Roots of the Invasive Species Carduus nutans L. and C. acanthoides L. Produce the Phytotoxin Aplotaxene, a Possible Allelochemical

Carduus nutans L. (musk thistle or nodding thistle), a member of the Asteraceae, is a Eurasian native plant, but is an invasive species in North America, Australia, and New Zealand where it is more successful in many habitats than in its native area; part of its success may be due to allelopathy. Several secondary compounds have been identified in the genus Carduus. Among them are sterols such as taraxasterol and sitosterol, and flavonoids such as kaempferol, apigenin, and rutin. The objective of this study was to identify the phytotoxic compounds present in Carduus acanthoides and C. nutans roots and aerial parts by systematically performing bioassay-directed isolation and subsequent identification of the bioactive constituents. No significant phytotoxic activity against Lactuca sativa or Agrostis stolonifera was detected in methanol, or water extracts when tested at 1.0 mg/mL; however, the dichlormethane root extract of C. acanthoides was active. Further fractionation using a hexane:diethyl ether step gradient was performed with the DCM extract. The active compound was isolated and identified by GC-MS and ¹H- and ¹³C-NMR. The isolated compound was identified as the moderately phytotoxic, highly lipophilic compound aplotaxene and was found in roots of both species C. canthoides and C. nutans.