Physiological Mechanisms And Metabolomics Analysis Associated With The Alleviation Of Drought Stress By CO₂ Enrichment In Cucumber

The experiment used‘Jinyou No.35’cucumber was chosen as the test material,which were arranged in a randomized complete block split-plot design.The main plot was CO₂,including an ambient concentration of approximately 400μmol·mol^-1（designated A）and an elevated concentration of 800±20μmol·mol^-1（designated E）,while the subplot was water treatments consisting of control conditions（nutrient solution;designated 0）,moderate drought stress（nutrient solution+5%polyethylene glycol 6,000;designated 1）,and severe drought stress（nutrient solution+10%polyethylene glycol 6,000;designated 2）.The effects of physiological characteristics and metabolome of cucumber under elevated CO₂ and drought stress were investigated.The purpose of this study is to explore the physiological mechanisms associated with the alleviation of drought stresses by CO₂ enrichment in cucumber,and screen differential metabolites in response to CO₂ enrichment under drought stresses.The chief results are showed as follows:1.CO₂ enrichment significantly increased the pigment content of leaves under severe drought stress,and improved the activities of ribulose-1,5-bisphosphate carboxylase（RuBPCase）and rubisco activase（RCA）under moderate drought stress,and improved the net photosynthetic rate under moderate and severe drought stresses,and significantly improved the contents of sucrose,starch,reducing sugar,soluble sugar,soluble protein and the activities of sucrose synthetase（SS）and nitrate reductase（NR）,and significantly decreased the cytomembrane permeability and MDA content,and enhanced the activities of SOD,POD,CAT,and reduced the content of H₂O₂ and O₂^·-generating rate.2.Based on UHPLC-QTOF-MS,analyze the metabonomics of cucumber seedling leaves,and the results show that there were 79 differential metabolites identified in response to elevated CO₂ under moderate drought stress.proline,4-aminobutyric acid,glutamate,citrulline,argininosuccinic acid,succinate,α-ketoglutarate,urea and glycerol 3-phosphate that are involved in alanine,aspartate and glutamate metabolism;arginine and proline metabolism;TCA cycle;and glycerophospholipid metabolism were down-regulated.Salicyluric acid,isoferulic acid,and m-coumaric acid were up-regulated in our study.These metabolites are associated with the biosynthesis of isoquinoline alkaloids,alkaloids derived from the shikimate;isoflavonoid biosynthesis;and flavone and flavonol biosynthesis.Twenty-six differential metabolites that were identified in response to elevated CO₂underseveredroughtstress.Succinicacidsemialdehyde,glutamate,S-adenosyl-L-homocysteine,trehalose,maltose and uracil nucleotide were up-regulated,and pyruvate,malic acid,N-acetyl-glutamate,S-methyl-5’-thioadenosine were down-regulated.The metabolites involved in alanine,aspartate and glutamate metabolism;pyruvate metabolism;arginine and proline metabolism;glyoxylate and dicarboxylate metabolism;cysteine and methionine metabolism;starch and sucrose metabolism;glycolysis or gluconeogenesis;and pyrimidine metabolism.3.Screened the two differential metabolites（L-gulonolactone and L-glutamate）that responded to elevated CO₂ under drought stress.Applying exogenous glutamate significantly increased pigment content and the net photosynthetic rate under moderate and severe drought stress,and reduced stomatal conductance and transpiratory rate under moderate drought stress,promoting the growth;and improved the SOD activity under severe drought stress,POD activity under moderate drought stress,and decreased CAT activity under moderate and severe drought stresses,and enhanced the contents of proline and soluble sugar under moderate and severe drought stresses,besides,increased soluble protein content under severe drought stress;reduced MDA content under moderate drought stress,and reduced the content of H₂O₂ and O₂^·-generating rate under moderate and severe drought stresses.In conclusion,CO₂ enrichment respond to drought stress by regulating metabolites and metabolic pathways in cucumber leaves,and improve photosynthetic characteristics,antioxidant capacity and osmotic regulation ability,to improve the adaption ability under drought environment.Applying exogenous glutamate improved photosynthetic characteristics,antioxidant capacity and osmotic regulation ability under the drought stress,and verified that CO₂ enrichment reduced the harm of cucumber seedling under drought stress by regulating glutamate content.