Morphological and physiological characterization of nitrogen fixing rhizobia isolated from country bean (Lablaba perpureus) of Narail, Bangladesh

Abstract

Nitrogen is one of the important constituents of the plants. It is fixed in legume plants from the atmosphere by the association of beneficial gram negative soil bacteria named rhizobia. This study aimed to distinguish the morpho-physiological characteristics of naturally growing rhizobia having higher environmental and external stress tolerance. Twenty-two isolates were isolated from country bean root nodules and characterized based on different tests. The isolates varied from one another with level of parameters of the treatments. Most of them exhibited cream color colonies and some isolates evolved white and pink colonies. The average colony size of the isolates was ranged between 0.83 to 2.83 mm after one day and 1.33 to 5.00 mm after two days. Among them, 17 isolates were alkali producers, three isolates were acid producer and only two isolates produced neutral mucous. All the isolates showed complete resistance at pH 5.0 to 10.0, 2% NaCl, 10 to 42oC temperature, and 0.1 mM of heavy metallic salt MnCl2, FeCl3 and PbCl3. Nevertheless, most of the isolates also tolerated pH 4.0; 3 to 7% NaCl; 47oC temperature; 0.1 mM CuSO4. On the other hand, a few numbers of the isolates resisted 100 µg/ml and 200 µg/ml ampicillin, kanamycin and tetracycline antibiotics. Therefore, exploration of country bean rhizobia for their environmental stress tolerance capacity might be a core study for the production of cost-effective bio-fertilizer to boost up country bean yield.