Bioassay-directed isolation and identification of phytotoxic and fungitoxic acetylenes from Conyza canadensis.
Conyza canadensis (L.) Cronquist syn. (horseweed) is a problematic and invasive weed with reported allelopathic properties. To identify the phytotoxic constituents of the aerial parts, a systematic bioactivity-guided fractionation of the dichloromethane extract was performed. Three active enyne derivatives, (2Z,8Z)-matricaria acid methyl ester, (4Z,8Z)-matricaria lactone, and (4Z)-lachnophyllum lactone, were identified. The lactones inhibited growth of the monocot Agrostis stolonifera (bentgrass) and the dicot Lactuca sativa (lettuce) at 1 mg mL-1, while the (2Z,8Z)-matricaria acid methyl ester was less active. In a dose-response screening of the lactones for growth inhibitory activity against Lemna paucicostata, (4Z)-lachnophyllum lactone was the most active with an IC50 of 104 µM, while the (4Z,8Z)-matricaria lactone was less active (IC50 of 220 µM). In a fungal direct bioautography assay, the two lactones at 10 and 100 µg/spot inhibited growth of the plant pathogenic fungi Colletotrichum acutatum, Colletotrichum fragariae, and Colletotrichum gloeosporioides. In a dose-response screening of the lactones against six different plant pathogenic fungi, (4Z,8Z)-matricaria lactone was more active than the commercial fungicide azoxystrobin on Col. acutatum, Col. fragariae, and Col. gloeosporioides at 30 µM and about as active as the commercial fungicide captan against Col. gloeosporioides, while (4Z)-lachnophyllum lactone was less active.