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 N₂-dependent plant growth between 9° and 12°C. There was wide variation in the abilities of R. meliloti isolates to grow and form nodules at 10°C. Culture doubling times (t_d) varied from 1 to 155h, 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°C, but there was no significant N₂-dependent plant growth at this temperature. However, several isolates of R. meliloti had the ability to nodulate alfalfa and produce N₂-dependent growth at root temperatures between 10° and 12°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.