Selection of bean (Phaseolus vulgaris L.) rhizobial strains for the Brazilian Cerrados

The common bean crop (Phaseolus vulgaris L.) occupies 5.5 million hectares in Brazil and provides about 30% of the population's protein needs. Yield remains low mainly due to limited availability of N and P, and to the often acid soil conditions. Surprisingly, there have been only very limited studies in Brazil in which the isolation and evaluation of efficient N-2-fixing bean strains has been attempted. This paper reports the selection of bean rhizobia for the Cerrados, the savannas that represent one-fourth of Brazilian land. Two-hundred strains were selected from large pink-colored nodules collected in the Cerrados region and biological nitrogen fixation was evaluated under optimal (27/21 degreesC, day/night) and high (37/21 degreesC) temperature conditions. Thirty-six strains were selected and their nitrogen-fixing capacity and competitiveness further evaluated with black-seeded Negro Argel, and colored Carioca cultivars. One-fifth of the strains showed low genetic stability of nodulation genes, but some strains were as or more competitive than the strains currently recommended for use in commercial bean inoculants in Brazil. The superior performance of five strains was confirmed under field conditions and reinoculation in the second year increased bean yield. The DNA fingerprintings obtained by the ERIC-REP-PCR analyses indicated a high level of genetic diversity, and among the 36 strains, six different patterns of RFLP-PCR of the 16S rRNA gene region were detected. The 16S rDNA sequences of the most efficient and competitive strains were genetically similar to Rhizobium tropici, suggesting that further studies on inoculant strains in the hot and acid soils of the Brazilian Cerrados and Africa should emphasize this species. (C) 2002 Published by Elsevier Science B.V.