| Abstract | Factors affecting the efficiency of selection for increased N₂(C₂H₂) fixation in Pisum sativum L. were examined. Ten genotypes, previously selected for variability in fixation rates when inoculated with commercial (mixed strain) Rhizobium leguminosarum, were examined using pure strain (128C52 and 10004) inocula. Strains and genotypes both affected fixation rates, but genotype × strain interaction was also found. Increases in relative growth rate (RGR) were associated with increases in fixation across strains but not with increases in fixation across genotypes. These differences in the response to changes of strain and genotype were further studied over a 4-week period using genotypes specifically selected for high or low rates of C₂H₂ reduction with both strains. Strain 128C52 and the genotypes selected for high rates of C₂H₂ reduction produced greater maximum rates of C₂H₂ reduction than their low counterparts, but the strains also differed in the time at which fixation began. Plants inoculated with strain 128C52 showed significant rates 3–4 days before those inoculated with strain 10004. This earlier availability of nitrogen in the former may have alleviated the early nitrogen starvation and hence increased RGR. All differences in fixation rate were related to nodulation. Although early nodulation and fixation are apparently important to plant growth under N-free conditions, the expression of genotype or strain variability might be reduced by combined N in the field owing to its inhibitory effect on fixation. |
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