Mesquite rhizobia isolated from the Sonoran Desert: Physiology and effectiveness

A mesquite Rhizobium isolated from the Sonoran Desert (strain AZ-M1) and a commercial mesquite Rhizobium obtained from the Nitragin Company (strain 31A5) were chosen as superior strains from among many evaluated in a screening study of the efficiency of mesquite rhizobia. Both strains were fast-growing and acid-producing in denned media. The desert strain AZ-M1 had the shortest mean generation time of 3.5 h. Strain 31A5 grew better in broth amended with various sugars and amino acids, but generally produced less acid. The desert strain showed greater resistance to various antibiotics than did 31A5. In a greenhouse study N applied at high rates inhibited N₂ fixation when either strain was used as inoculant for mesquite seed. At low N rates, AZ-M1 fixed more N than 31A5. Total N, nodule weight, C₂H₂ reduction, mesquite shoot weight and root weight were all significantly increased when AZ-M1 was the applied inoculant. This study shows that the mesquite Rhizobium AZ-M 1 isolated from the Sonoran Desert is infective and effective on mesquite seedlings. Fast growth rate, acid production and high resistance to antibiotics in laboratory media may indicate the adaptation of this organism to its microbial niche in the Sonoran Desert.