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 m orning-glory seedling biomass accumulation in Cecil B(t)-horizon soil. The amount of p-coumaric acid required for a given level of inhibitio n of shoot and seedling biomass accumulation decreased as the concentr ation 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 showe d that utilization of p-coumaric acid by microbes decreased linearly a s the concentration of glucose increased. The increased inhibitory act ivity of a given concentration of p-coumaric acid in the presence of g lucose 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 presen ce 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 microb es can modify, and in this case increase, the effectiveness of a given concentration of an inhibitor such as p-coumaric acid on the inhibiti on of seedling growth such as morning-glory.