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 0.50μol/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 0.1μmol/g soil material in one experiment, 0.000 or 0.025μimol/g soil material in a second) and then once with a highconcentration solution (acute treatment; 0.0, 0.5, or 1.0μmol/g soil material in the first experiment; 0.000, 0.125, or 0.250μmol/g soil material in the second).