Abstract
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.
Highlights
With the sampling and isolation studies, 28 bacterial isolates were obtained from soil samples and nodules: 14 isolates were obtained from soil samples (6 and 7) using lima bean as bait; 12 were isolated of nodules of Desmodium spp. collected in the field at location Eldorado do Sul; and 2 isolates were obtained from soil sample 4 using leucena as bait (Table 2)
For the production of acid alkali related to the reaction in YMA with bromothymol blue (YMA+BB) medium, 14 isolates of lima bean were observed, six produced acid reaction (Plu01, Plu02, Plu04, Plu07, Plu09, Plu13) and eight isolates produced alkaline reaction (Plu03, Plu05, Plu06, Plu08, Plu11, Plu12, Plu14)
Some bacterial isolates such as Plu03, Plu05, Plu06, besides having growth 7-8 days, generated alkali production in YMA+BB medium, characteristics that could be presumptive of Bradyrhizobium genera (Somasegaran and Hoben, 1994)
Summary
Due to its characteristics as drought and excess moisture resistance and protein potential, representing a source of food and income for small farmers (Fofana et al, 1997; Santos et al, 2009; Vieira, 1992). In the state of Rio Grande do Sul, the species has a diversity of genotypes, the variety “olho de cabra preto”, “olho de cabra vermelho” and “Branco”. This bean is cultivated by small producers and its production is limited, due to the tradition of consuming common beans and the lack of cultivars recommended for the subtropical climatic conditions of the region (Cavalheiro, 2012)
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