Symbiotic Characteristics of Rhizobium leguminosarum bv. trifolii Isolates Which Represent Major and Minor Nodule-Occupying Chromosomal Types of Field-Grown Subclover ( Trifolium subterraneum L.)

Abstract

The symbiotic effectiveness and nodulation competitiveness of Rhizobium leguminosarum bv. trifolii soil isolates were evaluated under nonsoil greenhouse conditions. The isolates which we used represented both major and minor nodule-occupying chromosomal types (electrophoretic types [ETs]) recovered from fieldgrown subclover (Trifolium subterraneum L.). Isolates representing four ETs (ETs 2, 3, 7, and 8) that were highly successful field nodule occupants fixed between 2-and 10-fold less nitrogen and produced lower herbage dry weights and first-harvest herbage protein concentrations than isolates that were minor nodule occupants of field-grown plants. Despite their equivalent levels of abundance in nodules on field-grown subclover plants, ET 2 and 3 isolates exhibited different competitive nodulation potentials under nonsoil greenhouse conditions. ET 3 isolates generally occupied more subclover nodules than isolates belonging to other ETs when the isolates were mixed in 1:1 inoculant ratios and inoculated onto seedlings. In contrast, ET 2 isolates were less successful at nodulating under these conditions. In many cases, ET 2 isolates required a numerical advantage of at least 6:1 to 11:1 to occupy significantly more nodules than their competitors. We identified highly effective isolates that were as competitive as the ET 3 isolates despite representing serotypes that were rarely recovered from nodules of field-grown plants. When one of the suboptimally effective isolates (ET2-1) competed with an effective and competitive isolate (ET31-5) at several different inoculant ratios, the percentages of nodules occupied by the former increased as its numerical advantage increased. Although subclover yields declined as nodule occupancy by ET2-1 increased, surprisingly, this occurred at inoculant ratios at which large percentages of nodules were still occupied by ET31-5.

Influence of Bovine Slurry Deposition on the Structure of Nodulating Rhizobium leguminosarum bv. viciae Soil Populations in a Natural Habitat

“…The proportion of each of the 24 groups was in no case more than 5% of the total number of patterns analyzed (Table 1). In contrast to earlier rhizobial population structure analysis (27,28,40), no dominant strain groups could be detected. Of the 24 strain groups, 19 were derived from nodules isolated from both plant cultivars used for the nodulation tests.…”

Genetic Structure and Symbiotic Characteristics of a Bradyrhizobium Population Recovered from a Pasture Soil

“…Since we have extensively characterized an indigenous population of R. leguminosarum in an Oregon soil (28, 29,30), a unique opportunity exists to address the comparative ecology of the two genera in the same soil once the Bradyrhizobium population is characterized. Although agricultural legumes that are Bradyrhizobium hosts are rarely grown in Oregon, we speculated that local soils should harbor lupine-nodulating Bradyrhizobium sp.…”

Competition for Nodulation

“…In two pea fields (named sites I and II) inoculated with R. leguminosarum bv viceae it was found that, although the rhizobial indigenous populations were similar, the inoculant resulted more competitive at site I but less competitive at site II, even when at this site the inoculum was applied at very high concentration; however, in laboratory tests of competition of the inoculant against the dominant strain from site II, both resulted equally competitive (Meade et al, 1985). Strains of R. leguminosarum bv trifolii that were the most abundant nodule occupiers in fieldgrown subclover were not the most competitive in laboratory experiments carried out in Leonard jars with perlite-vermiculite as substrate (Leung et al, 1994a). In particular, there were four isolates, classified according to their electrophoretic types (ET), which were the most abundant in the field crops.…”