02840naa a2200145 a 450000100080000000500110000800800410001910000200006024500970008026000090017752024290018665000120261570000180262777300490264510581442018-02-22       1975    bl uuuu     u00u1 u    #d1 aLABANDERA, C.A.  aCompetition between an introduced strain and native Uruguayan strains of Rhizobium trifolii.  c1975  aThe relative success of a commercial strain (TA1) competing with ineffective Uruguayan strains of Rhizobium trifolii for the colonisation of the root surface and in forming nodules has been tested with Trifolium subterraneum and T. vepens, and with a species indigenous to Uruguay (T. polymorphum). Competing pairs of strains were tested on hosts, raised from surface-sterilized seed and maintained on seedling agar in tube culture, so as to permit representation on the root surface and in each nodule to be determined. Relative nodulation success was estimated as a competitive index (C): the ratio of nodules formed by one strain to those formed by the other when equally represented on the root surface.  On T. repens and T. subterraneum the effective strain (TA1) dominated the ineffective Uruguayan competitor (U73) so that CTA1:U73 ranged from 18 to greater than 94, and its success was very little dependent on relative representation on the root surface. On T. polymorphum, with which TA1 and U73 were both ineffective, the former was still superior (CTA1:U73=2?4), but was not as dominant as it was on the other two hosts; its success was more dependent on its root-surface representation. A mixture of several, all ineffective, Uruguayan strains narrowed the competitive advantage of strain TA1 and caused its relative nodulating success to become more dependent on representation on the root.  The greater intrinsic nodulating competitiveness of strain TA1 was to some extent offset by its relatively poor colonisation of the root surface, where it could be outgrown by the Uruguayan strains, particularly when a medium or low level of inoculation permitted considerable growth. Delay in the addition of TA1 to an established population of U73 resulted in additional representation of the latter in nodules of T. subterraneum. However strain U73 was largely restricted to the earlier-formed nodules and the total, as distinct from the proportion, of TA1 nodules was largely unaffected by the start given its competitor.  Nodules due to mixed infections were regularly encountered (up to 18%), particularly in the case of T. subterraneum.  It was not possible to find in any of the strains' symbiotic properties a consistent explanation of their competitiveness in nodule formation. However it is perhaps significant that TA1 (the more successful strain) formed its first nodule slightly ahead of its competitors.  aURUGUAY1 aVINCENT, J.M.  tIn: Plant and Soil, 1975gv. 42, p. 327-347.