Effects of Pseudomonas fluorescens F113 on ecological functions in the pearhizosphere are dependent on pH

The aim of this microcosm study was to determine influence of the antibioti c 2,4-diacetylphloroglucinol (DAPG) on the effect of wild-type and function ally modified Pseudomonas fluorescens F113 strains in a sandy loam soil of pH 5.4 planted with pea (Pisum sativum var Montana). The functional modific ation of strain F113 was a repressed production of DAPG, useful in plant di sease control, creating the DAPG negative strain F113 G22; both were marked with a lacZY gene cassette. Lowering the soil pH to 4.4 significantly redu ced the plant shoot and root weights and the root length, whereas the bacte rial inocula had no significant effect. Both inocula significantly reduced the shoot/root ratio at pH 5.4, but this effect was not evident at the lowe red or elevated (6.4) pH levels. The decrease in pH significantly increased the Fungal and yeast colony-forming units from the rhizosphere (root extra ct), but did not affect the total bacterial c.f.u.'s. Inoculatioin with str ain F113 in the pH 4.4 soil resulted in a significantly greater total bacte rial population. The fungal and yeast c.f.u.'s were not significantly affec ted by the inocula at any pH studied. Increasing the pH significantly incre ased the indigenous Pseudomonas population in comparison to the reduced pH treatment and significantly increased both the introduced and total Pseudom onas populations. The antibiotic producing strain significantly reduced the total bacterial population and the NAGase activity (related to fungal acti vity) at pH 6.4 where the inocula population was the greatest. Alkaline pho sphatase, phosphodiesterase, aril sulfatase, beta-glucosidase, alkaline bet a-galactosidase, and NAGase activities significantly increased with increas ing in pH. The F113 inocula reduced the acid phosphatase activity at FH 5.4 and increased the acid beta-galactosidase activity over all the pH treatme nts. The results presented illustrate the variation in impact with Soil pH, with implications for variability in efficacy of Pseudomonas fluorescens b iocontrol agents with soil pH.