Growth patterns and metabolic activity of pseudomonads in sugar beet spermospheres: relationship to pericarp colonization by Pythium ultimum

Growth patterns of seed-inoculated fluorescent pseudomonads in the spermospheres of sugar beet in soil or sand maintained at -15 J/kg and 16 C were monitored over a 72-h time period. When incubated in soil, strains 33-2 (Pseudomonas putida), A1 (P. fluorescens), and ML5 (P. fluorescens-putida) exhibited relatively short lag phases (< 4 h), with most growth occurring during the first 12 h. Strains F42 (P. putida) and PGS12 (P. aureofaciens) exhibited long lag phases (8-12 h), and their populations increased mainly between 12 and 24 h. Doubling times during the exponential growth phases were 2-3 h for all strains except P. syringae strain B728a, which had the longest doubling time (5-7 h). Most strains reached the stationary phase within 24 h after planting of seed. Growth rates of bacteria in spermospheres of seed planted in sand were similar, except some strains continued to grow past 24 h. When seeds were inoculated with bacteria at 10(6)-10(8) cfu per seed, the incidence of pericarp colonization by Pythium ultimum within 48 h in soil was less in seeds treated with strains with short lag phases than in those treated with long lag phases. However, the efficaciousness of these strains was essentially negated when soil contained a high population of the fungus. The high bacterial inoculum density was important, as there was a linear relationship between inoculum densities and percentage of pericarp colonization by P. ultimum. To determine if bacterial cells were active or dormant (or dead), cells were incubated in a solution of 2-(p-indophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT). The percentage of INT-active cells on seed with initial population densities of 10(7)-10(8) cfu per seed ranged from about 17.2-55.9% at 0 h to 85.8% at 48 h. The percentage of INT-active cells peaked at 6-12 h for the short lag phase strains. No clear relationship between efficacy of strains in reducing pericarp colonization and the percentage of metabolically active cells was detected. On the other hand, the finding that a significant percentage of cells are metabolically active following inoculation of seed with high-density inocula explains in part why protection of the pericarp against P. ultimum is better even though multiplication cannot be detected. Thus, competition for nutrients and the production of secondary metabolites occurs in the absence of increasing cell number.