| Abstract | The effects of increasing concentrations of NO₃⁻-N (0, 5, 10, or 15 mol∙m⁻³) supplied at planting or 7 or 14 days after planting were studied in faba bean (Vicia faba L.) and pea (Pisum sativum L.) inoculated with Rhizobium leguminosarum biovar viceae isolate 175F19. Averaged over all times of NO₃⁻-N application, N₂ fixation (acetylene reduction) was inhibited significantly more in pea than in faba bean. Times of NO₃⁻-N application had little effect on N₂ fixation for faba bean. In contrast, N₂ fixation in pea was most strongly inhibited when NO₃⁻-N was applied at planting and showed a decreasing sensitivity as NO₃⁻-N applications were delayed to 7 and 14 days after planting. At all times of NO₃⁻-N application, as the levels of NO₃⁻-N increased, the proportion of total plant nitrate reductase activity contributed by leaves and roots remained relatively constant in faba bean, but these proportions increased and decreased, respectively, in pea. Nitrate was taken up at higher levels and at a greater rate in pea than in faba bean. The lower capacity for NO₃⁻ uptake in faba bean could contribute to the greater tolerance of the symbiosis to NO₃⁻-N compared with that in pea. |
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