Biochemical and physiological characterization of Bradyrhizobium japonicum

Abstract

Soybean [Glycine max (L.) Merrill] is cultivated intensively in India. Rhizobium sp. associated with it derives nitrogen (N) by biological nitrogen fixation under field conditions. Out of total 25 isolated sp. from functional root nodules of soybean plants on congo-red yeast extract mannitol agar medium, 20 slow grower Bradyrhizobium sp. and 5 fast grower Rhizobium sp. were found on growing YEMA media supplemented with bromothymol blue. The purified strains were aerobic, gram negative, non-spore forming and motile rods. The optimum growth kinetics for both sp. was 35 °C at neutral pH (7.0). Both fast and slow growing rhizobia were found to be positive for oxidase, KOH, starch hydrolysis, nitrate reduction and catalase activity. Among all the 25 isolates tested, 23 isolates showed positive reaction to gelatin hydrolysis except Bj-11 and Bj-21 while, 21 isolates were reacted positive to H2S production whereas, the four isolates viz., Bj-1, Bj-2, Bj-8 and Bj-23 were negative in reaction. While all the samples were found negative for IAA production. Pot culture experiment revealed that seed inoculation with these isolates recorded significant improvement in nodule number and plant height ranged from 3.00 to 22.00 nodules per plant and 35.00 to 65.00 cm respectively at flowering stage over uninoculated control. Isolates viz., Bj-9, Bj-10, Bj-14, Bj-17, Bj-19, Bj-23, Bj-25 could produce large and pink nodules indicating their effective symbiosis with soybean.