Effects of host plant origin on nodulin activities and nitrogen fixation in Phaseolus vulgaris L.

The metabolic activities of root nodules formed by Rhizobium tropici UM1899 were measured to test for the effects of geographical origin of the host b ean (Phaseolus vulgaris L.) plant. Under increasing levels of N (0 to 24 mM of NH4NO3), the optimum condition for nitrogen fixation based on nitrogena se activity and allantoin concentration, was obtained between 2 and 4 mM N. Cultivars, including wild accessions from the two major domestication cent ers in America (Middle America and Andes), were then grown under aseptic co nditions with 2 mM NH4NO3 and the rhizobial inoculant. Plant nodulins [legh aemoglobin (Lb), phosphoenolypyruvate carboxylase (PEPC) and glutamine synt hetase (GS)], bacterial nitrogenase (NIF) activities as well as allantoin ( ALA) concentration in the xylem sap, were assayed in flowering plants. Lb, PEPC, NIF activities and ALA concentrations were strongly affected by culti var and by the center of origin. GS activity did not vary significantly wit h either cultivar or center of origin. LB, NIF and ALA were directly relate d to plant growth and offer opportunities to select for efficient N-2-fixin g symbioses. There were slight increases in nodulin activities of the domes ticated cultivars, but the overall low variability within this material rel ative to landraces suggests that diversity for biological nitrogen fixation was reduced by domestication.