Variability in Symbiotic Effectiveness of Native Rhizobia in Acid Stress

In vitro experiments had been conducted to assess the effect of pH on different native isolates of Rhizobium from the lower Brahmaputra valley region of Assam, India. The growth rate of all the Rhizobium isolates were compared growing at three different pH levels viz. 4.5, 5.5 and 6.5. All the slow-growing Rhizobium isolates (AR1, BR5, BR8, BR12, GR13, AR10, BR13, GM16, GR21) were showed better growth at all the pH levels, whereas three fast-growing Rhizobium isolates (PR7, PR12 and PR16) failed to grow at pH 4.5 and 5.5 and could show growth only at pH 6.5. Experiments were also conducted in vitro to determine the symbiotic effectiveness of all the Rhizobium isolates with test legume, green gram (Phaseolus aureus Roxb.) at three different pH levels for 42 days and significant differences were observed on nodulation, nodule dry weight and nitrogenase activity. The acid-tolerant isolates could be used as bioinoculant for enhancement of growth of leguminous plants in the acid soils.