Residue allelopathy in Parthenium hysterophorus L. -: Does parthenin play a leading role?

Apart from its competitive ability, the invasiveness of the noxious weed Parthenium hysterophorits L. is thought to be due to an ability to displace other species by means of alielopatby. The sesquiterpene lactone parthenin that is biosynthesized by this species is thought to play a role in its allelopathic interference with surrounding plants. However, despite the fact that parthenin is released from various plant parts into the soil, little is known about its relative contribution to overall allelopathic effects. Because leaf residues are believed to deliver large amounts of parthenin to soils during decomposition, we investigated the level of involvement of parthenin in overall phytotoxicity of decomposing leaf material in a South African population of P. hysterophorus. The natural release of inhibitors during decomposition was simulated by aqueous extraction of fresh leaf material (FM) and the responses of test plant species (Ayeratum conyzoides L., Echinochloa crus-galli (L.) P. Beauv., Eragrostis curvula (Schrad) Nees, Eragrostis tef (Zucc.) Trotter, Lactuca sativa L.) to extracts were evaluated under laboratory conditions in dose-response bioassays. Species differed considerably in their sensitivity to leaf extracts, whereby A. conyzoides was most sensitive with ED50-values for root length of 24.8 mg FM/ml and 53.6 mg FM/ml for inhibition of germination. The quantities of parthenin within leaf extracts were determined using HPLC, and the phytotoxicity of quantified extract concentrations was assessed in pure compound dose-response bioassays. A. conyzoides was again most sensitive, with ED50-values for inhibition of root length and germination by parthenin of 51.8 and 289.9 mu g/ml, respectively. Furthermore, parthenin treatments proved to significantly delay germination and stimulate root growth at low doses. The contribution of parthenin to observed effects of leaf extracts was finally estimated by model comparisons of dose-response relations of parthenin in leaf extracts or as a pure compound. Results showed that the contribution of parthenin was highly dependent on its concentration within extract solutions and varied between 16% and 100% of overall phytotoxicity of leaf extracts. The inhibition could be completely reproduced by pure parthenin treatments in quantified amounts, when extract solutions with high levels of parthenin were tested on the most sensitive species, A. collyzoides. This suggested that the release of parthenin during decomposition of leaf material has a potential to play a leading role for allelopathy in P. hysterophorus; however, its significance in a natural setting will very much rely on the amount of leaf material accumulated on soil surfaces and the concentration of parthenin in residues. (c) 2006 Elsevier Ltd. All rights reserved.