Effect Of Exogenous Melatonin On The Antioxidant System And Nitrogen Metabolism Of Cucumber Seedling Under Nitrate Stress

Soil secondary salinization which was the widespread phenomenon in facilities, has become one of the main obstacle factors of vegetable cultivation in facilities. In the process of vegetable cultivation, the mail reasons of caused the soil secondary salinization are the using excessive nitrogen fertilizer, the lack of natural rain and the intense evaporation of soil moisture. The mail anion is the NO3– in the facilities soil salt and nitrate excessive accumulation in the soil will seriously affect the normal growth of vegetables. As a result, the research to alleviate the harm of cucumber which the high content of nitrate brings has important significance.In our syudy, cucumber which is one of facility cultivation vegetables was chosen as the experimented materials. Using MT solution to fill the roots and choosing the produce rate of free radicals, antioxidant enzymess and so on as indexs, we researched the physiological action of MT on cucumbers under nitrate stress, to provide the theoretical basis for the application of MT in greenhouse vegetable cultivation. The results are as follows:1. Under the nitrate stress, the superoxide radical(O2-) production rate, hydrogen peroxide(H2O2) and malonadehyde(MDA) contents in the cucumber seedling leaves increased significantly, and cell membrane permeability magnified. The contents of antioxidant substances such as ascorbic acid(AsA) and glutathione(GSH) decreased significantly while the DHA and GSSG contents increased dramatically; AsA/DHA, and GSH/GSSG ratio decreased markedly. The activity of antioxidant enzymes such as superoxide dismutase(SOD), peroxidase(POD), catalase(CAT), ascorbic acid peroxidase(APX) and glutathione reductase(GR) rised rapidly in the early nitrate stress, with the extension of nitrate stress time, the activity was restrained and gradually droped rapidly. Exogenous melatonin can effectively inhibit the generation of reactive oxygen species under nitrate stress, reduce the accumulation of MDA, alleviate the damage of cell membrane, increase the contents of cucumber antioxidants and improve the activity of antioxidant enzymes, improve the resistance of cucumber seedlings to the nitrate stress.2. The nitrate nitrogen and ammonium nitrogen can accumulate largely under the nitrate stress, and the activity of nitrogen metabolism enzymes such as nitrate reductase(NR), glutamine synthetase(GS), glutamate synthase(GOGAT) and glutamate dehydrogenase(GDH) was restrained. Exogenous melatonin can effectively reduce the nitrate nitrogen and ammonium nitrogen content, improve NR, GS, GOGAT and GDH activity, and promote the nitrogen metabolism of cucumber seedlings.3. Exogenous melatonin can significantly improve the net photosynthesis rate(Pn), stomatal conductance(Gs), intercellular CO2 concentration(Ci), transpiration rate(Tr), PSII practical quantum efficiency(ΦPSII), PSII maximum quantum efficiency(Fv/Fm), photochemical quenching coefficient(qP), and chlorophyll content of cucumber under the nitrate stress. Exogenous melatonin can also promote the photosynthesis efficiency of cucumber seedling leaf, increase leaf dry matter accumulation, and add production of cucumber.In summary, exogenous melatonin can effectly alleviate the damage of cucumber seedlings under the nitrate stress, and the effects was the best when the MT concentration was 100 umol/L.