Rhizobium specificity in Leucaena

Knowledge of the specificity of Leucaena for strains of rhizobia that form effective N-fixing associations is confined mostly to Leucaena leucocephala. However, recognition of the agroforestry potential of other species has stimulated an assessment of rhizobial requirements of all 22 species in the genus. In N-free systems, Leucaena exhibits very significant genotype interactions with rhizobial strains that form effective symbioses. Specificity for effectiveness was observed both between and within species. In two glasshouse experiments, all tested rhizobial strains formed nodules with all species, but the level of effectiveness, based on plant dry weights, ranged from totally ineffective to more effective than plants supplied with mineral nitrogen. Plant dry weight and nitrogen content responses were used to place host genotypes into three broad similarity groups. No strain of Rhizobium was universally effective on all host accessions, although several reasonably wide spectrum strains were identified and recommended for field testing. Need-forinoculation field experiments should be used to assess the effectiveness of background rhizobial strains and when found to be unsatisfactory, newly planted Leucaena should be inoculated with suitable Rhizobium. Strain CB3060 has demonstrated an intrinsic competitive ability in the field, but other elite strains (CB3126 and LDK4), identified in glasshouse trials require field testing. The main factors affecting nodulation and N2 fixation are identified as: 1) soil acidity and phosphorus deficiency; 2) high levels of nitrate-N in the soil; 3) moisture stress combined with high temperatures; and 4) the effects of defoliation. The importance of these observations to establishment of Leucaena in new areas, especially in acid soils, is discussed in relation to selection of suitable strains for successful early nodulation and plant growth. (Résumé d'auteur