Effect Of Melatonin On Cotton Seed Germination And Seedling Physiological Metabolism Under Salt And Drought Stress

【Objective】Drought and salinity are common abiotic stress factors in crop growth in arid areas,which have an important impact on crop growth and development.As the most valuable cash crop in Xinjiang,cotton is widely distributed in the north and south of Xinjiang.Drought and salt stress seriously affect its normal growth.Drought and salt stress seriously affect its normal growth.Plant melatonin plays an important role in resistance regulation.The resistance mechanism of melatonin to cotton under drought and saline-alkali stress was studied,It is of great value to promote the effective utilization of saline land in Xinjiang and realize high and stable yield of cotton.【Methods】1.Through the seed germination test,different concentrations of melatonin treatment were set to determine the seed germination characteristics under salt stress,drought stress and salt-drought compound stress,and the optimal melatonin concentration to improve seed germination under each stress treatment was determined.2.Through the pot test,set up 7 treatments of CK（blank）,S（salt）,D（drought）,S+D（sal+drought）,S+M（salt+melatonin）,D+M（drought+melatonin）,S+D+M（salt+drought+melatonin）,to determine the changes of various physiological indexes such as photosynthetic physiological characteristics,antioxidant enzyme activity,osmoregulatory substances and ion balance of cotton seedlings,and comprehensively analyze the adaptability and physiological response of melatonin to cotton under salt and drought stress.【Results】1.Melatonin can alleviate the germination index of cotton seeds under salt stress,drought stress and salt-drought combined stress,and the concentration of 150μmol·L^-1is the best.2.The photosynthetic pigment content,photosynthetic capacity and electron transfer rate of cotton seedlings were significantly inhibited under salt,drought stress and salt and drought combined stress.After spraying melatonin,the photosynthetic pigment content,gas exchange parameters（Net photosynthetic rate,Transpiration rate,Intercellular CO₂ concentration,Stomatal conductance）and chlorophyll fluorescence parameters（Initial fluorescence,Maximum fluorescence,Maximal photochemical efficiency,Actual photochemical efficiency,Photochemical quenching）.Among them,the net photosynthetic rate of S+M,D+M and S+D+M treatments was 41.81%,36.69%and 53.47%higher than that of the control（without melatonin）,the maximum photosynthetic efficiency was 10.87%,19.23%and 7.89%higher,The actual photochemical efficiency increased by 17.95%,4.35%and 19.44%,while the PSII regulatory energy dissipation decreased by 23.08%,10.00%and 29.79%,the non-photochemical quenching coefficient decreased by 50.00%,9.67%and 41.94%,and the PSII non-regulated energy dissipation decreased by19.15%,13.46%and 23.61%.3.Melatonin increased the activity of antioxidant enzymes in cotton leaves and other osmotic adjustment substances under salt and drought stress.S+M,D+M,S+D+M treatments increased Catalase enzyme activity by 24.01%,31.25%,33.74%,Proline content by 14.10%,11.85%,11.66%,Soluble sugar content by 17.17%,17.31%,17.72%,Free amino acid content by 10.35%,18.83%,18.46%,Malonaldehyde content by 5%,3.98%,6.73%,Hydrogen peroxide content by 18.48%,19.59%,17.81%compared with the control（without melatonin）.Melatonin increased the biomass of cotton under salt and drought stress.For example,the underground fresh weight of S+M,D+M and S+D+M treatments increased by 22.32%,13.33%and 7.49%,respectively,compared with that of the control,and recovered to 95.43%,83.24%and 71.82%of the control.4.The content of Na⁺in cotton plants increased,while the contents of K⁺,Ca²⁺and Mg²⁺decreased under salt,drought and combined stress of salt and drought.Exogenous spraying melatonin significantly reduced the content of Na⁺,increased the content of K⁺,Ca²⁺and Mg²⁺,and improved the ion transport capacity under salt,drought and salt-drought combined stress,so as to maintain the cell ion balance under salt and drought stress.【Conclusion】Salt and drought stress inhibited cotton seed germination and physiological metabolism at seedling stage,and the combination of salt and drought aggravated the degree of stress.Melatonin can alleviate the damage of cotton seeds and seedlings to salt and drought stress,150μmol·L^-1the seed soaking or coating with melatonin can improve the seed germination ability and seedling emergence rate of cotton under salt and drought stress.Spraying 150μmol·L^-1melatonin at the seedling stage of cotton can alleviate the physiological damage caused by salt and drought stress to the photosynthetic system,antioxidant enzyme system,osmotic regulation system and ion balance of the plant during the growth process,improve the salt and drought resistance of cotton seedlings,and provide a reasonable and effective solution for the cotton flower seed germination and physiological stress caused by salt and drought at the seedling stage in Xinjiang arid areas.