Effect of Chemical Mutagenesis on Salt Tolerance of Common Beans (Phaseolus vulgaris L.) Nodulating Rhizobium

The chemical mutation is one of the important factors in the enrichment of the salt resistance capacity of the rhizobium isolates. Therefore, this study focused on 10 isolates of rhizobium collected from soil samples of Babile, Hararghe region, Ethiopia. All the collected isolates turned into a moderately yellow, yellow, and deep yellow color in yeast extract mannitol agar medium (YEMA) containing bromothymol blue after 48 h of incubation. It indicated that all the isolates were acid-producing Rhizobia. Moreover, based on colony morphology and diameter, 70% of the isolates displayed large mucoid colonies, and 30% of the isolates showed large watery colonies in YEMA media. Among the 10 isolates, 9(90%), 9(90%), 6(60%), 6(60%), 2(20%), and 1(10%), were grown at different salinity levels such as 2%, 4%, 6%, 8%, 9% and 10% of NaCl, correspondingly. The most salt-resistant wild isolate was HUCR 6 collected from Babile soil grown at 10% NaCl salinity level. A total of six mutants were considered after chemical mutagenesis based on their capacity to survive in the extreme salinity levels (11 to 14%). Mutant isolates such as HUCRM 4, HUCRM 5, and HUCRM 6 were the most tolerant Rhizobium that grew at the salinity level of 11 to 14% NaCl. The most sensitive mutant isolate was HUCRM 10 followed by the isolates HUCRM 8 and HUCRM 9 was the next sensitive mutant Rhizobia that grew only at 11% of NaCl concentration. Compared to the Rhizobium wild isolates, the mutant isolates were observed to be more tolerant to a medium containing higher concentration of NaCl, as high as 11% to 14%. Besides, 80% of the mutant isolates demonstrated effective nodulation with the common beans. The mutant isolate (HUCRM 4) showed better performance in relation to root nodule performance of Rhizobium species and increased the plant biomass production. In this study, mutant isolates HUCRM 6, which is tolerated to 14% NaCl, and HUCRM 4, HUCRM 5, and HUCRM 6 isolates tolerated at 12% salinity level. Finally, based on their symbiotic efficiency and tolerance to extreme salt levels, these mutant isolates (HUCRM 4, HUCRM 5 and HUCRM 6) were encouraged to be used for the development of Rhizobium inoculants of common beans grown under extreme saline conditions. J. Bio-Sci. 30(2): 13-22, 2022