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

Although isotopic discrimination processes during nitrogen (N) transformations influence the outcome of N-15 based quantification of N-2 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 N-15 during N-2 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 mu mol P plant(-1) week(-1)) versus P-deficient (75 mu mol 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 N-15 over total N content (N-15/Nt) for shoots, roots and nodules were determined. The results showed lower N-15/Nt in shoots than in roots, both being much lower than in nodules. P deficiency caused a larger decrease in N-15/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 N-15/Nt was significantly related to both the quantity of N-2 fixed (R-2= 0.96***) and the P content of nodules (R-2= 0.66*). We conclude that the discrimination against N-15 in the legume N-2-fixing symbiosis of common bean with R. tropici CIAT899 is affected by P nutrition and plant genotype, and that the N-15/Nt in nodules may be used to screen for genotypic variation in P use efficiency for N-2 fixation. (C) 2013 Elsevier GmbH. All rights reserved.