DOES SIDEROPHORE PRODUCTION INFLUENCE THE RELATIVE ABUNDANCE OF RHIZOBIUM-MELILOTI IN 2 FIELD POPULATIONS

Populations of indigenous Rhizobium meliloti isolated from nodules of alfalfa grown at sites A and B (soil pH, 7.0 and 6.1, respectively) were previously characterized on the basis of phage sensitivity and divided into 55 and 65 phage types. The available iron content of soil at site B was significantly higher than that at site A. Isolates representing the phage types comprising each of these populations were tested for the production of siderophores. The frequency of siderophore-producing (sid+) phage types of R. meliloti, estimated from the distribution of types in the two field populations, was significantly higher-at site A (54%), where iron was less available, than at site B (18%). The distributions of frequency for sid+ and sid- phage types were similar at site A but differed (P < 0.005) at site B, where the soil was slightly acidic and contained more available iron. The apparent absence of a direct relationship between siderophore production and frequency of occurrence suggests that siderophore production may not influence the relative abundance of R. meliloti in these populations at sites differing in iron availability.