Growth and N2-fixation of soybeans inoculated with strains of Bradyrhizobium japonicum differing in energetic efficiency and PHB utilization

Abstract The hypothesis, that accumulation of poly-hydroxybutyrate (PHB) in bacteroids of soybean root nodules and utilization of this polymer during pod-filling, are factors in N 2 fixation during seed development, was tested in a glasshouse experiment. Lincoln soybeans, inoculated with strains of B. japonicum of different energetic efficiency (Hup + or Hup − ), were grown under bacteriologically-controlled conditions in open pots of combined-N-free medium, until the pod-filling stage. Nitrogen fixation was measured as accumulation of total N in shoots (including seeds). Bacteroids, prepared from tap-root nodules at intervals during growth, were assessed for PHB and protein content. During vegetative growth, all of the strains were of similar symbiotic effectiveness but during pod-filling, plants inoculated with the three Hup + strains fixed N 2 and grew at about twice the rates of plants inoculated with the three Hup − strains. Bacteroids of all three Hup + strains accumulated PHB (from 40 to >50% dry wt) during vegetative plant growth and utilized it during pod-filling. Two of three Hup − strains accumulated little PHB in the bacteroids and all three utilized little or none of it during pod-filling. These results are discussed in relation to a supply of energy-yielding substrates to nodules during pod-filling, to the influence of energetic efficiency of the bacteroids on PHB accumulation and to the role of PHB utilization in sustaining nodule O 2 -demand during seed development.