| This experiment was carried out with greenhouse cucumber (Cucumis sativus L. 9930) seedling grown in nutrient solutions with different concentration of NO3- in nutrient solutions for a short period treatment, and consisted of three treatments: (T1) NO3- 2 mmol/l, (T2) NO3- 5mmol/l, (CK) NO3- 11 mmol/l. On the 7th day of treatment, the cucumber seedlings were transferred to CK solution for recovery. The growth and development indicators, photosynthetic characteristics, and activity of enzymes related to the metabolism of ascorbic acid during treatment and recovery phases, yield and quality of cucumber plant were investigated. The experiment was aimed at exploring a new way which can provide technical support and theoretical basis for farmers to use nitrate fertilizer more reasonably. The main results are as follows:1. Excessive lack of nitrogen inhibited photosynthesis of cucumber seedlings significantly, however reducing supply of nitrogen appropriately could improve the photosynthetic rate of cucumber seedlings. In this study, T1's net photosynthetic rate, stomatal conductance, intercellular CO2 concentration and transpiration rate was 28.21%, 44.08%, 7.9%, and 40.72% lower than that of CK during treat phase, while after recovery all these differences had been reduced. It is noteworthy that, T2's stomatal conductance, intercellular CO2 concentration and net photosynthetic rate was 15.25%, 4.9% and 3.28% higher than CK.2. Short-term low nitrogen during seedling stage could regulate the redox state and antioxidant enzyme activity of cucumber. In this study, the AsA/DHA ratios of cucumber were on the rise during treat phase, which indicated that low nitrogen environment caused membrane lipid peroxidation in cucumber seedlings. On the 7th day of stress, AsA/DHA ratios of T1, T2 were 3.85 and 2.28 times higher than CK respectively. The GSH/GSSG ratios of cucumber had the same trend to AsA/DHA, but there were no significant difference. Regardless of treatment or recovery, the G-POD and CAT activity were increased first and then decreased, indicating that low nitrogen could activate protective enzymes. On the 5th day of stress, G-POD activity of T1,T2 were 85.46% and 59.06% higher than that of CK respectively.3. Short-term low nitrogen during seedling stage could regulate the metabolism of AsA in cucumber seedlings. During treat phase, low nitrogen treatment increased APX, AAO, DHAR, MDHAR and GR activity, which suggested that low N could activate protective enzymes and promote the metabolism of AsA. On the 5th day of stress, APX activity of T1,T2 were 65.01% and 18.21% higher than that of CK respectively. On the 3rd day of stress, DHAR activity of T1,T2 were 81.30% and 25.71% higher than that of CK respectively. In addition, the increase of relative expression levels of APX, DHAR, GR and GPX had been a further evidence that low nitrogen could regulate gene expression of the key enzymes in AsA metabolism. According to principle component analysis, DHAR, MDHAR and GR activity were the main indexes in leaves, and APX, AAO activity were the main indexes in roots induced by low nitrogen treatment. 4. The inhibition of growth and development of cucumber seedlings caused by short-term low nitrogen treatment can return to normal after a short recovery. In this study, T1's height increment and leaf area was 10.38% and 21.22% lower than that of CK during treat phase, while after recovery all these differences had been reduced.5. Short-term low nitrogen treatment during seedling stage could keep reasonable output meanwhile improving the nutritional quality of cucumber fruit obviously. In this experiment, as compared with CK, the yield in T2 only reduced by 1.6%, while VC content in T2 obviously increased by 14.94% and nitrate content reduced by 22.43%. Consequently, effects of frequency of low nitrogen treatments during growth period on hydroponic cucumber growth and yield should be further investigated.
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