The novel weapons hypothesis posits that invasive plant species gain an advantage in invaded habitats by possessing novel biochemical traits toward which naive native species from many taxonomic groups lack resistance or tolerance.  The invasiveness of the herbaceous biennial garlic mustard, Alliaria petiolata, in North America has been associated with the possession of novel chemical weapons, but the chemistry of the native community has not been examined sufficiently to know whether the putatively bioactive molecules of garlic mustard are indeed novel in the native community.  We compared the HPLC profile of flavonoids and glucosinolates, activities of trypsin inhibitor proteins, and concentrations of cyanide in leaves of garlic mustard with that in leaves of four North American biennial or perennial mustard species that occupy forest understories where garlic mustard invades.  Leaves were sampled from flowering plants in naturally growing populations from three natural areas.

Results/Conclusions

While profiles varied among North American Arabis and Cardamine species, no native mustard examined shared the same profile of glucosinolates or flavonoids in their leaves with garlic mustard.  Garlic mustard contained several metabolites that were not found in the native mustards.  Trypsin inhibitor activities were lower in undamaged leaves of garlic mustard than in the native species, which differed from each other, but these proteins are known to be highly inducible in garlic mustard.  Significant concentrations of cyanide were only found in garlic mustard.  Among the species examined here, the profile of putatively bioactive compounds and proteins of garlic mustard was distinct, supporting assertions of the novelty of the chemical weapons of garlic mustard in North America.