 was selected as the salt resistant test materialand Xin lu zao 51 as the salt sensitive test material.Exogenous application of 50-200 μM MT can significantly alleviate the inhibition of seed germination and significantly increase germination potential,germination rate,lipase activity and other indexes of Lu 7619 and Xin lu zao 51 under 150 mM NaCl stress.Moreover,the relative expression of GhGLIP gene in two cotton seeds were up-regulated to varying degrees after treatment with 50-200 μM MT under salt stress,of which Xin lu zao 51 was up-regulated more significantly.Soaking seeds with 25 μM MT or foliar spraying of 75 μM MT can alleviate the growth inhibition of Lu 7619 and Xin lu zao 51 seedlings under 200 mM NaCl stress,mainly by increasing fresh weight per plant and root vigor,reducing the relative electrolyte leakage rate and hydrogen peroxide content,enhancing antioxidant enzyme activity,reducing Na+,Cl-contents,Na+/K+ ratio and so on.After foliar spraying of 75 μM MT under salt stress,the relative expression levels of GhSOS1,Gohir.D04G077700 and Gohir.D11G062600 in two cotton leaves were significantly up-regulated on the second day,which was larger than that of salt treatment alone.Exogenous application of 50,100,and 200 μM MT had no difference in alleviating cotton seed germination under salt stress and was better than 10 μM MT.400 μM MT could inhibit seed germination under salt stress.Seed soaking with 10,25,and 75μM MT showed no significant difference in mitigation effects on the growth of Lu 7619 seedlings under salt stress.The mitigation effect of 25 or 75 μM MT soaking was better than 10 μM MT of Xin lu zao 51 seedings under salt stress,and there was no significant mitigation effect of two cotton seedings after seed soaking with 175 μM MT.Foliar spraying of 25 or 75 μM MT showed significant mitigation effect on the growth of two cotton seedlings under salt stress,and mitigation effect of foliar spraying of 75 μM MT was better than 25 μM MT of Xin lu zao 51 seedings.The mitigation effect of foliar spraying of 10 or 125 μM MT of two cotton seedings was not obvious under salt stress.In conclusion,exogenous application of melatonin can alleviate the salt damage of cotton in germination and seedling stage,mainly by regulating the lipase activity and lipase gene expression during cotton seed germination,as well as enhancing the antioxidant capacity and maintaining ion homeostasis during cotton seedlings.Moreover,melatonin has a dose-dependent mitigation effect on the germination and seedling stages of cotton.Treatment with too low or too high concentrations of melatonin has no mitigation or inhibition effect.Different melatonin treatment methods(soaking seed and foliar spray)showed different mitigation concentrations,and the same melatonin treatment method showed different mitigation concentrations for cotton varieties with different salt tolerance.These are of great significance for improving the salt tolerance of cotton in saline areas.