Podbel’skogo, St. Petersburg, 196608 Russia, e-mail provorov@newmail.ru Acknowledgements: The authors are grateful to L.A. Lutova for critical comments on the article and to S.N. Yurhel for mutant GL1. <...> Supported by Russian Science Foundation grant ¹ 14-26-00094 Received December 30, 2015 A b s t r a c t Symbiotic efficiency (SE) in nodule bacteria (rhizobia) characterizers their adaptive potential and practical significance. <...> This trait is closely related to fixation of molecular nitrogen (N2) which is implemented by rhizobia in root nodules and is controlled by genotypes of both partners. <...> The highest intensity of N2 fixation is typical for fast-growing rhizobia (Rhizobium, Sinorhizobium). <...> However, SE (impact of the rhizobia inoculation on the plant biomass) is determined by many gene systems of rhizobia including those not related to operation of nitrogenase. <...> Therefore, N2-fixing activity and SE should be considered as two different symbiotic features. <...> Specifically, rhizobia can synthesize biologically active substances (phytohormones, vitamins, lumichrome) which influence the development of host plants. <...> The aim of our research was to explore the impacts of Sinorhizobium meliloti strains which are fixing actively N2 but are contrasting in salt resistance on productivity and on the indices of habitus (ratios of different plant morphometric traits) in perennial tetraploid alfalfa (Medicago sativa). <...> We used S. meliloti strains: 1021 (standard laboratory strain), СХМ1-188 (UV-induced mutant of strain СХМ1 possessing an increased SE) and СА67 (strain isolated from the Tajikistan soils). <...> Besides, we used Tn5-induced mutants with a decreased SE: F (obtained from strain СА67 in this research) and GL1 (obtained from strain 1021 at Dalhouse University, Canada). <...> Seeds of alfalfa cultivar Agnia resistant to edaphic stresses were obtained from AllRussia Research Institute of Fodders named after V.R. Williams (Lobnya, Moscow Province). <...> The legume-rhizobia symbiosis was studied in the microvegetative trials by analyzing different morphometric traits (mass and length of shoots and roots) and their ratios as dependent on the salinity stress (0.6 % NaCl). <...> In the absence of stress, all S. meliloti strains possessing the high N2-fixing activities but contrasting in salt tolerance resulted in the increased mass and length of alfalfa shoots combined <...>