Effects Of CO₂ Enrichment And Exogenous ABA On Physiological Characteristics Of Cucumber Seedlings Under Drought Stress

In this experiment,hydroponic cucumber seedlings of’Jinyou No.35’were used as test material,and PEG-6000 was added to the nutrient solution to simulate drought stress.The experiment was designed as a split-plot,and the main factor was CO₂ concentration,and two levels were set:atmospheric CO₂ concentration(≈400μmol·mol^-1)and enriched CO₂concentration(800±40μmol·mol^-1),subplot factors were exogenous ABA and its inhibitors,four levels,respectively,deionized water（control）,20μmol·L^-1ABA,2 mmol·L^-1Na₂WO₄（the synthesis inhibitors of ABA）and 2 mmol·L^-1 Na₂WO₄+20μmol·L^-1ABA,the effects of CO₂enrichment and exogenous ABA on growth and development,photosynthesis characteristics,stomatal movement,carbon metabolism,antioxidant system and endogenous hormone levels of cucumber seedling under drought stress were studied to explore the physiological mechanism of drought tolerance in cucumber under the interaction of CO₂ enrichment and ABA.The main results were as follows:1.CO₂ enrichment and ABA improved the photosynthetic rate of cucumber seedlings under drought stress by promoting the accumulation of photosynthetic pigment,enhancing the key enzymes activity of photosynthetic carbon assimilation and alleviating the damage of drought stress to PSII,thereby alleviating the inhibition of drought stress on cucumber seedling growth.but after inhibiting the synthesis of ABA,the enhancement of CO₂ enrichment on photosynthetic performance and the promotion of photosynthetic electron transfer were weakened.2.CO₂ enrichment significantly increased the activities of calcium-dependent protein kinase（CDPK）and carbonic anhydride（CA）and up-regulated the expression levels of related genes in cucumber seedling leaves under drought stress.However,when ABA synthesis was inhibited,the expression of CsCA4 and CsCDPK were not significantly induced by CO₂.The activity of HT1 protein kinase and the expression of CsHT1 were significantly decreased by CO₂ enrichment,but not affected by ABA.ABA increased the activities of CDPK and CA as well.CsOST1,CsSn RK2.1 and CsSn RK2.2 were not sensitive to CO₂ enrichment,but could be significantly induced by ABA.CsCDPK and CsSLAC1 could be enhanced by CO₂ enrichment and ABA at the same time.3.CO₂ enrichment and ABA significantly promoted the accumulation of total sugar,sucrose,fructose,reducing sugar and starch in the leaves of cucumber seedlings under drought stress.ABA could significantly increase the activities of sucrose synthase（SS）and sucrose phosphate synthase（SPS）,but CO₂ enrichment could only significantly increase the activity of sucrose synthase（SS）.4.CO₂ enrichment and ABA could reduce the contents of reactive oxygen species and MDA,promote the accumulation of proline（pro）,ascorbic acid（ASA）,glutathione（GSH）and other osmotic regulators,and increase the activities of antioxidant enzymes such as organic dismutase（SOD）,peroxidase（POD）,catalase（CAT）,ascorbate peroxidase（APX）and so on in cucumber seedling leaves under drought stress,so as to alleviate the oxidative damage of cucumber seedlings caused by drought stress.5.Both CO₂ enrichment and ABA could increase the content of ethylene（ETH）,but CO₂enrichment could increase the contents of auxin（IAA）,cytokinin（CTK）and gibberellin（GA）,and reduce the content of abscisic acid（ABA）in the leaves of cucumber seedlings under drought stress,ABA had the opposite effect to CO₂ enrichment...