MORPHOLOGICAL AND BIOCHEMICAL CHARACTERIZATION OF Rhizobium STRAINS COLLECTED FROM WILD AND DOMESTICATED COMMON BEAN

Abstract

The Rhizobium-legume association is considered a highly efficient process in biological nitrogen fixation (BNF) and may supply up to 90 % of the nitrogen requirements for those plants. This research described the morphological and biochemical characteristics of 20 Rhizobium strains, nine from wild common bean (Phaseolus vulgaris L.) roots and 11 from domesticated bean roots from Western Mexico. Morphological characterization of colonies showed that all strains exhibited rapid growth (2 to 3 d), white color and smooth border; 14 had a convex shape, and 12 were translucent. Results from the biochemical characterization indicated that all rhizobia developed at pH values equal to or greater than 6.0, and four did not grow in pH from 4.0 to 5.5. Strains responded differently to concentrations of sodium chloride: all strains grew at 1 %, 13 grew at 3 % and none grew at 5 to 10 %. Resistance to heavy metals showed that all rhizobia were susceptible to mercury chloride and copper sulfate, eight were resistant to magnesium sulfate and only two to magnesium sulfate and zinc sulfate. Sixteen strains exhibited resistance to the 12 antibiotic products used. The 20 strains in this study showed variability and generally presented rapid growth, tolerance to acid pH values, tolerance to moderate concentrations of sodium chloride, susceptibility to heavy metals, resistance to antibiotics and correspond to bacteria of the genus Rhizobium. No differences were observed between strains from wild beans and those from domesticated ones.