Identification of dominant indigenous Rhizobium meliloti by plasmid profiles and intrinsic antibiotic resistance

Yields of alfalfa (Medicago sativa L.) in irrigated production agriculture in the southwest U.S.A. varies widely, but is generally higher than in other areas of the U.S.A. Since nitrogen fertilizer is rarely applied, high yields are likely to be due in part to biological N₂ fixation in the symbiotic association between alfalfa and Rhizobium meliloti. Dominant types of R. meliloti responsible for N₂ fixation were identified from nodule isolates collected from five locations throughout the state of Arizona, which were not known to have been inoculated. The locations were sampled in the winter of 1987 and the summer of 1988. The dominant types (≥20% nodule occupancy at each sampling site) were identified through plasmid profile analysis and intrinsic antibiotic resistance patterns. Four types were found to be dominant throughout the state. A single example of each of these four dominant types and a commercial strain (Nitragin Co., Milwaukee, Wis.) were of equal symbiotic effectiveness as M. sativa cv. Lew. No significant differences were found (P ≥ 0.05) in shoot weight, root weight, nodule weight, acetylene reduction and total plant N content. Several of the selected cultures may have potential as inoculants for use in arid lands due to their effectiveness and their ability to survive in the extreme environmental conditions prevailing in the soils from which they were isolated.