AMMONIUM AND NITRATE TOLERANT RHIZOBIAL M UTANTS

Mutants of Bradyrhizobium japonicum and Rhizobium phaseoli tolerant to ammonium and nitrate were isolated. Tolerant mutants were selected on a IPG-05 agar medium complete for rhizobia with glutamate, glutamine, and yeast extract. This medium contained growth inhibiting concentrat ions of ammonium and nitrate. All mutants isolated exhibited cross-res istance to both agents. Certain mutants of B. japonicum were shown to have a severalfold decrease in activity of the enzymes GSII and GOGAT. One mutant had the phenotype Nod(+)Fix(-) and two mutants exhibited d ecreased colony mucoidness on agar media. When mutants NOJ-18 and NHJ- 14 were grown in symbiosis with soybean and in conditions of plant wat ering with NH4NO3 solutions of different concentrations, tolerance of symbiotic nitrogen fixation to NH4NO3 (according to reduction of acety lene) was shown.