Allelochemicals isolated from tissues of the invasive weed garlic mustard (Alliaria petiolata)

Garlic mustard (Alliaria petiolata) is a naturalized Eurasian species that has invaded woodlands and degraded habitats in the eastern United States and Canada. Several phytotoxic hydrolysis products of glucosinolates, principally allyl isothiocyanate (AITC) and benzyl isothiocyanate (BzITC), were isolated from dichloromethane extracts of garlic mustard tissues. AITC and BzITC were much more phytotoxic to wheat (Triticum aestivum) than their respective parent glucosinolates sinigrin and glucotropaeolin. However, garden cress (Lepidium sativum) growth was inhibited to a greater degree by glucotropaeolin than BzITC, possibly due to conversion to BzITC by endogenous myrosinase. Sinigrin and glucotropaeolin were not detected in leaf/stem tissues harvested at the initiation of flowering, but were present in leaves and stems harvested in the autumn. Sinigrin levels in roots were similar for both sampling dates, but autumn-harvested roots contained glucotropaeolin at levels over three times higher than spring-harvested roots. The dominance of garlic mustard in forest ecosystems may be attributable in part to release of these phytotoxins, especially from root tissues.