Study On The Alleviation Effect Of Exogenous Melatonin On Tomato Seedlings Under NaHCO₃ Stress

Processed tomato（Solanum lycopersicum L.）is an important processed crop in Xinjiang.But at present,soil salinization has become one of the main obstacles restricting the production of processed tomatoes in Xinjiang.The damage of alkaline salts to plants is far greater than that of neutral salts.Therefore,it is of great significance to study the mechanism of tomato seedling tolerance under alkaline salt（NaHCO₃）stress.previous studies have shown that melatonin（MT）,as an endogenous signaling molecule,is involved in the regulation of plant growth and stress resistance.Therefore,in this study,the processed tomato variety’Ligel 87-5’was used as the material,and 100μmol·L^-1and 400μmol·L^-1MT were sprayed on the leaves of tomato seedlings under NaHCO₃stress to investigate the exogenous Mitigation effect and mechanism of MT on tomato seedlings under NaHCO₃stress.The main results are as follows:1.NaHCO₃stress led to the disruption of ion balance in tomato seedling leaves,the reduction of photosynthetic pigment content,the activity and efficiency of photosynthetic electron transport,the damage to both the donor side and the acceptor side of the photosynthetic electron transport chain,the reduction of antioxidant enzyme activities,the accumulation of ROS,and the loss of membrane lipids.Oxidation is increased,thereby inhibiting the growth of tomato seedlings.The increase of endogenous MT level and Pro content under NaHCO₃stress is the adaptive response of tomato seedlings under alkaline stress,while exogenous application of 100 and 400μmol·L^-1MT can promote tomato seedlings under non-stress conditions to varying degrees.growth and alleviation of NaHCO₃stress on tomato seedling growth inhibition.Under non-stress conditions,the promotion effect of 400μmol·L^-1MT was the best.Under NaHCO₃stress,the mitigation effect of 100μmol·L^-1MT was better than that of 400μmol·L^-1MT.Under NaHCO₃stress,exogenous application of 100μmol·L^-1MT significantly reduced Na⁺content,increased K⁺,Ca²⁺content,and K⁺/Na⁺,Ca²⁺/Na⁺and Mg²⁺/Na⁺ratios by mediating the increase of endogenous MT content to maintain Cell ion balance under NaHCO₃stress;increase the content of photosynthetic pigments,reduce the degree of damage to the donor side and acceptor of the photosynthetic electron transport chain,improve the activity and efficiency of photosynthetic electron transport,optimize the share of light energy distribution in the PSII reaction center,and upregulate the key enzyme FBA of the Calvin cycle,FBP and PGK gene expression levels,thereby improving the photosynthetic performance of tomato seedlings;regulating SOD,APX,POD and CAT activities,up-regulating soluble sugar,glucose and sucrose content,and improving antioxidant and osmotic regulation capabilities.Therefore,the inhibition of tomato seedling growth by NaHCO₃stress was alleviated,and the alkali tolerance of tomato was improved.2.Under non-stress conditions,the root medium environment of tomato seedlings was neutral,and the organic acid secreted by the roots was mainly oxalic acid;the pH value of the root medium environment of the tomato root system under NaHCO₃stress was higher than that of the control after 8 h of acid secretion,and it was alkaline.The pH value of the root medium environment decreased with the prolongation of acid secretion time（pH was 8.48-8.74）,but it was still alkaline.The main organic acid is succinic acid.Under NaHCO₃stress,exogenous application of 100μmol·L^-1MT could significantly reduce the pH value of the root medium environment,and the pH value decreased to acidic after 20 h of acid secretion.There was no significant effect on the content of total organic acids,but the content of oxalic acid was increased to different degrees.It indicated that exogenous 100μmol·L^-1MT could improve the damage of high pH value caused by alkali stress by regulating the content of organic acid components excreted by the root system and reducing the pH of the rhizosphere medium.3.Transcriptomic analysis showed that compared with NaHCO₃stress,N+100MT and N+400MT treatments identified 160（68 up-regulated,98 down-regulated）and 524（210 up-regulated,314 down-regulated）DEGs,respectively Under N+100MT treatment,GO analysis showed that biological processes such as negative regulation of peptidase and endopeptidase activities,redox processes and hydrolase activities played important roles.KEGG cluster analysis showed that DEGs involved in signal transduction,amino acid synthesis and secondary metabolism play a key role in alleviating alkali stress by exogenous MT,and mainly concentrated in phenylalanine metabolism,carotenoid biosynthesis,and MAPK signaling pathway-plant.The results indicated that 100μmol·L^-1MT alleviated the damage of NaHCO₃stress and improved the tolerance of tomato seedlings by regulating the above pathways.