Influence of Phenolic acids on microbial populations in the rhizosphere of cucumber

Experiments were conducted to determine whether changes in soil microbial populations that occur in response to additions of certain allelopathic phenolic acids to bulk soil also occur in the rhizosphere. Cucumber seedlings were transplanted into cups containing a nutrient-enriched mixture of Portsmouth B₁, soil and sand and were watered five times (once every 48 hr) with aqueous solutions of ferulic,p-coumaric, or vanillic acid (each at 0, 0.25, or 0.50ol/g soil material). Nutrient solution was applied on alternate days. Leaf growth was suppressed by up to 42% by phenolic acids, but changes in root growth varied with the compound and concentration in solution. Significant increases (over 600% relative to controls) in populations of fast-growing bacteria in the rhizosphere were detected after two but not after five treatments, and increases (400% relative to controls) in numbers of fungal propagules were detected after five treatments. Such increases suggested that chronic exposure to a phenolic acid might resuit in high populations of rhizosphere microorganisms that could metabolize the compounds and thus alter observable responses by the plant. To test this, plants were watered repeatedly with a low-concentration solution of ferulic acid (chronic treatments; 0.0 or 0.1mol/g soil material in one experiment, 0.000 or 0.025imol/g soil material in a second) and then once with a highconcentration solution (acute treatment; 0.0, 0.5, or 1.0mol/g soil material in the first experiment; 0.000, 0.125, or 0.250mol/g soil material in the second).Paper No. 12385 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, North Carolina. The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service or the United States Department of Agriculture of the products named, nor criticism of similar ones not mentioned. Acute treatments and some chronic treatments suppressed leaf growth, but results were inconsistent for root growth. Acute treatments increased numbers of several types of bacteria in the rhizosphere but had inconsistent effects on fungi. Chronic treatments had no effect on numbers of bacteria or fungal propagules in the rhizosphere. Furthermore, chronic treatments did not alter responses of plants or microbial populations to the subsequent acute treatment. Results demonstrated that phenolic acids in soil, which must pass through the rhizosphere before interaction with plant roots can occur, alter the microbial ecology of the rhizosphere. However, microbially mediated acclimation of plants to relatively high concentrations of ferulic acid was not observed.