SYMBIOTIC CHARACTERISTICS OF RHIZOBIUM-LEGUMINOSARUM BV TRIFOLII ISOLATES WHICH REPRESENT MAJOR AND MINOR NODULE-OCCUPYING CHROMOSOMAL TYPES OF FIELD-GROWN SUBCLOVER (TRIFOLIUM-SUBTERRANEUM L)

The symbiotic effectiveness and nodulation competitiveness of Rhizobiu m leguminosarum bv. trifolii soil isolates were evaluated under nonsoi l greenhouse conditions. The isolates which we used represented both m ajor and minor nodule-occupying chromosomal types (electrophoretic typ es [ETs]) recovered from field-grown subclover (Trifolium subterraneum L.). Isolates representing four ETs (ETs 2, 3, 7, and 8) that were hi ghly successful field nodule occupants fixed between 2- and 10-fold le ss nitrogen and produced lower herbage dry weights and first-harvest h erbage protein concentrations than isolates that were minor nodule occ upants of field-grown plants. Despite their equivalent levels of abund ance in nodules on field-grown subclover plants, ET 2 and 3 isolates e xhibited different competitive nodulation potentials under nonsoil gre enhouse conditions. ET 3 isolates generally occupied more subclover no dules than isolates belonging to other ETs when the isolates were mixe d in 1:1 inoculant ratios and inoculated onto seedlings. In contrast, ET 2 isolates were less successful at nodulating under these condition s. In many cases, ET 2 isolates required a numerical advantage of at l east 6:1 to 11:1 to occupy significantly more nodules than their compe titors. We identified highly effective isolates that were as competiti ve as the ET 3 isolates despite representing serotypes that were rarel y recovered from nodules of field-grown plants. When one of the subopt imally effective isolates (ET2-1) competed with an effective and compe titive isolate (ET31-5) at several different inoculant ratios, the per centages of nodules occupied by the former increased as its numerical advantage increased. Although subclover yields declined as nodule occu pancy by ET2-1 increased, surprisingly, this occurred at inoculant rat ios at which large percentages of nodules were still occupied by ET31- 5.