The Response Mechanism Of Cucumis Sativus L.Seedling Growth To Drought Stress And Re-Watering Based On Physiological And Transcriptomics Analysis

Cucumber is an important vegetable crop.However,drought in cucumber cultivation has become increasingly serious with the global warming and the intensification of human activities.Drought stress has become one of the important environmental factors affecting cucumber yield and quality.The physiological and transcriptomics response mechanisms of cucumber seedlings to mild,extreme drought and re-watering are rarely reported in China.Therefore,the Jinguawang F1 cucumber seedling was used in a greenhouse experiment with three treatments:the control treatment?watering every two days?,the drought treatment?watering every five days?and the extreme drought treatment?no watering?to analyze how drought stress affects cucumber seedling growth.Two more treatments was added in this study:the control treatment?watering every two days?and the re-watering treatment?no watering at the early stage,and watering at the later stage?to examine whether re-watering has compensation effect on growth and physiological-biochemical characteristics and the intensity of compensating effect on cucumber seedlings under limited water deficit.The Illumina second-generation high-throughput sequencing platform(Hi Seq^TM2500/4000)was used to analyze differentially expressed genes using the DESeq method,the GOseq software and the genome encyclopedia database?KEGG?and KOBAS software.The main results are as follows:?1?The leaf area,plant height,stem diameter,root length and aboveground biomass of cucumber seedlings were all reduced under different intensity of drought stress.With the aggravation of drought,physiological indicators such as relative water content,photosynthetic rate,transpiration rate,stomatal conductance,intercellular CO₂ concentration and chlorophyll content of cucumber seedling leaves decreased further.?2?Drought stress resulted in the accumulation of malondialdehyde?MDA?,proline?Pro?,ascorbic acid?AsA?and glutathione?GSH?,and the enhancement of superoxide dismutase?SOD?and peroxidase?POD?activities in cucumber seedling leaves.?3?Compared with the extreme drought treatment,re-watering after drought can effectively alleviate the significant decrease of growth indexes,such as leaf area,plant height,stem diameter,root length and aboveground biomass,but these indexes could not recover to the levels in the control treatment.However,the photosynthetic rate,transpiration rate and stomatal conductance in the re-watering treatment could return to the levels in the control treatment.?4?The contents of MDA and Pro,SOD and POD as well as AsA and GSH in cucumber seedling leaves were reduced to the control level after re-watering,which alleviated the damage caused by drought stress to some extent.?5?About 753 differentially expressed genes in cucumber seedling leaves were screened in the control and drought treatments,of which 487 were up-regulated and 266 were down-regulated,indicating that drought stress significantly affected gene expression in cucumber seedling leaves.?6?The GO analysis showed that differentially expressed genes were mainly involved in biological processes,such as chemical stimulation reaction,the precursor metabolites and energy produce and nitrogen utilization,and molecular functions,such as nucleic acid binding transcription factor activity,DNA specific sequence binding transcription factor activity and glycosyltransferase activity.The KEGG analysis showed that the differentially expressed genes were mainly involved in metabolic pathways,such as synthesis of secondary metabolites,photosynthesis,carbon fixation and phenylpropane metabolism.The above results indicated that the damage caused by drought stress to the growth of cucumber seedlings was probably due to the drought-induced change of some processes,such as glucose metabolism,nitrogen utilization,metabolite biosynthesis and photosynthesis.Timely re-watering after drought can effectively alleviate the negative effects of drought stress on the growth and development of cucumber seedlings,and has a certain compensation effect.