MECHANISM BY WHICH NONINHIBITORY CONCENTRATIONS OF GLUCOSE INCREASE INHIBITORY ACTIVITY OF P-COUMARIC ACID ON MORNING-GLORY SEEDLING BIOMASS ACCUMULATION

Noninhibitory levels of glucose-C ]less-than-or-equal-to 72 mug carbon (C)/g soil[ increased the inhibitory activity of p-coumaric acid on morning-glory seedling biomass accumulation in Cecil B(t)-horizon soil. The amount of p-coumaric acid required for a given level of inhibition of shoot and seedling biomass accumulation decreased as the concentration of glucose increased. Soil extractions with neutral EDTA (0.25 M, pH 7) after addition of combinations of p-coumaric acid and glucose (concentrations ranging from 0 to 1.25 mumol/g soil) to the soil showed that utilization of p-coumaric acid by microbes decreased linearly as the concentration of glucose increased. The increased inhibitory activity of a given concentration of p-coumaric acid in the presence of glucose was not due to a reduction in soil sorption of p-coumaric acid or effects of nitrogen-limited microbial growth. Noninhibitory levels of phenyl-alanine and p-hydroxybenzoic acid slowed the utilization of p-coumaric acid by microbes in a similar manner as glucose. The presence of methionine, however, did not affect the rate of p-coumaric acid utilization by microbes. These observations suggest that differential utilization of individual molecules in organic mixtures by soil microbes can modify, and in this case increase, the effectiveness of a given concentration of an inhibitor such as p-coumaric acid on the inhibition of seedling growth such as morning-glory.