Residue allelopathy in Parthenium hysterophorus L.â€"Does parthenin play
a leading role?

Regina G. Belza, Corresponding Author Contact Information, E-mail The
Corresponding Author, Carl F. Reinhardtb, Llewellyn C. Foxcroftc and
Karl Hurlea
aUniversity of Hohenheim (360), Department of Weed Science, 70593
Stuttgart, Germany
bUniversity of Pretoria, Department of Plant Production and Soil
Science, Pretoria 0002, South Africa
cScientific Services, Kruger National Park, P/Bag X402, Skukuza 1350,
South Africa
Received 9 January 2005; accepted 16 June 2005. Available online 12
October 2006.

Abstract

Apart from its competitive ability, the invasiveness of the noxious
weed Parthenium hysterophorus L. is thought to be due to an ability to
displace other species by means of allelopathy. 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
(Ageratum 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 doseâ€"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
doseâ€"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 μg/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
doseâ€"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. conyzoides.
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.

Keywords: Allelopathy; Aqueous leaf extract; Doseâ€"response modeling;
Hormesis; Parthenin; Petri dish assay


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