Mechanisms Of Cucumber High Temperature Tolerance In Greenhouse

Global warming leads to many extreme weather disasters.The high-frequent extreme hot weather has seriously affected the production of greenhouse vegetables in recent years.Cucumber?Cucumis sativus L.?,which is one of the most widely cultivated vegetable in greenhouse in China,is a thermophilic species that grows well in warm temperature but is intolerant to high temperature.Heat stress seriously suppresses the cucumber growth,development,yield and quality.Therefore,the study of the growth,physiological changes,and the tolerance of cucumber under the high temperature stress is critical to provide theoretical and practical supports in solving the problems of low yield and poor quality of cucumber cultivation in hot season.In this dissertation,we studied the correlation between greenhouse environmental variables and cucumber yields,and the changes of different greenhouses temperature and its effect on cucumber photosynthetic characteristics.Meanwhile,we studied the tolerance of cucumber seedlings to different manners of high temperature,and revealed the tolerance and physiological adaptation mechanism of cucumber in high root-zone temperature?HT,35°C?stress mitigated by exogenous glutathione?GSH?.The effect of metabolic pathways and the differentially expressed genes?DEGs?on cucumber growth in HT stress by transcriptome analysis were studied as well.The main results are as follows:1.Study on the correlation between greenhouse environmental variables and cucumber yield formation.Cucumber was grown over four years in the experimental greenhouse and data of yields?mean,highest and lowest?,environmental variables including total radiation,air temperature,relative humidity,carbon dioxide?CO₂?concentration and the amount of irrigation nutrient solution were collected weekly.Regression analyses were applied to develop the relationships and build best regression models of yields with environmental variables by using the first two years'data.Data collected in the 3^rd and 4^th year were used for model validation.Results showed that total radiation,nutrient,temperature,CO₂ concentration and average nighttime relative humidity had significant correlations with cucumber yields.The best regression models fit the mean,lowest and highest yields well with R² values of0.67,0.66,and 0.64,respectively.Total radiation and air temperature had the most significant contributions to the variations of the yields.Low temperature and weak radiation in winter and high temperature in summer are important factors that inhibit cucumber growth and yield.Our results of this study provide useful information for improving greenhouse climate management and yield forecast in semi-closed greenhouses.2.Changes of greenhouse temperature and cucumber photosynthetic characteristics were studied in summer season.We have studied the temperature changes of cooling and CK?non-cooling?greenhouses,and change of different vertical heights in semi-closed greenhouse in summer by cooling through air treatment unit?ATU?.Meanwhile,we also studied the changes of photosynthetic characteristic for cucumber in cooling and CK greenhouses.The results showed that the daily average temperature,maximum temperature and minimum temperature of cooling greenhouse were obviously lower than those of CK greenhouse.ATU clearly decreased the ten days'average greenhouse temperature by 6.89?,5.82?,3.45?,1.45?,0.29?for different vertical heights of 0m,1m,2m,3m,4m above ATU which compared to the outdoor temperature.ATU had good performance on cooling of 2m height and lower zone in the greenhouse during day time.Cucumber photosynthesis in cooling greenhouse is clearly higher than cucumber in CK greenhouse.Lowing the plants can also increase the leaf photosynthesis compared to the plants not lowing in cooling greenhouse.3.The inhibition of cucumber growth and photosynthesis by high temperature stress were studied.Effects of high root-zone temperature,high air temperature and their interaction?overall heat stress?on relative chlorophyll content,chlorophyll fluorescence and photosynthesis parameters in leaf were studied.The results showed that relative chlorophyll content,The efficiency of excitation energy capture by open PSII reaction centres?F_v'/F_m'?,Electron transport rate?ETR?,actual photochemical efficiency of PSII(?