| Abstract | Combined nitrogen (N) has adverse effects on virtually all stages of the Rhizobium–legume symbiosis. Tolerance to combined N varies among legume hosts and rhizobial isolates, but the contribution of each symbiotic partner is not well established. The effects of combined N were studied in faba bean (Vicia faba L.) and pea (Pisum sativum L.), using the same Rhizobium leguminosarum isolates for both hosts. In one experiment, faba bean and pea were inoculated individually with four rhizobial isolates and grown for 28 days in the continuous presence of 0, 2.5, 5.0, or7.5 mol m−3 NH₄NO₃. For each isolate, faba bean was consistently more tolerant to combined N than pea as shown by significantly smaller rates of decrease in N₂-fixing activity (acetylene reduction) in faba bean than in pea. The results were substantiated by those of a similar experiment in which increasing levels of ¹⁵N-labeled NO⁻₃-N (5, 10, or 15 mol m⁻³) were supplied continuously to faba bean and pea inoculated individually with two rhizobial isolates. Comparisons of the different symbioses based on the proportion of total plant N derived from N₂ fixation confirmed the conclusions reached using acetylene reduction activities. |
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