SUMMARY: Graded doses of six phytopathogenic bacteria, namely Pseudomonas lachrymans, P. morsprunorum, P. phaseolicola, P. syringae, P. tabaci and P. tomato, were introduced into the leaves of one of their natural (homologous) hosts and of other (heterologous) plants. Probit slopes for the infectivity of any of the six bacteria did not exceed the value b = 2 in the homologous combinations, but were much greater than 2 in the heterologous ones. In the homologous combinations, the mean probability, P, per inoculated bacterium of multiplying to induce host response was invariant to inoculum dose, d. Hence the probability, Q, that a host would not respond after inoculation with d bacteria was given by the simple exponential, Q = e−pd. In the heterologous combinations, the value of P increased with increasing d.
When a streptomycin-sensitive and a streptomycin-resistant variant of the same bacterium were mixed together in a I: I proportion in the challenge dose, plants responding after inoculation with a heterologous pathogen yielded a mixture of both variants in the same proportion, whereas those responding to inoculation with a homologous pathogen yielded either a I : I mixture of both variants or a large predominance of a single variant, depending on whether the dose contained more or less than i ED 50. It was concluded from these experiments that the relationships between inoculated cells of homologous and heterologous bacteria during growth in vivo were described best by the hypothesis of independent action and the hypothesis of co-operative action, respectively.
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