C-13 ISOTOPE DISCRIMINATION CORRELATES WITH BIOLOGICAL NITROGEN-FIXATION IN SOYBEAN (GLYCINE-MAX (L) MERRILL)

A preliminary study was conducted on greenhouse-grown soybean plants to investigate if carbon isotope (C-13) discrimination (DELTA) is related with the capacity to fix atmospheric nitrogen (N2). Seven soybean (Glycine max (L.) Merrill) cultivars (Goldsoy, A-100, Lee, Ada, D61-2694, Alto and Evans) inoculated with a mixture of Bradyrhizobium japonicum were tested. A salinity treatment imposed on one set of plants increased the range in DELTA (18.72 x 10(3) - 19.56 x 10(3) without salinity to 18.72 x 10(3) - 20.36 x 10(3) with salinity) and N2 fixation (79.4%-84.9% without salinity to 73.6%-84.9% with salinity) attainable in the seven cultivars studied. For the full range of treatments, DELTA was negatively correlated (r = -0.62*) with N2 fixation. Thus, subject to confirmation from further study, DELTA may prove a useful tool in the selection of legume genotypes with a high capacity for N2 fixation.