This study was devoted to isolate and assess the symbiotic efficiency of faba bean (Vicia faba L.)-nodulating rhizobia isolate at few faba bean growing areas of the eastern Hararghe highlands of Ethiopia. A total of 50 isolates were obtained from soil samples of three Woredas (districts) of the eastern Hararghe highlands using the host trap method. Out of these 50 isolates, 40 of them were presumptively identified as rhizobia. Among these 40 rhizobia isolates only 31of them were successful to nodulate faba bean, and authenticated as true faba bean nodulating rhizobia. The physiological tests revealed that among the 31 isolates 31(100%), 27(87.1%), 23(74.2%), 4(12.9%) and 1(3.2%), were grown at 2%, 4%, 6%, 8% and 9% of NaCl concentration, respectively. All of the isolates grew in the range of pH 5.5-7.5. Nine point seven percent of the isolates were adapted to grow at pH 4.5, whereas 87% of the isolates managed to grow at pH 8. Concerning the symbiotic effectiveness about 52%, 35%, and 13% of the isolates were found to be highly effective, effective, and lowly-effective, respectively. The correlation data on the sand experiment displayed that nodule dry weight was associated positively and significantly (r = 0.494, p<0.05) with shoot dry weight while shoot dry weight was associated positively and significantly(r=0.41, p<0.05) with plant total nitrogen. Positive correlations were also observed concerning shoot dry weight and dry weight of nodules (r = 0.7, p<0.05) on unsterilized soil. Among the observed rhizobium isolates, EHHFR (4A, 6A) showed the highest symbiotic efficiency above 110%, tolerated NaCl concentration ranging from 2% to 6% and 2% to 8%, respectively, and a pH range of 4.5 to 8 and 5 to 8, respectively. Thus, based on their symbiotic efficiency at the greenhouse level and relative tolerance to extreme conditions these faba bean nodulating rhizobia isolates were recommended to be used as candidates for the future development of faba bean rhizobial inoculants after being tested on field conditions.
Keywords: Biological nitrogen fixation (BNF), Faba Bean, Rhizobia, Symbiotic efficiency.