Discrimination against 15N among recombinant inbred lines of [i]Phaseolus vulgaris[/i] L. contrasting in phosphorus use efficiency for nitrogen fixation

Although isotopic discrimination processes during nitrogen (N) transformations influence the outcome of 15N based quantification of N2 fixation in legumes, little attention has been given to the effects of genotypic variability and environmental constraints such as phosphorus (P) deficiency, on discrimination against 15N during N2 fixation. In this study, six Phaseolus vulgaris recombinant inbred lines (RILs), i.e. RILs 115, 104, 34 (P deficiency tolerant) and 147, 83, 70 (P deficiency sensitive), were inoculated with Rhizobium tropici CIAT899, and hydroaeroponically grown with P-sufficient (250 μmol P plant−1 week−1) versus P-deficient (75 μmol P plant−1 week−1) supply. Two harvests were done at 15 (before nodule functioning) and 42 (flowering stage) days after transplanting. Nodulation, plant biomass, P and N contents, and the ratios of 15N over total N content (15N/Nt) for shoots, roots and nodules were determined. The results showed lower 15N/Nt in shoots than in roots, both being much lower than in nodules. P deficiency caused a larger decrease in 15N/Nt in shoots (−0.18%) than in nodules (−0.11%) for all of the genotypes, and the decrease in shoots was greatest for RILs 34 (−0.33%) and 104 (−0.25%). Nodule 15N/Nt was significantly related to both the quantity of N2 fixed (R2 = 0.96***) and the P content of nodules (R2 = 0.66*). We conclude that the discrimination against 15N in the legume N2-fixing symbiosis of common bean with R. tropici CIAT899 is affected by P nutrition and plant genotype, and that the 15N/Nt in nodules may be used to screen for genotypic variation in P use efficiency for N2 fixation