COBALT AND NITROGEN-FIXATION IN LUPINUS ANGUSTIFOLIUS L .2. NODULE FORMATION AND FUNCTION

The effects of Co deficiency on nodule formation and function in sweet lupin (L. angustifolius L. cv. Unicrop) were studied in Co-deficient Lancelin sand in the glasshouse. Bacteroid [Rhizobium lupini] densities in Co-deficient nodules were lower than in normal nodules. Recovery from Co deficiency in inoculated treatments was associated with increases in bacteroid density and Co accumulation in lateral nodules. Such changes did not occur in treatments infected with rhizobia from the soil. Acetylene-reducing activity of Co-deficient plants was not initiated until plants were nearly 6 wk old, at which time Co-treated plants were at their peak of activity. Specific activities of Co-deficient nodules remained very low even when nitrogenase did develop. Their large mass of nodules allowed Co-deficient plants to reach 20-50% of the normal activity per plant, but specific activities were only 5-13% of peak activities in Co-treated nodules. Nodule bacteroid content and leghemoglobin content were linearly related to cobalamin content, each with a single relationship. Plotting acetylene-reducing activity against cobalamin content or leghemoglobin content generated 2 different linear response curves in each case. The slopes of the lines were different, depending on the presence or absence of cobalt. There may be a function in N2 fixation in legume nodules for a non-cobalamin form of Co.