Effects Of Exogenous Polyamines On Cucumber Seedlings' Physiology Responses To Salt Steress

In protected cultivation, that the fertility used unreasonably, water special movements and salinity unbalanced accumulation etc, lead to the soil salinization. So the protected cultures commonly suffered salinity stress that induced the decrease of plants growth. Polyamines are ubiquitous low-molecular-weight aliphatic amines that are involved in regulation of plant growth and development. It was reported that polyamines were involved in the defense reaction of higher plants to various environmental stresses, especially the relationship between Spd metabolism and salt tolerance of vegetables and the physiological regulation function of Spd is researched systemically, but the relationship between Put metabolism and salt tolerance is still not well known.In the present study, two cultivars of cucumber (Cucumis sativus L.) cvs. Changchun mici and Jinchun No.2 were used. It has been reported that cv. Jinchun No.2 is salinity-sensitive as in most cucumber cultivars, wheres cv. Changchun mici is comparatively tolerant to high salinity. we investigated the effects of exogenous Spd, Put and D-Arg on salt stress on cucumber seedlings' growth, photosynthesis, nitrogen metabolism, polyamine metabolism, reactive oxygen metabolism, proton pump activity and gene expression. The objectives of the present study were to elucidate the relationship between the nitrogen metabolism, photosynthesis and salt tolerance, the relationship between he changes of the polyamine metabolism in leaves and roots of cucumber and salt tolerance, the relationship between the reactive oxygen metabolism and salt tolerance, and the physiology regulation function of exogenous Put in improving salt tolerance of cucumber plants. Main research results were as follows:1.NaCl suppressed the growth of cucumber seedlings:dry and fresh weight, Pn, Gs, Tr, Ci decreased while WUE increased significantly under salt stress. NaCl treatment to cucumber induced Fv'/Fm', qP andΦPSⅡdecrease, NPQ increase. While Fv/Fm have any significant changes. These results indicated that the decrease of Pn under salt stress is influenced by stomata and non-stomota restricts.2. NaCl decrease the contents of NO3--N content and soluable protein content in leaves and roots of cucumber seedlings. NaCl increase the Pro content and NH4+-N content, also NR and GS activities increased under NaCl stress. NaCl disturbed the polyamine metabolism homeostasis in leaves and roots of cucumber:During exposure to salt stress, free and conjucatd Put, Spd and Spm and the ratio of (free-Spd+free-Spm)/free-Put increased significantly, also DAO and PAO activities increased under NaCl stress. Free Put and Spd contents, conjucatd Put, Spd and Spm contents and PAO activity increased greater in cv. Changchun mici than in cv. Jinchun No.2. These results indicated that the increase of PAs contents, especially conjucated PAs contens inhanced the salinity tolerance of cucumber seedlings.3. NaCl induced increase of SOD, POD and CAT activities, also induced increase of As A, DHA, GSH and GSSG contents, and the ratios of AsA/DHA and GSH/GSSG. Under NaCl stress, APX, GR and DHAR activities also increased. Eventhough the increases of antioxi-dansts contents, O2-·and H2O2 production and the MDA content increased significantly. SOD, POD, CAT, APX, GR and DHAR activities and the ratios of AsA/DHA and GSH/GSSG increased greater in cv. Changchun mici than in cv. Jinchun No.2. As a result, O2-·and H2O2 production and the MDA contents increased marked in cv. Jinchun No.2 than in cv. Chang-chun mici. These results indicated that the increases of antioxidansts activities/contents play important role in cucumber seedlings' salinity tolerance.4. Under salt stress, exogenous Spd inhanced photosynthesis ability of salt stressed cucumber seedlings, Pn, Gs and Tr increased. Spd increased SOD, POD and CAT activities, as a result, O2-·production decreased. These results indicated that Spd take part in the ROS metabolism and also nitrogen metabolism, so decreased the ROS and NH4+ stress to plants.5. Put (0.1-15mmol·L-1) alleviated the salt stress to cucumber seedlings, and 10mmol·L-1 Put is the best concentration to alleviated salt stress.Plant height, fresh and dry weight, Pn significantly increased. High concentration(20-30mmol·L-1) aggravated the salt stress.6. Put takes part in the nitrogen metabolism and photosynthesis of cucumber seedlings under salt stress. Put alleviated the decrease of the Pn, Fv'/Fm',ΦPSⅡand qP and the increase of NPQ. Aggravated the decrease of Ci, but has no effects on Gs,Tr and Fv/Fm. These results indicates that Put could induce the PSII photochemical efficiency to alleviate the salt stress to plants. Put takes part in the regulation of nitrogen metabolism of cucumber plants under salt stress:Put improved the absorption and translocation of NO3--N in cucumber plants under salt stress, induced the increase of NR activity, and improved the deoxidization of N03--N. Exogenous Put also improved the assimilation of NH4+-N and decreased the accumulation of NH4+-N. As a result, the homeostasis of nitrogen metabolism in cucumber plants was alleviated.7. Put takes part in the polyamines metabolism. Put induced the increase of the free Put, Spd and Spm contents, especially PAs contents. The ratio of (free-Spd+free-Spm)/free-Put also induced by exogenous Put. In addition, exogenous Put reversed the detrimental effects of salt stress on plant growth of cucumber. These results indicate that the alleviation of salt injury could be achieved by increasing the conversion of free Put to free Spd and Spm, and the conversion of free PAs to conjucated PAs. Put takes part in the regulation of reactive oxygen metabolism of cucumber plants under salt stress:Exogenous Put elevated the activities of antioxidant enzymes, SOD, POD and CAT suppressed O2-·and H2O2 production, decreased the MDA content and leakage of electrolytes. A positive correlation was found between the contents of bound PAs and the activities of SOD, POD and CAT. These results indicate that exogenous Put can enhance antioxidant enzyme activities and reduce reactive oxygen production and membrane damage by increasing the bound PAs content, resulting in the improvement of the salt tolerance of cucumber plants.8. NaCl stress aggravated the peroxidation of membrane lipid, the H+-ATPase activity in plasma membrane and visicle increased significantly. PAs synthesis was restrained under D-Arg treatments and thus PAs contents in plants decreased siginificantly. Also H+-ATPase activity, CsVH-A, CsVH-c expression was decreased uner D-Arg treatment. While exogenous Put treatment induced PAs synthesis much more under salt stress, H+-ATPase activity and CsVH-A, CsVH-c expression were increased, and so allivated the salt stress to plants. Put makes up the metabolism disorder induced by D-Arg.