Mitigative Effects Of Melatonin On Chilling Stress Of Cucumber In Solar Greenhouse

Cucumber（Cucumis sativus L.）is a typical cold-sensitive plant and also is one of the main planting vegetables in northern China.Low or suboptimal temperature is an important stress factor which limit the growth,yield and quality of cucumber plants.Therefore,we investigated the mitigative effect of exogenous melatonin on low temperature stress of cucumber.‘Jinyou 35’cucumber was used as experimental material and plants were pretreated with melatonin at 0μM,25μM,50μM,75μM and 100μM,and 0μM was as the control.The main results are as follows:1.The net photosynthetic rate（Pn）,transpiration rate（Tr）,stomatal conductance（Gs）,actual photochemical efficiency（ΦPSΠ）and maximal photochemical efficiency of PSII in darkness（Fv/Fm）,the activities of Ru BPCase and RCA as well as the m RNA abundance decreased significantly,whereas intercellular CO₂concentration（Ci）in cucumber seedlings increased during chilling stress days.The growth,Pn,Tr,Gs,ΦPSΠ,Fv/Fm,the activities of photosynthetic enzyme and m RNA abundance of cucumber seedlings pretreated with melatonin were obviously higher and Ci was significantly lower than those of control.2.The chilling injury index,electrolyte leakage（EL）,malondialdehyde（MDA）,hydrogen peroxide（H₂O₂）,and superoxide anion（O₂.^-）contents of cucumber seedlings gradually increased,while the content of ascorbic acid（As A）of cucumber seedlings gradually decreased under chilling stress.Meanwhile,the content of glutathione（GSH）as well as the activities and gene expressions of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）,ascorbate peroxidase（APX）and glutathione reductase（GR）of cucumber seedlings increased first and decreased afterwards during chilling stress days.Compared with the control,the melatonin treatment showed lower chilling injury index,EL,MDA,H₂O₂,and O₂^·-and higher activities and gene expression of the antioxidant enzyme as well as the contents of GSH and As A,indicating that melatonin can relieve the chilling damage to cucumber and higher activities and gene expression of the antioxidant enzyme as well as the contents of GSH and As A,indicating that melatonin can relieve the chilling damage to cucumber membrane system through enhancing antioxidant ability which accelerates the removal of ROS.3.The promotion effect of exogenous melatonin on the growth and cold tolerance of cucumber seedlings were related to its application concentration under low temperature stress.At some extend,the promotive effect of exogenous melatonin increased at first and then decreased with the increase of melatonin concentration.75μM was the optimal application concentration,which showed the highest cold tolerance of cucumber seedlings under the condition of this experiment.4.Before cold snaps,foliar spray with 50μM and 75μM melatonin could notably promote the antioxidant capacity and accelerate ROS removal to alleviate membrane lipid peroxidation,meanwhile,maintain high photosynthetic performance and increase dry matter accumulation of cucumber in solar greenhouse after cold snaps.Compared with the control,50μM and 75μM melatonin treatment increased the yield by 13.89%and16.32%.