The nature of non-protein nitrogen accumulation in Trifolium subterraneum root nodules

Abstract

Non-protein nitrogen accumulated in nodules formed on Trifolium subterraneum cv. Tallarook by Rhizobium trifolii strain NA30, but not in nodules formed by strain TA1. Studies with six R. trifolii strains and four T. subterraneum cultivars indicated that the accumulation of non-protein nitrogen was a characteristic of certain strains and that it was accompanied by a greater development of nodule tissue. With normal symbiotic associations, approximately 6 per cent of the total plant nitrogen was located in the nodule system whereas nodules accumulating non-protein nitrogen contained, on average, 12 per cent of the total nitrogen in the plant. The principal component of the accumulating non-protein nitrogen was identified as “bound” γ-aminobutyric acid. Moderate to high concentrations of γ-aminobutyric acid (0.3–1.7 mmoles/g nodule dry weight) were found in nodules formed by 10 strains (of 36 examined) on Tallarook. With two “accumulating” strains, higher concentrations of γ-aminobutyric acid were found in nodules formed on the cultivar Clare (2.0 mmoles/g nodule dry weight) than in nodules formed on Tallarook or Yarloop (1 1.4 mmoles). No γ-aminobutyric acid was found in cultured cells of either an accumulating strain (NA30) or a nonaccumulating strain (TA1) of R. trifolii. The accumulation of non-protein nitrogen as γ-aminobutyric acid, and the accompanying increase in nodule tissue, each resulting in the export of a lower proportion of the nitrogen fixed to the host, is considered to be a factor causing a lower degree of symbiotic effectiveness.