The fate of allelochemicals after their release into the environment and the relationship between the presence of allelochemicals in soil and their phytotoxic activity are not fully understood. This is also true of dehydromatricaria ester (DME), an allelochemical compound produced by Solidago altissima L. The effects of DME on the growth of rice seedlings in agar and soil cultures were compared. The DME concentration which led to 50% growth inhibition was ten or 20 times greater in soil (100 mug g-1 soil) than in agar culture (5 mug ml-1). Most of the DME was adsorbed to soil solids and little DME was present in soil water. Time-course studies of DME concentration in the soil showed that DME in soil water and adsorbed to soil solids decreased and almost disappeared within about 10 d. Microbial activity must have been involved in DME degradation because the breakdown of DME was slowed by autoclaving the soil. It is concluded that plant growth inhibition in soil depends on the DME concentration of soil water. Lower inhibition of rice seedling growth in soil than in agar at the same initial concentrations of DME is due to the low concentration of DME present in soil water because most DME in the soil is either adsorbed to soil solids or degraded by microbes. Although DME was found in soils with S. altissima (maximum concentration 4.5 mug g-1), it was not detected in the soil water in natural fields. Therefore, DME released by S. altissima has little allelopathic activity in natural stands.
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