INOCULUM DENSITY-DEPENDENT MORTALITY AND COLONIZATION OF THE PHYLLOSPHERE BY PSEUDOMONAS-SYRINGAE

Pseudomonas syringae inocula containing cell concentrations ranging fr om 10(5) to 10(9) cells per ml were applied to the primary leaves of b ean plants. The plants were incubated under conditions of high tempera ture and illumination and low relative humidity. Bacterial mortality r ates and the proportional population decline of the inoculum were lowe st at the highest inoculum concentrations. Addition of a high concentr ation of heat-killed cells to the inoculum containing a low concentrat ion of viable cells significantly reduced both the mortality rate and the proportional population decline of the viable cells: The mechanism s underlying this density dependent mortality may include cooperative protective effects of extracellular factors, such as bacterial extrace llular polysaccharides, and physical protection by neighboring cells. Although epiphytic populations derived from inoculum concentrations of 10(8) or 10(9) cells per mi tended toward 10(6) CFU/g, the presumed c arrying capacity of the leaf, populations derived from lower inoculum concentrations never achieved this carrying capacity. Assuming that ep iphytic populations of P. syringae reside in discrete protected sites, our results suggest that at low inoculum concentrations, following a period of environmental stress, the number of viable cells may have dr opped to zero in some sites; hence, the carrying capacity of the leaf could not be achieved.