Rhizobia that nodulate tree legumes: specificity of the host for nodulation and effectiveness

Rhizobial specificity, defined in terms of nodulation and nitrogen-fixing effectiveness characteristics of a group of rhizobia on a host legume, serves as a basis for predicting the need to inoculate, selecting species for most probable number plant-infection assays, and preparing rhizobial inoculants suitable for a range of legume species. A series of inoculation experiments were performed under growth room and greenhouse conditions to delineate rhizobial specificity of a variety of tree legumes. Gliricidia sepium, Calliandra calothyrsus, and Leucaena leucocephala nodulated effectively with rhizobia isolated from each of the three genera. With a few exceptions, Sesbania grandiflora and Robinia pseudoacacia nodulated effectively only with rhizobial strains isolated from each genus respectively. A range of specificity was found among species that nodulate with Bradyrhizobium. Whereas Acacia mearnsii nodulated with most strains but fixed N2 effectively with relatively few, Acacia mangium and Lysiloma latisiliqua were specific for both nodulation and effectiveness. Key words: nitrogen fixation, effectiveness, nodulation, rhizobia, tree legumes.