Effects Of Exogenous NO On Cucumber Seedlings Growth And Physiological Characteristics Under NO₃^- Stress

Soil salinization has been a main barrier in protected vegetable production and seriously hampered the sustainability of production. Cucumber (Cucumis sativus L.) is widely planted in field and facilities, but soil secondary salinization has restricted cucumber production. The main stress anion is nitrate. Researches indicated that exogenous nitric oxide (NO) could improve the resistance of plants to a certain degree. In this study, effects of exogenous NO 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 NO on cucumber plant growth and leaf antioxidant enzymes activity, photosynthetic characteristics, nitrogen metabolism related enzymes, and hormones 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 1 and 7 days of 140 mmol·L^-1 NO3- stress, application of 0.1 mmol·L^-1 sodium nitroprusside significantly increased soluble protein contents and superoxidase (SOD), catalase (CAT) and ascorbic acid peroxidase (APX) activities, and significantly decreased the accumulation of malondialdehyde (MDA) in cucumber leaves, which suggested exogenous NO could enhance the capacity of scavenging active oxygen species and protect cucumber seedlings from the peroxidation of membrane lipids and promoted the growth and development of cucumber seedlings. On the contrary, application of 0.3 mmol·L^-1 sodium nitroprusside significantly decreased activities of SOD, POD and CAT, and resulted in higher accumulation of MDA of cucumber leaves under 140 mmol·L^-1 NO₃^- stress. The result indicated that exogenous NO could increase cucumber tolerance to NO3- stress, while the function depends on the level of NO, excessive application of NO could aggravate the NO₃^- stress in cucumber seedlings. 2. Under NO₃^- stress, the plant height, leaf area, shoot and root dry weight, leaf Pn, Ci, Gs and Tr of the two cucumber cultivars were all decreased significantly except root-shoot ratio were all increased. The inhibition of NO₃^- stress to the two cultivars cucumber seedlings photosynthesis was stomatal limitation, and the inhibition degree to photosynthesis, plant height and leaf area increment of'Shennongchunwu'was larger than to that of'Xintaimici'. Application of 0.1 mmol·L^-1 sodium nitroprusside (SNP) promoted'Shennongchunwu'stem growth more significantly than'Xintaimici', and showed a rule contrary to leaf growth.3. With time prolonged, leaf NO₃^- and NH4+ content of cucumber seedlings were increased while NR activities were decreased gradually under NO₃^- stress. 140 mmol·L^-1 NO₃^- treated for 3 days, cucumber seedlings GS, GOGAT and GDH activities in leaf and GS and GDH activities in root were all increased significantly except GOGAT activity in root showed a falling trend. When treated for 7 days, the two cucumber cultivars leaf and root GS and GOGAT activity continued to decrease. Adding 0.1 mmol·L^-1 SNP could increase cucumber seedlings leaf and root NR, GS, GOGAT and GDH activity in different degree. The effects of 0.1 mmol·L^-1 SNP on decreased degree of NO₃^- content and increased degree of GDH activities in'xintaimici'leaf and root were more significantly that those of'shennongchunwu'.4. Under 140 mmol·L^-1 NO₃^- stress, ABA content in leaf and root of the two cultivars cucumber seedlings increased while ZT, GA, 6-BA, IAA and IBA content decreased. Adding 0.1 mmol·L^-1 SNP could increase ZT, GA, 6-BA, IAA and IBA content, and further increased ABA content. Therefore, exogenous NO could promote endogenous hormone synthesis and promote them to a balance level. SNP increased leaf and root GA content of'shennongchunwu'more significantly than that of'xintaimi ci', and showed a contrary rule in leaf 6-BA content.