Autotoxicity in Parthenium hysterophorus and its possible role in control of germination

The autotoxic effect of water soluble secondary plant metabolites, sesquiterpene lactones and phenolic acids present in the aggressive weed Parthenium hystrophorus were examined. It was found that the germinaion rates of achenes of P. hysterophorus increased with increasing distance between them (decreasing achene density) and increasing washing period preceding the germination trials. The inhibition of germination processes by water soluble autotoxins is temporary and its duration is determined by the concentration of autotoxins in the environment. Sesquiterpene lactones, parthenin and coronopilin, which are present in large amounts in P. hysterophorus, are autotoxic to seedlings and older plants at a concentration of 0.1%. It is suggested that in P. hysterophorus, the water soluble plant metabolites play an important role not only in allelopathy and defense against herbivorous predators and diseases but also as autotoxins in population regulation and the timing of the germination processes.