Effects of time of nitrate application on nitrate reductase activity, nitrate uptake, and symbiotic dinitrogen fixation in faba bean and pea

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Abstract
The effects of increasing concentrations of NO3--N (0, 5, 10, or 15 mol .cntdot. 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.