Nodulation, nitrogen fixation, and Rhizobium strain affinities in the genus Leucaena.

Abstract

All 10 species of Leucaena have been reported to nodulate readily, but some doubt remains in the case of L. retusa, which has failed to nodulate in Hawaii soils. The infection process has not been studied. Nodule shape is variable, being astragaloid, desmodioid, or muconoid in form. Attachment to the root is narrow and fragile. Annual nitrogen accumulation rates of 600 kg/ha and as high as 1 t/ha are on record, puts annual nitrogen fixation has not been determined with precision. One approximation, based on the acetylene-reduction technique, puts annual fixation at 110 ± 30 kg. The specific designation for the nodule bacterium is Rhizobium loti. These rhizobia are typically fast growing, gummy, and translucent on yeast-mannitol agar (YMA). They are neutral, or slightly acidic, in the pH reaction on YMA Leucaena rhizobia will nodulate other tropical legumes usually nodulated by fast-growing rhizobia and will also nodulate some species normally nodulated by slow growers. Slow-growing strains of Rhizobium from other tropical genera do not nodulate Leucaena.
Leucaena usually nodulates spontaneously with indigenous soil rhizobia without specific inoculation, but the documented cases of nodulation failure are sufficient to prompt adoption of legume inoculation when Leucaena is introduced to locations where it has not been grown before. Although Leucaena performs very poorly in acid-soil sites, rhizobial strains for Leucaena are available that exhibit tolerance to low pH. Acid tolerance in a strain is not dependent on an alkali-producing capability. One particular strain, TAL 1145, has proved to be competitive for nodule formation, persistent in the soil microflora, and fully effective as a nitrogen-fixing partner for Leucaena in acid soils.