In a series of greenhouse experiments, isolates representing serotypically
distinct subpopulations of Rhizobium leguminosarum bv. trifolii recovered from root
nodules of field-grown subclover Trifolium subterraneum cv. "Nangeela"), were
evaluated for their symbiotic effectiveness potential, their effect on regrowth
potential of subclover, and for their competitive abilities to form nodules. Isolates
from the nodule dominant serotypes AS6, AS6-21, and AS21 were shown to be
suboptimally effective at fixing nitrogen gas (N₂) relative to RT162X95, a highly
effective commercial inoculant strain. Although the majority of isolates
representing the minor nodule occupying serotypes AS27, AG4, and AP17 were
also shown to be of moderate effectiveness, isolates ADS14 and RW11 of
serotype AS27 were equally as effective as RT162X95. Subclover regrowth was
influenced by the severity of the first harvest. Plant-isolate combinations
possessing superior N₂-fixing capabilities were more adversely affected by severe
clipping than were the less effective combinations. Isolate 88FL3,5, a mediocre
N₂-fixing member of the nodule dominant serotype AS6, was not universally more
competitive at forming nodules than other isolates. Although 88FL3,5 formed more
nodules than many of the other isolates when mixed in equal numbers with each
of them separately, isolates ADS14 and ADS16 significantly outcompeted 88FL3,5
even though they represented a minor nodule occupying serotype, AS27. The
nodule occupying successes of 88FL3,5 or ADS16 could be overcome if their
respective competitor had a numerical advantage ranging between 10:1 and 25:1.
Since the soil-borne population of 88FL3,5 has been shown to outnumber other
subpopulations of rhizobia in the same soil by at least 10:1, it is not surprising that
it can occupy the majority of nodules on field-grown plants even in the presence
of more competitive types. Why the highly effective and highly competitive
members of serotype AS27 are rarely recovered from nodules of field-grown
Trifolium subterraneum L. is a question that merits further study. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/37399 |
| Date | 27 January 1992 |
| Creators | Wanjagi, Francis Ndung'u |
| Contributors | Bottomley, Peter J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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