Genistein, a major root-secreted isoflavone of soybean ( Glycine max (L.) Merr), is critical for the legume- Bradyrhizobium symbiosis as it induces several bacterial nod-gene systems. An experiment with soybean grown under salt stress was conducted to evaluate the effect of exogenous genistein addition to the Bradyrhizobium culture medium on subsequent nodulation, nitrogen fixation and selected plant physiological attributes. Five day-old plants (in pots) were inoculated with a liquid B. japonicum broth culture and irrigated with B&D solution containing either 0, 25, 50 and 100 mM NaCl. Four weeks after inoculation, maximum photochemical efficiency of PSII (Fv/Fm), photosynthetic rate, stomatal conductance, and transpiration rate were measured. Number of nodules per plant and apparent nitrogen fixation (as acetylene reduction activity) were determined. Salt stress decreased nodule number/plant and nitrogenase activity/plant and induced large changes of both photosynthetic parameters and antioxidant enzyme activity, compared to the control, genistein reversed the effect in each level of salinity tested. Moreover, pre-treatment of the microsymbiont with genistein enhanced maximum photochemical efficiency, photosynthetic rate, stomatal conductance and transpiration rate, while the enzymatic activities of catalase, superoxide dismutase and peroxidase in leaves and roots were not affected. It can be concluded that preincubation of the B. japonicum inoculant with genistein probably contributed towards growth in soybean via enhancement of nodulation and nitrogen fixation under both normal and salt stress conditions.
