Effects Of Exogenous Melatonin On Growth And Fruit Yield And Quality Of Tomato Under Drought Stress

By the impact of climate warming,water resources crisis further worsened,drought has become one of the prominent problems restricting agricultural production.Tomato is one of the main horticultural crops in protected cultivation and field cultivation,and it often encounters water deficit in the production process.Drought stress is one of the main factors limiting plant productivity,which affects the growth and development of tomato.Its external morphology,physiological and biochemical indexes will have a series of changes,and then affect its reproductive growth,it has certain influence on yield and quality index.Melatonin（MT）is a kind of indoleamine substance which is necessary to plant,and it plays an important role in plant resistance to stress.In this experiment,the seedlings of AC tomato were used as subjects,and the root was treated with 100μmol/L melatonin solution,the biomass,relative water content of leaves,active oxygen,antioxidant enzyme activity,osmotic adjustment substance,root morphology,endogenous hormones and elements were measured under drought stress,the effects of exogenous melatonin on the growth,root morphology and ABA content of tomato seedlings were studied.The main findings are as follows:（1）Melatonin could alleviate the damage caused by drought stress on tomato seedlings,especially at the concentration of 100μmol/L.the method of applying melatonin was 10 ml per root per day for 3 days.（2）Under drought stress,plant height,stem diameter,aboveground and underground biomass and relative water content of tomato seedlings decreased significantly,compared with drought treatment,root application of melatonin increased plant height,stem diameter,aboveground biomass and relative water content by 16.67%,7.39%,10.58%and 13.31%,respectively.Root application of melatonin improved the growth of tomato seedlings.（3）Compared with drought treatment,the content of chlorophyll increased by 40.51%,and the net photosynthetic rate increased by 1.2 times,stomatal conductance and transpiration rate increased by 74.11%and 50.86%,respectively,and intercellular CO₂concentration decreased by 12.63%.（4）Drought stress significantly increased the content of reactive oxygen species（ROS）,relative conductivity and malondialdehyde（MDA）,and increased the activities of antioxidant enzymes and osmotic regulators,the activities of antioxidant enzymes further increased and the content of osmoregulatory substances decreased.Melatonin decreased the content of reactive oxygen species（ROS）under water deficit,which indicated that melatonin reduced the damage of membrane by increasing the activity of antioxidant enzymes and regulating the content of osmotic regulators.（5）Under drought stress,the ABA content in leaves and roots of tomato seedlings was significantly increased by 4.3-fold and 16.4-fold,respectively,and the expression levels of ABA synthesis genes NCED1,NCED2 and NCED3 were also significantly increased,compared with T2 treatment,the ABA content in leaves and roots of T3 treatment was significantly lower,and the expression of genes in T3 treatment was also significantly lower than that of T2 treatment.Melatonin can enhance the resistance to drought stress by regulating the content of abscisic acid in tomato seedlings.（6）Melatonin increased the root activity of tomato seedlings under drought stress.The root growth and development of tomato seedlings were inhibited by drought stress.The root length,root surface area,root mean diameter,lateral root number and root volume were not significantly increased by melatonin treatment.Melatonin can improve plant growth and development under drought stress.It may regulate root growth by mediating endogenous ABA biosynthesis and metabolism,and alleviate the adverse effects of drought on lateral root development.（7）Drought treatment could improve the yield and quality of tomato fruit under drought stress.Compared with T2 treatment,the average yield and single fruit weight per plant were increased by 61.92%and 37.79%respectively.Moderate drought stress can improve fruit quality,and melatonin treatment can further improve fruit quality.In summary,melatonin improved plant growth and development under drought stress,and improved the resistance of tomato seedlings to drought stress by regulating osmotic stress substance content and antioxidant enzyme activity,it may also regulate root growth by mediating endogenous ABA biosynthesis and metabolism,which is beneficial to improve the efficiency of water and nutrient absorption and utilization,thus improving the yield and quality of tomato fruit under drought stress,at the same time,melatonin can also promote the growth of seedlings and fruit quality under normal water conditions.