Morphological and biochemical characterization of soybean nodulating rhizobia indigenous to Zambia

Abstract

Soybean [Glycine max (L.) Merrill] is known for symbiotic nitrogen fixation by rhizobia present in the soil with which it establishes an efficient symbiosis. In Zambia, current rhizobial inoculants used in soybeans production are based on non-indigenous strains; this creates a need to isolate local strains that can be used for the development of local inoculants for soybeans in Zambian soils. This study reports the isolation and characterization of rhizobial isolates from virgin and cultivated soils of the three agroecological regions of Zambia. The study was conducted in the greenhouse during the 2013/2014 season. Rhizobia were isolated using the Trap Method; a promiscuous soybean genotype (magoye) was grown for eight weeks in replicates of three arranged in Completely Randomised Design. Morphological and biochemical markers were used to characterize. A total of 61 isolates were isolated on Yeast Extract Mannitol (YEM) agar medium. The isolates were circular, irregular and punctuate in form with convex elevation; entire, undulate and lobate colony margins; with cream, white, yellow, transparent and pink coloured. All isolates produced mucous, were gram negative and rod shaped, a characteristic of rhizobial cells. None of the isolates could tolerate extremes of pH (4 and 9) in growth medium but grew well at pH 6.8. All isolates utilized glucose as a source of carbon. Based on the Bromothymol Blue (BTB) assay, 59 isolates were fast growing while two isolates from cultivated soils of region II were slow growing. The fast growing 59 isolates showed an acidic reaction changing the medium from green to yellow, while the others showed an alkaline reaction. Based on results, the 59 fast-growers could be Ensifer fredii or/and Rhizobium tropici rather than Bradyrhizobium. However, further tests to confirm these findings using ketolactose, genetic characterization and inclusion of reference strains, are still needed and are being recommended here.