Evaluating the Symbiotic Effectiveness of Common Bean (Phaseolus vulgaris L.) Nodulating Rhizobial Mutants on Extreme pH and High Salt Soil Conditions

Abstract

The intent of this study was to evaluate the symbiotic effectiveness of common bean (Phaseolus vulgaris L.)-nodulating mutant rhizobia isolates to extreme pH and high salt soil condition around Babile, eastern Ethiopia. After mutagenesis, from 50 identified wild rhizobia isolates, a total of 8 mutants were selected based on their ability to survive at extreme salt and pH conditions. The nodule number of mutants were positively and significantly correlated with nodule dry weight (r=0.85, p<0.0001) on sand culture. Six of the highly effective mutants were tested on unsterilized soil in controlled growth chamber. The correlation data on soil experiment displayed that nodule number was positively associated and significant (r=0.73, p<0.0001) with nodule dry weight (NDW) while shoot dry weight (SDW) was positively correlated with present N (r=0.8, p<0.0001) and total nitrogen content (r=0.9, p<0.0001). Physiological test of mutants also showed that- 5 (63%) and 3 (36%) of mutants were able to grow at salt concentrations of 11% and 12%, respectively. Notably, 3 (38%), 4 (50%), 2 (25%), and 2 (13%) of the mutants were able to grow at pH 4, 11, 11.5 and 12, respectively. Only the mutant isolates HUCRM2D (which tolerated 12% NaCl, pH4, and pH12), HUCRM5C (which tolerated 12% NaCl and pH 4), HUCRM3B (which tolerated 12% NaCl) and HUCRM9C (which tolerated 11% NaCl) were growing successfully at the indicated extreme conditions. Thus, on the basis of their symbiotic effectiveness and tolerance to extreme environmental conditions, these mutant isolates were recommended to be used as candidates for future development of rhizobial inoculants of common bean grown under saline and extreme pH conditions.