_PSII),photochemical quenching coefficient?qP?and leaf net photosynthetic rate?P_n?have dropped to some extent,and the treatment of high root-zone temperature stress dropped mostly.The different heat stress enhanced the relative electrolyte leakage rate of leaves,and high root-zone temperature treatment increased mostly.Stomatal conductance?G_s?,intercellular CO₂ concentration?C_i?and transpiration rate?T_r?of the high air temperature and overall heat stress treatment showed clearly increase,but the high root-zone temperature treatment dropped.The results indicated that high root-zone temperature was the most serious stress for cucumber growth.4.The physiological mechanism of glutathione?GSH?enhancing cucumber tolerance under high root-zone temperature stress?HT,35°C?were studied.Cucumber seedlings were exposed to HT,plant physiological variables,growth and antioxidant enzymes related to gene expression were measured.Importantly,the results showed that spraying or irrigation GSH could clearly relieve the harm of the HT,but the effect of GSH treated in irrigation was better.The GSSG treatment had no effect on cucumber growth.HT significantly decreased GSH content,the ratio of reduced to oxidized glutathione?GSH/GSSG?,chlorophyll content,photosynthesis,related gene expression,shoot height,stem diameter and dry weight.The exogenous GSH treatment clearly lessened the HT stress by increasing the above variables.Meanwhile,HT significantly increased soluble protein content,proline content,malondialdehyde?MDA?content,O₂?^-production rate,the gene expression and activities of antioxidant enzymes.The GSH treatment remarkably improved soluble protein content,proline content,antioxidant enzymes activities,and antioxidant enzymes related gene expression,and reduced the MDA content and O₂?^-production rate compared to no GSH treatment under HT condition.Interestingly,the results suggested that exogenous GSH enhanced cucumber seedling tolerance to HT stress by modulating the photosynthesis system,antioxidant enzyme activities,osmolytes systems and enhancing GSH/GSSG of redox signals to improve cucumber growth and physiological tolerance.5.Transcriptome analysis on the main metabolic pathways and differentially expressed genes in cucumber under high root-zone temperature stress were studied.The dissertation conducted comprehensive analyses for 2h and 4d of high root-zone temperature stress?HT,35°C?in cucumber by RNA sequencing?RNA-Seq?.The results demonstrated that a large amount of differentially expressed genes?DEGs?were found in treatments 2hNT vs 2hHT?798?,2hNT vs 2hHT+GSH?695?,4dNT vs4dHT?1002?,4dNT vs 4dHT+GSH?870?.Treatments of long time stress had more DEGs than short term treatments and the treatments of HT+GSH had less DEGs than HT.From Gene Ontology?GO?analysis and Kyoto Encyclopedia of Genes and Genomes?KEGG?pathway analysis of high root-zone temperature stress?2h and 4d?,it was found that Phenylpropanoid biosynthesis,Stilbenoid,diarylheptanoid and gingerol biosynthesis,Nitrogen metabolism,Plant hormone signal transduction,Drug metabolism-cytochrome P450 and Metabolism of xenobiotics by cytochrome P450were clearly enriched in 2hNT vs 2hHT,2hNT vs 2hHT+GSH,4dNT vs 4dHT and4dNT vs 4dHT+GSH.The cucumber tolerance to high root-zone temperature stress may have more relevance to metabolic pathway of protein processing in endoplasmic reticulum,glutathione metabolism and the upregulation DEGs of HSPs and LAPs,which could be the main reason that exogenous GSH alleviated high root-zone temperature stress for cucumber.In conclusion,the paper studied the correlation between greenhouse environment variables and cucumber yield in semi-closed greenhouse in China,and built the greenhouse cucumber yield model.Meanwhile,the paper studied the changes of different greenhouses temperature and its effect on cucumber photosynthetic characteristics.It showed that the photosynthesis of cucumber was seriously inhibited by high temperature.Importantly,we studied the tolerance of cucumber seedlings to different manners of high temperature,and found that the high root-zone temperature stress was the most serious harm.However,exogenous GSH enhances cucumber seedling tolerance of HT stress by modulating the photosynthesis system,antioxidant enzyme activities,osmolytes systems and enhancing GSH/GSSG of redox signals to improve cucumber growth and physiological tolerance.The study found that metabolic pathway Protein processing in endoplasmic reticulum,Glutathione metabolism and the upregulation DEGs of HSPs and LAPs,which could be the main reason that the tolerance formed under high root-zone temperature stress for cucumber.Our findings highlight the theoretical and technical method for forecasting and improving cucumber yield and managements in greenhouse.