Effects Of Exogenous Sucrose On Cucumber Seedlings Growth And Physiological And Biochemical Characteristics Under NO₃^- Stress

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 sucrose could improve the resistance of plants to a certain degree. In this study, effects of exogenous sucrose on cucumber seedlings growth and physiological characteristics under nitrate stress were studied. And there are sugar-specific signal transduction pathways affecting gene transcription and translation. 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 sucrose on cucumber plant growth and leaf and root antioxidant enzymes activity, photosynthetic characteristics, nitrogen metabolism related enzymes of key enzymes 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 0.5⁵ mmol·L^-1 scrose significantly increased growth potential, photosynthetic capacity and superoxidase (SOD), peroxidase(POD), catalase (CAT) and ascorbic acid peroxidase (APX) activities, and significantly decreased the accumulation of malondialdehyde (MDA) but increased soluble protein contents in cucumber leaves and roots, which suggested exogenous sucrose could enhance the capacity of scavenging active oxygen species, to the benefit of nitrogen metabolism and protect cucumber seedlings from the peroxidation of membrane lipids and promoted the growth and development of cucumber seedlings. However, with the increase of sucrose concentration, the mitigative effect to excess NO₃^- stress is rising early but declining later. The result indicated that 1 mmol·L^-1 scrose is the optimal concentration to alleviate NO₃^- stress by improving activities of antioxidant enzymes and photosynthetic capacity.2 Under 140 mmol·L^-1 NO₃^- stress,with time prolonged, NO₃^- and NH₄⁺ content of cucumber seedlings were gradually increased, while leaf and root NR,GOGAT and root GS activity was decreased. After the second and six day of 140 mmol·L^-1 NO₃^- stress,leaf GS activity was higher than control. Application of 1 mmol·L^-1 sucrose increased NR, GS, GOGAT activity in leaf and root of cucumber seedlings under NO₃^- stress, decreased NO₃^- and NH₄⁺ content.3,Under 140 mmol·L^-1 NO₃^- stress, soluble sugar content increased markedly than CK, and accumulation of soluble sugar in leaves shows more than in roots. Excess nitrate significantly reduced SS and SPS activity in cucumber leaves. Adding 1 mmol·L^-1 Sucrose in 140 mmol·L^-1 NO₃^- increased soluble sugar content and activities of SS and SPS.4,After 6 days of 140 mmol·L^-1 NO₃^- stress, MDA content increased significantly in cucumber seedlings. The activities of SOD,CAT,APX were all decreased except SOD in roots and APX in leaves increased than CK. Adding 1 mmol·L^-1 Sucrose could increase cucumber seedlings leaves and roots SOD,CAT,APX activities. It indicate that exogenous sucrose increased the ability of scavenging ROS in cucumber.