Studies On Physiological Regulation Role Of Polyamines In Salt Tolerance Of Cucumber Seedlings

Salt stress is a global problem in crop production, and secondary salinization in soilsof protected cultivation has become one of major factors adversely affecting horticulturalcrops. Salt stress affects every aspect of physiology in crops, as a result virtually reducestheir productivity and adversely affects their product quality. As adaptive response, plantscan effectively adjust their physiology and metabolism to reduce stress injury. As secondarymetabolites in organisms, polyamines, mainly including putrescine (Put), spermidine (Spd),spermine (Spm), have long been recognized to be linked with growth, differentiation,senescence of plants and defense reaction of plants to various environmental stresses.Under many kinds of stress conditions, polyamine content in plants changes rapidly.However, the mechanism is still not well established in physiological regulation role ofpolyamine in its adaptation to salt stress in plants.In the present study, salt-tolerant cucumber cultivar 'Changchun Mici' andsalt-sensitive cultivar 'Jinchun No.2' were chosen as experimental materials. In order toelucidate the mechanism of exogenous polyamines in improving tolerance of plants tosalinity, and develop suitable methods to mitigate the adverse effects of salinization onhorticultural crops in protected cultivation, we investigated the effects of salt stress ongrowth, water uptake efficiency and distribution of K⁺, Na⁺ and Cl^- in cucumber seedlingsby adding NaCl to the nutrient solution. Meanwhile, by using exogenous polyamines,polyamine biosynthesis inhibitor (D-Arg and MGBG) and polyamine oxidase (PAO) AG,the relationship between polyamines and seedling growth, reactive oxygen metabolism anddistribution of K⁺, Na⁺ and Cl^- in cucumber seedlings under salt stress were discovered, andthe physiological regulation roles of polyamines in adaptation of cucumber seedlings to saltstress were discussed. Main research results were as follows:After treating with different levels of NaCl (50, 75 and 100 mmol·L^-1), both netphotosynthetic rate (P_n) and transpiration rate (T_r) drastically decreased, root activity andWUE initially increased but decreased later during the period of salt treatment. Undersaline stress, absorption surface area of root system declined, however, membrane permeability of root system increased significantly. And leaf number per plant under saltstress decreased compared with the control. Under salt stress, K⁺ level in all of organsdecreased, companied with the accumulations of Na⁺ and Cl^-, so K/Na ratio declined incucumber seedlings. Among all the NaCl concentrations, 25 mmol·L^-1 inhibited little togrowth of cucumber seedlings during short-term stress, 50 mmol·L^-1 inhibited the growth ofcucumber seedlings obviously but all the plants could survive, however, 100 mmol·L^-1arrived to the ultimate limit under which the survival percent of cucumber seedlingsdecreased obviously. Under the same level of NaCl exposure, all the parameters ofseedlings in the sensitive cultivar 'Jinchun No.2' varied more significantly than those in thetolerant cultivar 'Changchun Mici', and their growth was inhibited more than those of theresistant cultivar.Under salt stress, exogenous foliar spraying Put at the concentration of 1.0 mmol·L^-1and 5.0 mmol·L^-1, Spd of 0.2 mmol·L^-1 and 1.0 mmol·L^-1 or Spm of 0.2 mmol·L^-1 and 1.0mmol·L^-1 could enhance growth of cucumber seedlings significantly. Under salt treatment,contents of free, conjugated and bound polyamine in leaves and roots of two cucumbercultivars seedlings increased at different degrees, and the ratio of free (Spd+Spm)/Put in'Changchun Mici' seedlings was higher than that in 'Jinchun No.2' seedlings. Compared tothe simple salt stress, the treatments combined with exogenous Put, Spd or Spm changedthe contents of free polyamines and the proportion of polyamine kinds, increased thecontents of both conjugated and bound polyamine in leaves and roots of cucumberseedlings. Furthermore, exogenous Put, Spd or Spm increased the activities of SOD, POD,CAT and APX significantly, decreased the O₂·^-production rate, H₂O₂ content and MDAcontent, and alleviated the loss of K⁺ level, reduced the accumulations of Na⁺ and Cl^-,increased K/Na ratio and the absorption and transporting selectivity of K⁺ to Na⁺. As aresult, under salt stress, exogenous Put, Spd or Spm improved biomass of cucumberseedlings significantly. Regulation role of Put, Spd or Spm on salt-tolerance of cucumberseedlings was related to transform ablitily to its subsequent polyamines, to conjugated andbound polyamines. Exogenous Spd or Spm inhibited Na⁺ accumulation more effectivelythan exogenous Put, whereas exogenous Put inhibited Cl^- accumulation more effectively.Under salt stress, the activities of ADC, ODC and SAMDC in leaves and roots ofcucumber seedlings increased at different degrees. Compared to the simple NaCl exposure,D-Arg decreased levels of all kinds of polyamines in leaves and roots significantly, MGBGdecreased contents of Spd and Spm while increased Put content, and each of these treatments decreased seedling growth under salt stress significantly. Either D-Arg orMGBG caused the lower K/Na ratio in all of organs especially in leaf and root, higher Cl^-accumulation in leaf, and decreased the activities of SOD, POD, CAT and APXsignificantly, increased the O₂·^- production rate, H₂O₂ content and MDA content. Theinhibiting effect of D-Arg could be reversed partly by the combination with exogenous Put,which indicated that ADC pathway was important for Put biosynthesis in salt-stressedcucumber seedlings, and the higher endogenous polyamines levels could improve salttolerance. The combination of exogenous Spd or Spm could reverse the effects of MGBGon salt-stressed cucumber seedlings and spd was the more effective. These results indicatedthat higher level of free-Spd or free-Spm had the great benefit to improve the tolerance tosalt stress for cucumber seedlings, and free-Spd was the more effective.Exogenous foliar spraying AG caused the inhibition of the activities of DAO and PAOin leaves of cucumber seedlings, increased the levels of all kinds of polyamines in leavesand roots, increased the K/Na ratio, and reduced the accumulations of Na⁺ and Cl^-.Moreover, compared with simple salt treatment, AG increased the activities of SOD, APXand GR, decreased the O₂·^-production rate, H₂O₂ content and MDA content, and improvedbiomass of salt-stressed cucumber seedlings significantly. Treatment of NaCl+AG+MGBGdecreased contents of Spd and Spm while increased Put content further, reduced the ratio of(Spd+Spm)/Put, decreased the activities of SOD, APX and GR, increased the O₂·^-production rate, H₂O₂ content and MDA content, resulted in lower biomass than that ofsimple salt treatment. These results indicated that lower DAO activity was advantageous tosalt tolerance of cucumber seedlings by improving the endogenous polyamines levels.Excess Put accumulation was harmful to plant, and the effective transform from free Put toSpd and Spm and the higher (Spd+Spm)/Put benefited the tolerance to salt stress forcucumber seedlings.