Symbiotic specificity of tropical tree rhizobia for host legumes

• The host range and specificity is reported of a genetically diverse group of rhizobia isolated from nodules of Calliandra calothyrsus, Gliricidia sepium, Leucaena leucocephala and Sesbania sesban. • Nodule number and nitrogen content was measured in seedlings of herbaceous and woody legume species after inoculation with rhizobial strains isolated from tropical soils, to establish symbiotic effectiveness groups for rhizobial strains and their hosts. • Specificity for nodulation and N₂ fixation varied greatly among the legumes. Symbionts of all four legumes exhibited a wide range of promiscuity and symbiotic effectiveness with isolates of S. sesban having the narrowest host range. N₂ fixation varied greatly; although some strains fixed large amounts of N₂ with more than one host, none was effective with all hosts. Rhizobial isolates of C. calothyrsus, G. sepium and L. leucocephala were able to effectively cross-nodulate each others' hosts as well as a number of other species. • The complex nature of cross-nodulation relationships between diverse rhizobial strains and legume hosts is highlighted. Host plants inoculated with effective rhizobial strains showed better nitrogen use efficiency than plants supplied solely with mineral nitrogen.