Effects Of Exogenous Polyamines On Growth And Physiological And Biochemical Characteristics Of Cucumber Seedlings 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 polyamines (PAs) could improve the resistance of plants to a certain degree. In this study, effects of exogenous PAs 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 PAs on cucumber plant growth and leaf antioxidant enzymes activity, photosynthetic characteristics, nitrogen metabolism related enzymes, and polyamines metabolism 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. Under 140 mmol·L^-1 NO₃^- stress, exogenous application of 1 mmol·L^-1 spermidine (Spd), spermine (Spm) and putrescine (Put) can anvance the growth of cucumber seedling in various degree. The plant growth, leaf area and Pn increased firstly and then decreased with the Spd, Spm and Put concentrations increased. After 7 days of 1 mmol·L^-1Spd, Spm and Put treatment, the plant growth, leaf area and Pn have remarkable difference compared with 140 mmol·L^-1 NO₃^- stress. We can conclude that the concentration of polyamine can improve the resistance to salt srtess. But when the concentration of Spd, Spm and Put up to 2 mmol·L^-1, the plant growth, leaf area and Pn decreased. So application of a certain concentration of Spd, Spm and Put up can improve the cucumber growth, too high concentration inhibited the growth.2. After 7 days of 140 mmol·L^-1 NO₃^- stress, application of 1 mmol·L^-1 Spd, Spm and Put significantly increased soluble protein and proline contents and nitrate reductase (NR), glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) activities, and significantly decreased the contents of NO₃^--N and NH4+-N in cucumber leaves. These findings suggested exogenous Spd, Spm and Put enhanced nitrogen metabolism level,decreased the accumulation of NO₃^--N and NH4+-N, intensified the accumulation of osmotic adjustment materials, for example, soluble protein and proline and so on , promoted the ability of osmotic adjustment and growth of cucumber seedings, and increased resistance to high concentration of NO₃^- stress.3. Under 140 mmol·L^-1 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. 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 1 mmol·L^-1 Spd promoted'Shennongchunwu'growth more significantly than'Xintaimici'.4. After 1 days of 140 mmol·L^-1 NO₃^- stress, catalase (CAT) activity and malondialdehyde (MDA) content increased rapidly in two cucumber seedling leaves, while ascorbic acid peroxidase (APX) activity declined significantly. Superoxidase (SOD) and peroxidase (POD) activities of Xintaimici had no significant difference with CK. After 9 days of 140 mmol·L^-1 NO₃^- stress, CAT, POD and APX activities declined significantly, while the content of MDA and SOD activity of'Xintaimici'increased. Application of 1 mmol·L^-1 dicyclohexylamine (DCHA) could intensify the harm by the stress of NO₃^-. In the condition of T2 and T3, application of exogenous 1 mmol·L^-1 Spd could increase the activities of SOD, CAT, POD and APX in these two kinds of cucumber breeds and reduce the content of MDA and the peroxidation of membrane lipid.5. The content of Spm in leaves of cucumber seedling increased and the contents of Spd and Put reduced under 140 mmol·L^-1NO₃^- stress. Compared with 140 mmol·L^-1NO₃^- stress, adding 1 mmol·L^-1 DCHA significantly reduced the contents of Spd and Spm. The content of Put in the leaves of'Xintaimici'increased with extending process time, while the Put content change of'Shennongwuchun'was opposite. Adding 1 mmol·L^-1Spd increased the contents of Spd, Put and Spm significantly and reduced the activities of diamine oxidase (DAO) and polyamine oxidase (PAO). It also promotes the synthesis of endogenous polyamines and maintains polyamine metabolic balance.