Variation in ability of Rhizobium leguminosarum isolates to fix dinitrogen symbiotically in the presence of ammonium nitrate

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Abstract
To assess the variation in symbiotic effectiveness of N2-fixing R. leguminosarum isolates in the presence of NH4NO3, peas were inocualted with 38 isolates and grown for 4 wk with weekly additions of 2 mM NH4NO3. Acetylene reduction was inhibited relative to a N-free control in all isolates; inhibition was 60-100%. 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 wk) to characters associated with N2 fixation and plant growth. Plant age, NH4NO3 treatment, and isolate were all significant sources of variation (P .ltoreq. 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 N.