Soil degradation of parthenin-does it contradict the role of allelopathy in the invasive weed Parthenium hysterophorus L.?

The invasive success of Parthenium hysterophorus L. is thought to be partially attributable to allelopathy mediated by the plant metabolite parthenin. To assess the ecological significance of parthenin release from plant material, its persistence and phytotoxicity in soil was studied. Results show parthenin is rapidly degraded with an average DT (50) of 59 h under standard experimental conditions. Degradation was delayed in sterilized soils, at lower soil moisture, and higher parthenin concentrations. Higher temperatures, higher CEC(pot)/clay content of soils, soil preconditioning with parthenin, and P. hysterophorus infestation accelerated degradation. Physico-chemical and biological processes are, therefore, expected to govern the fate of parthenin in soil. Parthenin exhibited low soil phytotoxicity and did not accumulate over time. Along with the indicated reduction in bioavailability and development of hormetic effects, results suggest that for parthenin to have detrimental allelopathic effects, it requires high P. hysterophorus densities that result in high soil levels of parthenin and soil conditions that favor the persistence of parthenin. In light of this, the ecological significance of parthenin is discussed.