Effects Of Melatonin Pretreatment On Salt Tolerances Of Cucumber Seedlings

Salinity stress inhibites the plant growth and development, and is one of the main reasonsfor the decline of agricultural crop production. Cucumber is the major vegetables in theprotected culitivation and easily affected by the salinity stress. Salinity stress can cause theplants harmed and a series of changes in external morphology, physiological and biochemicalcharacteristics to plants. These characteristics resulted in the growth and development affectedand a reduction in the economic benefits. In recent years, salinity becomes the importantfactors in affecting the protected culitivation with the continuous croppings.In our study, cucumber (Jinchun No.4) was chosen as the experimented materials. Thepaper investigated the effects of melatonin pretreatment on the plant growth, photosynthesischaracteristic, active oxygen metabolism and accorbate metabolism under salinity treatment.The results showed as followed:1. Melatonin pretreatment significantly improved the plant height, stem diameter, leavewidth, leaf area and seedling index and increased the dry quality, fresh quality and dry mattercontent under salinity treatment.2. Melatonin pretreatment can significantly increased the net photosynthesis rate (Pn),stomatal conductance (Gs), intercellular CO2concentration (Ci), transpiration rate (Tr), PSIIpractical quantum efficiency (ΦPSII), PSII maximum quantum efficiency (Fv/Fm),photochemical quenching coefficient (qP), and chlorophyll content of cucumber under salttreatment. These results suggest that melatonin pretreatment can effectively protect thephotochemical reaction center of cucumber seedlings under salt treatment, improve theefficiency of energy conversion leaves, promote the accumulation of photosynthetic pigments,enhance photosynthesis, increase the growth of cucumber seedlings, and improve the salttolerances of cucumber seedlings.3. Melatonin pretreatment can significantly decreased the MDA and H2O2content andrelative leakage conductivity and increased the activity of SOD, CAT, POD and solubleproteins contents of cucumber during the salinity treatment. These results demonstrated thatmelatonin pretreatment may inhibite the generation of active oxygen species, improve theactivity of antioxidant enzymes and antioxidants contents, decrease the membrane lipidperoxidantion and leakage of electrolyte to protect the interity of lipidmembranes. Therefore it can ease the damage by salinity treatment and increase the salinity tolerances of cucumberseedlings.4. During the salt treatment of12days, the content of AsA and GSH decreasedintensively while the content of DHA and GSSG increased largely, AsA/DHA andGSH/GSSG ration decreased under melatonin pretreatment. The activity of APX, DHAR,MDHAR and GR were increased on3days, and then decreased under salt treatment. Theresult showed that the addition of melatonin can increased the AsA and GSH content, whichmay improve the expression of APX, MDHAR, DHAR and GR activities in AsA-GSH cycleand enhance the ability of scavenging H2O2and inhibite the accumulation of active oxygenspecies, alleviate the damage degree caused by salinity treatment and improve the salttolerances.In summary, Melatonin pretreatment could improve the growth under salinity treatment,increase the photosynthesis, enhance the activity of antioxidant enzymes and the contents ofantioxidants, inhibite the generation of active oxygen species, alleviate the damage by salinitytreatment and increase the salinity tolerances of cucumber seedlings.