Efficiency of rhizobia selection in Rio Grande do Sul, Brazil using biological nitrogen fixation in Phaseolus lunatus

The aim of this research was to evaluate the capacity of symbiotic efficiency of native rhizobia from soils of the state of Rio Grande do Sul (RS), Brazil in Phaseolus lunatus L. plants. Soil and nodule samples from predominant legume plants such as Desmodium species were collected in seven locations of the state of RS. For the isolation of the rhizobia from the soil samples, lima bean “olho de cabra preto” variety and “leucena” (Leucena leucocephala) and plants were used as baits. Subsequently, the symbiotic characterization of the isolates was performed by inoculation on lima bean plants in vitro conditions. The selected bacterial isolates were evaluated for biological nitrogen fixation efficiency in a greenhouse experiment, being determined after: shoot dry mass, root dry mass, nodule dry mass, nitrogen (N) accumulated shoot and calculated the N fixation relative efficiency index. Among the 28 rhizobia isolates tested, 11 induced nodule formation in lima bean. The rhizobia Plu03 and Plu14 stimulated a greater increase of plant dry mass, nodule dry mass and nitrogen accumulation in the shoot, reflecting in a higher relative efficiency index. These results suggest that the isolates Plu03 and Plu14 are more efficient in promoting growth of lima bean, which could be recommended for future agronomic efficiency studies. Key words: Rhizobia, lima bean, biological nitrogen fixation.