Cobalt status and its effects on soil populations of Rhizobium lupini, rhizosphere colonization and nodule initiation

Abstract

The role of cobalt in the processes leading to nodule initiation in Lupinus angustifolius L. cv. Uniharvest was studied. Survival and multiplication of Rhizobium lupini were examined in Lancelin soil and in the rhizosphere of L. angustifolius grown in it, with or without inoculation with antibiotic-resistant derivatives of R. lupini strain WU425. A clearing and staining technique was then used to record the incidence of nodule initiation in L. angustifolius seedlings. Within the constraints of experimental techniques, soil and rhizosphere numbers of R. lupini were unaffected by the cobalt status of the soil. Cobalt limitation was, however, inhibitory to nodule initiation in L. angustifolius, with reductions of up to 13 and 30% in nodule initiation for inoculated and uninoculated seedlings respectively. The severity of cobalt deficiency in L. angustifolius seems to be due solely to the plant's inability to provide cobalt to the rhizobia within the plant and not to any external limitation on their growth in soil or rhizosphere.