Stimulation of witchweed germination by sesquiterpene lactones: a structure-activity study

Abstract Twenty-four natural and synthetic sesquiterpene lactones were tested as witchweed germination stimulants to determine the essential structural requirements for the stimulation of witchweed germination by dihydroparthenolide. All of the 10 germacranolides tested induced 40–65% germination of witchweed across a broad concentration range, from 10 −5 to 10 −9 M. The highest activity was observed with a mixture of the two eudesmanolides, santamarin and reynosin. These findings indicate that it is the spatial arrangement of the germacrane-eudesmane skeleton in conjunction with the five-membered lactone ring which is important in the promotion of witchweed germination, rather than the presence of any specific functional group on the molecule. The structure of these compounds is similar to portions of the active molecule (+)-strigol.