| Cotton as a main economic crop and it is distributed in more than 150 countries in the world.There are Yellow River Basin,Yangtze River Basin and Northwest Inland cotton area in China.About one third of cotton fields in Yellow River Basin are suffering from drought stress.Seed germination and early seedling establishment are fragile stages in cotton development,which are easily affected by drought stress.It is of great theoretical and practical significance to improve the drought resistance of cotton seeds and seedlings.Melatonin,as a small molecular substances with multiple physiological regulation functions widely found existed in plants,which plays important roles in resisting a variety of abiotic stresses for plants.However,the regulation mechanism of melatonin on cotton seed germination and seedling development under drought stress has not been well studied.Therefore,in this study,ND601 and GX9 as experimental materials.10%PEG-6000 and(40±5)%relative soil water content treatments were used to simulate drought stress.By measuring the morphological and physiological characteristics of cotton seeds and seedlings,the alleviation effect of exogenous melatonin on cotton seed germination and seedling development under drought stress was explored.It provides a theoretical basis for the development and utilization of melatonin and drought resistance of cotton.The main research results are showed as follows:1.Effects of soaking seeds with melatonin on cotton seed germination under drought stress(1)Exogenous melatonin effectively enhanced the germination ability of cotton seeds under drought stress.Drought stress significantly reduced the germination potential,germination rate,radicle length and fresh weight of cotton seeds.The effects of melatonin at low concentration(10 μM,50 μM)was weak on seed germination,while high concentrations of melatonin(200 μM,500 μM)inhibited cotton seed germination.Among them,100 μM melatonin had the highest germination rate and reached a significant level compared with the 0 μM melatonin treatment.The treatment of 100 μM melatonin increased germination potential and germination rate by 8.9%and 9.8%.(2)Melatonin increased the activities of antioxidative enzymes and enhanced the scavenging ability of ROS and MDA of cotton seed germination under drought stress.Drought stress decreased the the antioxidant enzyme activity,and increased the the membrane lipid peroxidation damage during seed germination.100 pM melatonin treatment significantly increased the activities of POD,SOD and CAT.Compared with W+ DS treatment,the activities of SOD and POD increased by 2.9%and 36.2%,respectively,meanwhile,the CAT increased by 46.08%.The contents of O2-,H2O2 and MDA in MT+DS treatment were significantly lower than those in W+DS treatment.(3)Melatonin promoted the synthesis of proline,soluble sugar and soluble protein in cotton seeds under drought stress.The contents of proline,soluble sugar and soluble protein increased under drought stress.Soaking seeds with melatonin increased the content of osmoregulation substances in cotton seeds under drought stress.(4)Melatonin can regulate the content of hormones in cotton seeds under drought stress.Drought reduced ABA catabolism and GA3 biosynthesis,and inhibited cotton seed germination.After the application of 100 μM melatonin,the content of ABA decreased,the content of GA3 increased.After 4 days of seed germination,the 100 μM melatonin treatment significantly increased GA3 content in seeds under drought stress.Compared with W+DS treatment,MT+DS treatment increased by 16.09%(P<0.05).(5)Melatonin can promote cotton seed germination by changing stomatal structure of seed coat under drought stress.Compared with dry seeds and water soaked seeds,soaking seeds with melatonin increased the number of seed coat stomata on cotton,accelerated the opening of seed coat stomata,alleviated the mechanization degree of seed coat,and enhanced the water absorption capacity of seeds.2.The effect of soaking seeds with melatonin on the growth and development of cotton seedlings under drought stress(1)Soaking seeds with melatonin increased the plant height,leaf area,chlorophyll relative content and stem leaf biomass of cotton seedling under drought stress.Drought stress significantly decreased plant height,leaf area,SPAD value and material accumulation of roots,stems and leaves.After seed soaking with exogenous melatonin,the plant height,leaf area,SPAD value and dry matter accumulation of the two cotton varieties were significantly increased.At 10 days after drought stress,melatonin treatment effectively promoted plant height,leaf area and SPAD value of GX9 under drought stress,MT+DS treatment significantly increased by 5.51%,37.03%,and 12.09%,respectively,compared with W+DS treatment.(2)Melatonin treatment promoted the growth of total root length,total surface area and volume of cotton seedling under drought stress,and increased root biomass.Drought stress inhibited the root elongation of cotton seedlings.After soaking seeds with melatonin,the total root length,total surface area and volume of seedlings were significantly increased.At 17 days after drought stress,compared with W+DS treatment,the root length,surface area and volume of GX9 increased by 26.84%,42.09%,and 67.47%respectively,under MT+DS treatment.The root length and surface area of ND601 in MT+DS treatment were higher than those in W+DS treatment,and the difference was significant(P<0.05).(3)Exogenous melatonin improved the photosynthetic capacity of cotton seedling main stem under drought stress.Drought stress decreased the photosynthetic parameters,including net photosynthetic rate,stomatal conductance,intercellular CO2 concentration and transpiration rate,the maximum conversion efficiency(Fv/Fm)of PSII was reduced,the variable fluorescence parameters,such as Wk and V,increased,the probability that the exciton trapped in the reaction center pushes the electron transfer to the electron acceptor whose electron transfer chain exceeds QA(ψo)and the performance indices based on absorption of light energy(PIabs)decreased significantly,which resulted in the decrease of photosystem Ⅱ reaction center activity and the inhibition of photosynthesis.Compared with W+DS treatment,MT+DS treatment significantly increased Fv/Fm,Wk and Vj decreased,and ψo and PIabs increased significantly,which was more significant on the 10th and 17th days after the stress during seedling growth.(P<0.05).After the soaking seeds with melatonin,the net photosynthetic rate,stomatal conductance,intercellular CO2 concentration and transpiration rate increased significantly.In view of the above results,exogenous melatonin enhanced the antioxidant enzyme activity of cotton seeds,reduced the accumulation of ROS and MDA,increased the content of osmoregulation substances,and promoted ABA decomposition and GA3 synthesis,therefore,increased the germination rate of cotton seeds under drought stress.Soaking seeds with melatonin can promote the growth and development of seedling shoot and root of cotton under drought stress,improved the photosynthetic performance of seedling leaves,and enhanced the drought stress resistance of cotton seedling. |
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