Mechanism Of Green Light Partial Replacement Of Red And Blue Light Enhancing Tolerance Of Drought Stress In Cucumber(Cucumis Sativus L.)

Cucummber（Cucumis sativus L.）is very sensitive to water variation.Water supply has great influence on its growth,yield and quality.Light is one of the most important environmental signals in plant growth,development and stress response.Green light has been proved to enhance plant defense against biotic and/or abiotic stress.To illustrate the effects of green（G）light partially replaced red（R）and blue（B）light on plant under drought condition,cucumber（‘Xinchun’No.4）seedlings were treated with short-term drought stress and were concomitantly exposed to four treatments,which were set up by adjusting the relative amount of green light as 0（RB）,25（RBG25）,50（RBG50）,and 75（RBG75）μmol·m^-2·s^-1,respectively,with a total Photosynthetic photon flux density（PPFD）of 250μmol·m^-2·s^-1 and a fixed red-to-blue ratio of 4:1.Analyze the impact of green light partial replacement of red and blue light on growth and development,photosynthetic system,antioxidant system,water and stomatal status of cucumber seedlings under drought conditions.Finally,transcriptome sequencing technology was used to further analyze differential gene expression patterns and explore the effects of partial replacement of red and blue light by green light on drought tolerance of cucumber seedlings,to elucidate the physiological and molecular mechanisms of green light replacing partial red and blue light in regulating drought stress tolerance of cucumber seedlings.The main test results are as follows:（1）Compared with control（RB）,RGB50 could significantly improve the growth of seedlings under drought stress,including plant height,stem diameter,leaf area and leaf dry mass per unit area（LMA）,fresh and dry weight of aboveground and dry weight of underground,and promote the root morphogenesis,significantly increase the average root diameter,total root volume,and number of root tips.These results indicated that green light with appropriate intensity partly instead of red and blue light could alleviate the inhibition effect of drought stress on the growth of cucumber seedlings and improve the drought tolerance of cucumber.（2）Under drought stress,compared with RB,RGB50 could significantly increase the the synthesis and synthesis of light-trapping pigments（Chlorophyll a,b）,improve the activities of key enzymes in the Calvin cycle Rubisco,FBPase and FBA,and maintain relatively high net photosynthetic rate（P_n）.High maximal quantum yield of PS II（F_v/F_m）,actual photochemical efficiency of PS II in the light[Y（II）]and electron transport rate（ETR）were maintained,non-photochemical quenching coefficient（NPQ）was decreased,photosynthetic capacity of photosystem II（PS II）was improved,and photosynthetic products contents of fructose,glucose and sucrose were increased.At the same time,the intrinsic water use efficiency（WUE）and instantaneous water use efficiency（i WUE）of cucumber seedlings were increased by decreasing stomata conductance（g_s）and transpiration rate（T_r）,so as to alleviate the damage of drought stress on the photosynthetic system of cucumber seedlings.（3）Under drought stress,compared with RB,RGB50 could significantly increase the activities of antioxidant enzymes such as super oxide dimutese（POD）,superoxide dismutase（SOD）,catalase（CAT）and ascorbate peroxidase（APX）in cucumber seedlings.The contents of osmotic coordination substances,such as proline,soluble sugar and soluble protein,were significantly increased,and the osmotic coordination ability of plants was improved.At the same time,malondialdehyde（MDA）,content and relative conductivity were decreased,and the degree of membrane lipid peroxidation caused by drought was reduced.Moreover,the accumulation of superoxide anion（O₂^-）and hydrogen peroxide（H₂O₂）was decreased,the antioxidant capacity of cucumber plants was enhanced,and the relatively stable REDOX environment of cells was maintained,thus alleviating the oxidative damage of cucumber seedlings under drought stress.（4）Under drought stress,compared with RB,RGB50 and RGB75 by up-regulating Glutamate Decarboxylase 2（Cs GAD2）transcription level,increasing GABA content,and then negatively regulating Aluminium-Activated Malate Transporter 9（Cs ALMT9）transcription level,negatively regulating stomatal opening.At the same time,stomatal density was increased,and stomatal size was decreased by increasing abscisic acid（ABA）andγ-aminobutyric acid（GABA）and plasma membrane H⁺-ATPase concentrations.Thus,the free water content and total water content of cucumber seedling leaves were significantly increased,and the water potential of leaves was increased,which reduced the water loss of plants caused by drought stress.（5）Transcriptomic analysis of cucumber seedlings at 0 d,6 d and 9 d was performed by RNA-Seq technique.The results showed that cucumber seedlings responded to drought stress mainly through photosynthesis and catalytic activity with the extension of treatment time.Compared with RB,partial replacement of red and blue light by green light may regulate the drought tolerance of cucumber seedlings by regulating the expression of genes related to stomata,stress and lignin synthesis.