Carbon fractions in the rhizosphere of pea inoculated with 2,4 diacetylphloroglucinol producing and non-producing Pseudomonas fluorescens F113

The aim of this work was to determine the effect of wild type and functionally modified Pseudomonas fluorescens strains on C fractions in the rhizosphere of pea. The lacZY marked F113 strain produces the antibiotic 2,4 diacetylphloroglucinol (DAPG) useful in plant disease control. The modified strain of F113 was repressed in production of DAPG, creating the DAPG negative strain F113 G22. The F113 treatment resulted in a significantly lower shoot/root ratio. The F113 G22 treatment had a significantly greater indigenous and total fluorescent Pseudomonas population than the control and F113 (DAPG+) treatment. Both strains significantly increased the water soluble carbohydrates and the total water soluble carbon in the pea rhizosphere soil. Strain F113 significantly increased the soil protein content relative to the control but not in relation to the F113 G22 treatment. The F113 treatment had a significantly greater organic acid content than the control and F113 G22 treatments, whilst the F113 G22 treatment was also significantly greater than the control. Both inocula resulted in significantly lower phosphate contents than the control. The F113 inocula significantly increased alkaline phosphatase, sulphatase and urease activities, and reduced  glucosidase activities indicating increased carbon availability. Both inocula increased C availability, however, antibiotic production by strain F113 reduced the utilisation of organic acids released from the plant resulting in differing effects of the two strains on nutrient availability, plant growth, soil enzyme activities and Pseudomonas populations.