Nitrate inhibition of N2 fixation in Phaseolus vulgar is L. cv. OAC Rico and a supernodulating mutant

summary Time-integrated, 15N–based determinations of N2 fixation were undertaken on common bean (Phaseolus vulgaris L.) cv. Rico and its supernodulating mutant RBS15. Plants were cultured in pots filled with quartz sand and experienced three different (0.5, 3.0, 6.0 mol m3) constantly maintained levels of nitrate for a total period of 65 d. Irrespective of nitrate treatment, biomass production of RBS15 was consistently lower compared to Rico. Nodule numbers of the wild type were only slightly elevated in response to a rise in nitrate concentration from 05 to 30 mol m3, while nodule numbers of RBS15 increased substantially, indicating that nodulation of the mutant was N limited in the 0.5 mol m 3 treatment. A further rise in nitrate concentration failed to induce any more change in the mutant, but caused earlier senescence in Rico nodules. Both genotypes decreased their dependency on atmospheric N2 in the presence of increasing quantities of nitrate in the rooting medium, with relatively small differences between genotypes. Calculated in absolute terms, total N2 fixation was mostly lower in the mutant despite its higher nodule numbers. Much larger differences appeared when the amount of N., fixed per unit nitrogen invested into nodules was compared. Here the mutant exhibited significantly lower values. The data suggest that nodulation of the supernodulating mutant is tolerant to nitrate, while N, fixation in both genotypes is detrimentally affected.