COEXISTENCE AMONG EPIPHYTIC BACTERIAL-POPULATIONS MEDIATED THROUGH NUTRITIONAL RESOURCE PARTITIONING

The levels of coexistence between Pseudomonas syringae and various non pathogenic epiphytic species in the phyllosphere of beans (Phaseolus v ulgaris) mere assessed by using replacement series. The epiphytic spec ies Pseudomonas fluorescens, Pantoea agglomerans, Stenotrophomonas mal tophilia, and Methylobacterium organophilum were all capable of exhibi ting higher levels of coexistence with P. syringae than was observed w ith a near-isogenic P. syringae strain pair. The ecological similarity of the epiphytes was estimated with niche overlap indices derived fro m in vitro carbon source utilization profiles. The level of coexistenc e of the epiphytes was inversely correlated with the ecological simila rity of the strains. Hence, the level of coexistence between the epiph ytes was proportional to the degree of niche differentiation, defined as the ability to utilize carbon sources not utilized by a competing s train. Comparisons of utilization profiles for groups of carbon source s (amino acids, organic acids, and carbohydrates) indicated the types of carbon sources for which the strains likely competed in the bean ph yllosphere. P. fluorescens and P. syringae strains probably competed f or most carbon sources. S. maltophilia and M. organophilum strains pro bably competed with P. syringae for most organic acids but few amino a cids or carbohydrates. P. agglomerans strains probably competed with P . syringae for most amino acids and organic acids but few carbohydrate s. A variable level of coexistence observed between P. agglomerans and P. syringae probably reflected the variability in abundance in the be an phyllosphere of the carbohydrates that P. agglomerans utilized excl usively,