| Abstract | Combined nitrogen inhibits nodule development and nitrogen fixation in Rhizobium -legume symbioses. Isolates of R. leguminosarum which had been shown to vary in symbiotic effectiveness in the presence of NN₄NO₃ were used as inoculum for peas grown with NH₄NO₃ KNO₃ or NH₄Cl and harvested between 14 and 24 days after planting. Plants grown in 2mm NH₄NO₃ showed inhibition of nodule development and function 17 days after seeding but there were no significant differences (SDs) between isolates of high and low effectiveness. In the presence of 5 mM NH₄NO₃ nodule development and C₂H₂ reduction were severely inhibited from 14 days onwards. Plants inoculated with isolates of high effectiveness had more tap-root nodules, higher C₂H₂ reduction rates and higher leghemoglobin content than those inoculated with isolates of low effectiveness. In plants grown with 5 mm KNO3 or 5 mm NN₄Cl, NO₃⁻ had the major inhibitory effect on nodulation and nodule development, but NH₄⁺ began to show inhibitory effects on nodule development after 17 days. There were no SDs between isolates of low and high effectiveness at these concentrations of NO₃⁻ and NH₄⁺. Differences in symbiotic response of R. leguminosarum to combined nitrogen are small and appear during nodule development rather than at nodule initiation. |
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