| Special environment, intensive fertilizer input, and no rainwater leaching and other cause during protected cultivation, often led to soil salinization. Soil salinization not only seriously influenced plant growth, but also decreased yield and quality. Cucumber(Cucumis sativus L.) is widely planted in greenhouse. Cucumber roots are shallowly distributed and show preference for fertilizer, but no tolerance to excess fertilizer, so cucumber can be easily affected by salinization. Soil salinization has been a main barrier of protected cucumber production. The main stress anion is nitrate. Silicon (Si) is generally considered as a beneficial element for the growth of higher Plnats. Many previous studies have reported the roles of Si in alleviating both abiotic and biotic stresses in plants. Salinity is a agricultural and eco-environmental problem worldwide, and is becoming one of the most serious facters limiting the productivity of agricultural crops. Therefore, the investigation on the mechanisms of alleviating salt toxicity by Si has been received more attention recently. In this study, effects of exogenous Si on cucumber seedlings growth and physiological characteristics under nitrate stress were studied. It has important theoretical and practical meaning to guide sustainable production and increase utilization efficiency of protected cultivation.On the basis of existing research, effects of exogenous Si on cucumber plant growth and leaf antioxidant enzymes activity, photosynthetic characteristics, nitrogen metabolism related enzymes, and Si content were studied by nutrient solution culture under 140 mmol·L-1 NO3- stress. In order to establish theoretical formelation and propose new measures for alleviating salinity injury. The main results were presented as follows:1. After 7 days of 140 mmol·L-1 NO3- stress, application of 0.52 mmol·L-1 Si significantly increased growth potential, photosynthetic capacity, antioxidant enzymes and nitrate reductase (NR) activities and soluble protein contents, and significantly decreased the accumulation of malondialdehyde (MDA) in cucumber leaves and roots. However, with the increase of Si concentration, the mitigative effect to excess NO3- stress is rising early but declining later. Adding 1 mmol·L-1 Si to 140 mmol·L-1 NO3-, the growth potential, photosynthetic capacity, antioxidant enzymes and NR activities and soluble protein contents were all increasing dramatically than those of excess NO3- stress, in the mean while, the accumulation of malondialdehyde (MDA) was decreasing markedly. With adding Si concentration to 2 mmol·L-1, growth potential of cucumber seedling, photosynthetic capacity, antioxidant enzymes and nitrate reductase (NR) activities and soluble protein contents began to decrease markedly, in addition, the accumulation of malondialdehyde increased appreciably. The result indicated that 1mmol·L-1 Si is the optimal concentration to alleviate NO3- stress by improving activities of antioxidant enzymes and nitrogen metabolism enzymes and decreasing membrane permeability and lipid peroxidation.2. Under 140mmol·L-1 NO3- stress, the plant height, leaf area and net photosynthesis rate (Pn), stomatal conductance (Gs) and intercellular carbon-dioxide concentration (Ci) of salt-tolerance cultivar'Xintaimici'and salt-sensitive cultivar'Shennongcxhunwu'were all decreased significantly, which indicated that the inhibition of NO3- stress to the two cultivars cucumber seedlings photosynthesis was stomatal limitation, and the inhibition degree to'Shennongchunwu'was larger than to that of'Xintaimici'. Application of 1 mmol·L-1 Si improved cucumber seedling growth potential, and it promoted'Shennongchunwu'stem growth more significantly than'Xintaimici', and showed a rule contrary to leaf growth, and the improvement extent of photosynthetic capacity to'Shennongchunwu'was larger than'Xintaimici'.3. Under 140 mmol·L-1 NO3- stress, activities of antioxidant enzymes increased significantly, and the inhibition degree to every antioxidant enzyme of salt-sensitive cultivar'Shennongcxhunwu'was larger than that to salt-tolerance cultivar'Xintaimici'except POD activity in leaves. Exogenous Si increased activities of SOD, POD, CAT and APX evidently, and the improvement extent of'Shennongchunwu'was larger than'Xintaimici', except SOD activity in roots and CAT activity in leaves. It is indicated that silicon increased the ability of scavenging ROS in cucumber, resulting in lower H2O2 content and lipid peroxidation, and higher membrane stability under salt stress.4. With time prolonged, NO3- and NH4+ content of cucumber seedlings were increased, and the increase degree in'Shennongchunwu'was much larger than in'Xintaimici'. While Si content were decreased gradually under NO3- stress. Application of 1 mmol·L-1 Si decreased NO3- and NH4+ content of cucumber seedlings, and it is very intuitive to see that Si content in these treatments shows N+Si>CK>N. 140 mmol·L-1 NO3- treated for 4 days, cucumber seedlings NR, GOGAT and GDH activities in leaves and roots were all decreased significantly except GS activity and GDH activity in roots had no obvious changes, and the decreased range in salt-tolerance cultivar'Shennongchunwu'was much larger than in salt-sensitive cultivar'Xintaimici', except of NR activity in leaves. When treated for 8 days, activities of nitrogen metabolism enzymes were all decreased distinctly in the two cucumber cultivars leaves and roots under 140 mmol·L-1 NO3- stress. Adding 1 mmol·L-1 Si could increase cucumber seedlings leaves and roots NR, GS, GOGAT and GDH activity in different degree. The effects of 1 mmol·L-1 Si on increased degree of activities of nitrogen metabolism enzymes in 'shennongchunwu'leaf and root were more significantly than those of'xintaimici', except of NR activity in leaves.
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