Efficiency of Indigenous Soil Rhizobia on Growth Performance of the Common Bean ( Phaseolus vulgaris L.)

Phaseolus vulgaris L (Common bean) is an essential pulse crop for smallholder farmers in sub-Saharan African countries like Ethiopia. However, crop growth, development, and production are limited by poor soil fertility due to poor soil management practices. Our study evaluated the effect of indigenous rhizobia inoculants on common bean root nodulation, and growth on the Gofta , a local common bean variety (G-2816) in Fedis District, Eastern Oromia Region in Ethiopia. The study, conducted at Haramaya University, isolated and characterised the rhizobia from the root nodule of a common bean. A total of five bacterial isolates were isolated from the root nodules of the Gofta on YEMA media after four days of incubation. All five isolates were identified as rhizobia based on authentication tests (Congo red test and catalase test). Based on morphological properties, all the isolates were round and slimy white with raised elevation and smooth surfaces indicating rhizobia. All the isolates were pink in colour in Gram's reaction and were rod -shaped indicating gram -negative bacteria. The ability of isolates to utilize alternative sources of carbon was assessed. Mannitol was found to be the best source followed by maltose, with sucrose being the least favoured for the selected isolates. Different tolerance levels were noticed for stress conditions: high or low pH; salt concentration; and temperature above ideal levels. The five isolates were able to grow in acidic media of pH 5.5, while three isolates (HURFB1, HURFN2, and HURFM3) showed weak growth in alkaline conditions of pH 9.5. However, all five isolates were able to grow well at salt concentrations of 2% and 3% NaCl, but reduced growth was noted for 3%; only one isolate (HURFB1) was tolerant to 5% salt concentration. At 45 degrees C temperature, only 20% of the isolates experienced good growth. The isolates were assessed for symbiotic effectiveness (SE) through greenhouse experiments. All isolates were symbiotically effective(SE) with 80% of the isolates recording a SE of >80% with the remaining 20% exhibiting a SE of 51 to 80%. They produced root nodules and the effects on nitrogen fixation were noticed by changes in growth and development features which revealed efficiency of rhizobia isolated. Common beans inoculated by native rhizobia exhibited better performance than those fed with nitrogen fertilizer. Among the selected five isolates, HURFM3 isolate showed highest symbiotic efficiency (187%) when compared with the nitrogen fed plants. In conclusion, this study confirmed the presence of indigenous rhizobia in Ethiopian soil, especially in the study site, which have the ability to nodulate the common bean more effectively and enhance crop growth and development.