Occurrence, diversity and effectiveness of mid-acid tolerant alfalfa nodulating rhizobia in Uruguay

The occurrence, diversity and symbiotic properties of mid-acid tolerant (MAT) alfalfa nodulating strains (isolates able to grow at pH 5.5 but not at pH 5-0) in Uruguay were analysed. Uruguay has an indigenous alfalfa-nodulating rhizobial population, 10% of which is tolerant to acidic conditions. Although some genotypes were found repeatedly, PCR analysis of the MAT isolates suggests that considerable diversity is present. In addition, we provide evidence that strains from acid soils are not necessarily acid-tolerant. Effectiveness performance studies showed that Medicago species differ not only in their ability to achieve nodulation when growing on acidic soil, but also that distinct genotypes of alfalfa have different abilities to achieve nodulation. Symbiotic analysis of the isolates carried out in pots confirms that they have the potential to improve the growth of alfalfa in acidic and neutral soils. We suggest that a population of native alfalfa-nodulating strains found in Uruguayan soils is able to establish alfalfa in acidic soils.