SYMBIOTIC EFFECTIVENESS OF RHIZOBIUM-MELILOTI AT LOW ROOT TEMPERATURE

Laboratory, growth chamber and field experiments were conducted to select among 226 isolates of Rhizobium meliloti for the ability to grow, nodulate alfalfa (Medicago sativa L.) and support N-2-dependent plant growth between 9 degrees and 12 degrees C. There was wide variation in the abilities of R. meliloti isolates to grow and form nodules at 10 degrees C. Culture doubling times (t(d)) varied from 1 to 155 h, and the number of nodules formed on alfalfa in growth pouches in 2 weeks varied from 0 to 3.8 nodules per plant. Nodulation occurred at 9 degrees C, but there was no significant N-2-dependent plant growth at this temperature. However, several isolates of R. meliloti had the ability to nodulate alfalfa and produce N-2-dependent growth at root temperatures between 10 degrees and 12 degrees C. Six isolates, including strain NRG-34, produced significantly more N-2-dependent plant growth at 12 degrees C root temperature than did 14 other isolates tested. In field experiments, inoculation with strain NRG-34 resulted in greater nodule numbers, nodule weight, proportion of nodules occupied by the inoculant strain and plant weight than did inoculation with a commercial strain (NRG-185). These results permitted selection of a strain with better low-temperature competitive abilities than the currently available commercial strains.