Variation inRhizobium leguminosarum response to short term application of NH4NO3 to nodulatedPisum sativum L.

Abstract

This study was conducted to determine the effect of short term application of NH4NO3 on nodule function and to determine whether the rhizobial isolate used was a significant factor in this effect. Pea plants were inoculated with 10 differentRhizobium leguminosarum isolates and grown for 3 weeks in N-free medium before addition of 0, 1, 2 or 5 mM NH4NO3 for 2 to 7 days. Acetylene reduction and leghemoglobin content decreased with increasing exposure time to NH4NO3 and with increasing concentration of NH4NO3. NH4+ and NO3− depletion from the nutrient medium were assayed in plants exposed to 5 mM NH4NO3 and mean uptake rates were similar for each ion. There were significant differences among isolates in the rate of decrease of C2H2 reduction with increasing NH4NO3 concentration (C2H2 reduction responsiveness to NH4NO3) 4 and 7 days after addition of NH4NO3 but no differences after 2 days of exposure to NH4NO3. There were significant differences among isolates in NH4+ depletion from the nutrient medium but these differences were not correlated with the differences observed in C2H2 reduction. Ranking of the isolates for C2H2 reduction responsiveness to NH4NO3 applied to plants with nodules was different from that obtained when NH4NO3 was applied at seeding. Isolates with varying sensitivity to NH4NO3 may be useful tools for determining the mechanisms responsible for inhibition of symbiotic N2 fixation by combined nitrogen.