Analysis of Phaseolus-Rhizobium interactions in a subsistence farming system

Poor bean yields in the Cunha region of the Mata Atlantica ecosystem in the state of Sao Paulo, Brazil, are associated with low agronomic inputs, plant disease, and soil erosion. To identify sustainable farming practices that increase production and maximize biological N-2 fixation (BNF), the effects of soil fertility and plant cultivar on seed yield and root nodule formation were measured under standard agronomic practices. Results from 16 sites showed that fertilizing with lime and molybdenum increased seed yields to 370% for the landrace Serro Azul. In addition to increased yields, plants grown with fertilizer had more nodules. Marked strains of Rhizobium tropici were tested under controlled environments. An indicator strain of Rhizobium containing the gusA marker gene was used. Our results verify that the indicator strain CM-255 GusA(+)Hup(+) had a high capacity to associate with the five bean varieties tested. Fertilization with P, K, S + micronutrients and liming were essential for better nodulation by the indicator strain. Under low fertility conditions, the landrace variety Serro Azul was poorly nodulated, when associated with native strains or with the indicator strain. However, under better soil fertility conditions, nodulation of Serro Azul by the marked Rhizobium strain was increased. The commercial variety Carioca 80SH showed no increase in nodulation (nodule number).