SUMMARY: Serological typing has been used to study competition between five strains of in growth and nodulation with four species of host. Although a proportion of non-reacting antigenic variants appears to have been encountered, the serological method remains uniquely suitable for studies of this kind.

Irrespective of host species or level of inoculum the population of bacteria in the tube of seedling agar approached a maximum of approximately 4 × 10 viable cells per ml. No consistent relationship was found between host species and the proportion of the several strains of rhizobia in the population outside the plant. The relative frequency of strains was, however, markedly affected by level of total inoculum and time of growth after inoculation. Some strains grew better at first but reached a lower maximum than others which fared better at higher population levels, whether from a heavier inoculum or prolonged growth. The population on the root surface itself failed to reflect any marked host influence in the balance between strains.

The proportions of strains found in the nodules were unrelated to their representation in the root’s external environment. Each host exercised a specific selective effect in this regard.


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