| Abstract | To assess the variation in symbiotic effectiveness of N₂-fixing Rhizobium leguminosarum isolates in the presence of NH₄NO₃, peas were inoculated with 38 isolates and grown for 4 weeks with weekly additions of 2 mM NH₄NO₃. Acetylene reduction was inhibited relative to a N-free control in all isolates but the inhibition varied from 60 to 100%. There were also significant differences (p = 0.05) between isolates with varying inhibition of C₂H₂ reduction in H2 uptake, H2 evolution, and leghemoglobin content. Ten isolates were selected for further study of the importance of NH₄NO₃ concentration (0, 1,2, and 5 mM) and plant growth stage (3,4, and 5 weeks) to characters associated with N₂ fixation and plant growth. Plant age, NH₄NO₃ treatment, and isolate were all significant sources of variation (p ≤ 0.01) for each character measured but interactions between these factors were also significant. There were significant differences between isolates in the rate of decrease in C₂H₂ reduction with increasing NH₄NO₃ concentration (C₂H₂ reduction responsiveness to NH₄NO₃) and this responsiveness term was correlated with C₂H₂ reduction rates at 2 and 5 mM NH₄NO₃ (p = 0.01). Acetylene reduction rates were correlated with shoot dry weights at 0.1, and 2 mM NH₄NO₃ (p = 0.001). These results suggest the possibility of enhancing N₂ fixation in peas through selection of rhizobia with maximum effectiveness at low levels of combined nitrogen. |
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