Modification of allelopathic effects ofp-coumaric acid on morning-glory seedling biomass by glucose, methionine, and nitrate.

Studies of allelopathy have emphasized primarily the identification and quantification of phytotoxins in soils, with only limited attention directed toward how organic (carbon) and inorganic constituents (nutrients) in the soil may modify the action of such phytotoxins. In the present study, increasing carbon (C) levels (up to 108μg C/g soil) supplied as glucose, phenylalanine, orp-hydroxybenzoic acid did not alter morning-glory biomass, but similar C levels supplied as leucine, methionine, orp-coumaric acid were inversely related to morning-glory biomass. Similar joint action and multiplicative analyses were used to describe morning-glory biomass response to various C sources and to generate dose isolines for combinations ofp-coumaric acid and methionine at two NO3-N levels and for combinations ofp-coumaric acid and glucose at one NO3-N level. Methionine, glucose, and NO3-N treatments influenced the inhibitory action ofp-coumaric acid on biomass production of morning-glory seedlings. For example, results from the multiplicative analysis indicated that a 10% inhibition of morning-glory biomass required 7.5μgp-coumaric acid/g soil, while the presence of 3.68μg methionine/g soil thep-coumaric acid concentration required for 10% inhibition was only 3.75μg/ g soil. Similar response trends were obtained forp-coumaric acid and glucose. The higher NO3-N (14 vs. 3.5μg/g) treatments lowered the methionine and increased thep-coumaric acid concentrations required for 10% inhibition of morning-glory biomass. These results suggested that allelopathic interactions in soil environments can be a function of interacting neutral substances (e.g., glucose), promoters (e.g., NO3-N), and/or inhibitors (e.g., methionine andp-coumaric acid) of plant growth.