Abstract: Endophytic diazotrophic bacteria are the microorganisms that colonizing various living plants tissues and establishing symbiosis relationship with the host，these bacteria are believed having features such as nitrogenfixing，plant growth promoting，enhancing nutrient absorption，stress resistance of host plant．In this paper，the effects of inoculation endophytic diazotrophic bacteria on physiological characters of Tamarix ramosissima Ledeb.seedlings were studied using pot experiments under saltdrought stress，and the resistance mechanisms were discussed．The experimental design included four levels of water stress (75%,60%,40% and 20% of soil relative water content) and four levels of salt stress (0 g/L,4 g/L,6 g/L and 8 g/L of soil salt content)，compared with plant tolerance to stress on inoculated and uninoculated endophytic diazotrophic bacteria strain，the results showed that the activities of superoxide dismutase (SOD) increased 68.3%，52.0% under saltdrought stress and nonstress，respecively，in addition，the activities of SOD can maintain in 275.82-561.49 U·g-1 with prolong the period and intensive of stress．When saltdrought stress lasting 60 days，the activities of SOD can maintain high level under 0 g/L and 4 g/L salt in 60% water treatment．The activities of peroxidase (POD) showed increase firstly，then decrease and increase rapidly again，which mainly enhanced in middle and late stage of saltdrought stress and showed a greater difference in growth period，through the stress period，the activities of peroxidase (POD) can maintain high level under 0 g/L and 8 g/L salt concention in 60% water treatment at 60 days，moreover，various protective enzyme may play different role at different stages of stress．At the same time，inoculation endophytic diazotrophic bacteria also can efficiently reduce the cell membrane permeability and slow down the root activity dropping，so to keep normal physiological activities．After 20 and 40 days saltdrought stress，the cell membrane permeability of all treatment maintained low level，while the root maintained vitality steadily at 20 days stress．In conclution，under saltdrought stress，inoculation endophytic diazotrophic bacteria can enhance the host’s ability to adapt to stress environment，effectively heighten the root vitality，protect structure and weaken injury of plant cell membrane，therefore increase plant adversity resistance．

Key words: endophytic diazotrophic bacteria, Tamarix ramosissima Ledeb., saltdrought stress, protective enzyme, membrane permeability, root activity