Compettive ability of Rhizobium leguminosarum bv. phaseoli strains.

Abstract

Phaseolus vulgaris L. is an important crop species in Latin America. Beans have the potential to satisfy their nitrogen needs through N2 fixation, but since the two proposed centers of origin of this crop are in Latin America, soils in this area often contain large populations of Rhizobium. Many of these are weak in N2 fixing ability. In order to enhance inoculant strain success in these soils,and thereby the nitrogen and grain yield of this crop, it is important to select strains which combine efficiency in N2 fixation with competitiveness for nodule sites. The nod+nif- strain of R. leguminosarum bv. phaseoli UMR 1116 was used to test 62 representative &.icobiun strains for competitive abiLity, Each strain was inoculated with UMR 1116 at a cell ratio 1:1 onto seedlings of P. vulgaris L. growing in pots containing sterile sand and nutrient solution. Only strains which combined competitiveness and effectiveness induced active plant growth, with the strains UMR 1073, 1084, 1102, 1125, 1165,1378, and 1384 being outstanding in both characteristics. Bean cultivars also showed variation in their association with rhizobia. Flor de Mayo, Algarobo Arbustivo, and Canario 72, had the greatest responses with the effective (nod+nif-) strains tested, whereas Iguaçú and bayomex had the least. The root–tip marking (RTM) procedure was used to evaluate the speed in nodulation of selected strains. Competitive strains had the greatest ability to induce nodulation above the RTM.High temperature affected Rhizobium competitiveness and also the ability of strains to induce nodules in the RTM region. Under a regime of 91/26℃ day/night temperature, the correlation between strain competitiveness and ability to induce nodulation in the root system was still evident, but the zone in which the correlation between these traits occurred moved down the root system, below the RTM. Thus, while the nod+nif- strain UMR 1116 and the RTM procedure proved useful in selecting outstanding bean rhizobia for effectiveness and competitiveness, it is essential that such tests be undertaken under conditions similar to the field, with the best strains then verified by field testing.