Mitigative Effects Of Melatonin Pretreatment On Cinnamic Acid And Phenylpropionic Acid Stress In Cucumber

Cucumber（Cucumis sativus L.）is one of the main vegetables in China,with both raw and cooked use,large cultivation area,and mainly cultivated in facilities.However,due to the high degree of intensive and continuous cultivation,the obstacles to continuous cropping of cucumbers are more serious.The self-toxicity of cucumber root exudates such as Cinnamic acid（CA）and phenylpropionic acid（PA）is one of the main causes of cucumber continuous cropping disorders.Melatonin（MT）is a strong antioxidant that can promote plant resistance to abiotic stress and biological stress,improve plant antioxidant capacity and stress resistance.Therefore,it is of great significance to study the mitigating effect of melatonin on the double stress of cucumber cinnamic acid and phenylpropionic acid.In this study,the cucumber was irrigated with melatonin as the material,and then the root was irrigated with a mixed solution of cinnamic acid and phenylpropionic acid to simulate the self-toxic environment of cucumber,and the alleviation effect of melatonin pretreatment on the double stress of cucumber cinnamic acid and phenylpropionic acid was explored,which provided a reference for alleviating continuous cropping disorders.The test results are as follows:1.Melatonin pretreatment can reduce the effect of cinnamic acid and phenylpropionic acid double stress on cucumber growth by increasing cucumber plant height,stem thickness,plant dry fresh quality,leaf number,root vitality and leaf yellowing,and melatonin can reduce the relative conductivity（REL）,malondialdehyde（MDA）and superoxide anion（O₂^-）content of cucumber leaves under stress,and enhance superoxide dismutase（SOD）,peroxidase（POD）,by increasing the content of proline（Pro）,superoxide dismutase（SOD）,peroxidase（POD）,Ascorbate peroxidase（APX）,catalase（CAT）,polyphenol oxidase（PPO）activities and upregulated the expression of antioxidant enzyme-related regulatory genes Cs SOD,Cs POD,Cs APX,and Cs CAT to alleviate the damage caused by cell membrane lipid peroxidation to plants.In addition,melatonin pretreatment can increase the content of auxin（IAA）,gibberellin（GA）,zeatin nucleoside（ZR）and melatonin（MT）in cucumbers under stress,reduce the accumulation of abscisic acid（ABA）in cucumbers,upregulate the expression of MT synthesis-related regulatory genes Cs TDC,Cs T5H,Cs SNAT and Cs ASMT,and promote the normal growth and development and antioxidant capacity of cucumbers under stress;2.Melatonin pretreatment can reduce the damage of cucumber chloroplast structure by cinnamic acid and phenylpropionic acid double stress,reduce the accumulation of intercellular carbon dioxide,improve gas exchange parameters and chlorophyll fluorescence parameters,enhance the activity of 1,5-bisphosphate ribonulose carboxylase（Rubisco）and upregulate its related regulatory genes Csrbc L,The expression of Csrbc S to alleviate the effect of cinnamic acid and phenylpropionic acid double stress on the photosynthetic capacity of cucumbers;3.The double stress of cinnamic acid and phenylpropionic acid disrupts the carbon and nitrogen metabolism of cucumbers.Melatonin pretreatment can increase the activity of sucrose synthase（SS）,sucrose phosphosynthase（SPS）,acid invertase（AI）and neutral invertase（NI）under stress,upregulate the expression of SS and SPS synthesis-related regulatory genes Cs SUS3 and Cs SPS4,reduce the content of sucrose,glucose and fructose under stress,and enable normal transport and utilization of sucrose.In addition,under the dual stress of cinnamic acid and phenylpropionic acid,melatonin pretreatment can increase the activity of nitrate reductase（NR）,glutamine synthase（GS）,glutamate synthase（GOGAT）and glutamate dehydrogenase（GDH）,and upregulate the expression of its related regulatory genes Cs NR,Cs GS,Cs GOGAT,and Cs GDH,increase the nitrate nitrogen content,reduce the accumulation of ammonium nitrogen,and enable plants to carry out normal nitrogen metabolism;4.Melatonin pretreatment can improve the survival rate of root border cells（RBCs）and root viability（retention of RBCs）of cucumber under double stress of cinnamic acid and phenylpropionic acid,reduce the apoptosis rate of RBCs and the activity of apical pectin methylesterase（PME）,further increase the thickness of the adhesive layer of RBCs,improve the resistance of cucumber root tips to stress,and make the root system grow normally;5.Under the double stress of cinnamic acid and phenylpropionic acid,the content of nitrogen（N）,phosphorus（P）and potassium（K）in various organs of cucumber decreased,and the yield and quality of fruits decreased,but melatonin pretreatment could alleviate these phenomena,increase fruit yield and improve fruit quality.In summary,melatonin pretreatment can enhance the antioxidant capacity and stress resistance of plants,improve photosynthesis capacity,promote carbon and nitrogen metabolism of plants,improve the activity of root edge cells and the content of elements of various organs of plants,alleviate the effects of cinnamic acid and phenylpropionic acid double stress on cucumbers,and make cucumbers grow normally,thereby improving fruit yield and quality.