Interactive effects of garlic mustard metabolites on jewelweed plants and their mycorrhizae

Garlic mustard (Alliaria petiolata) is a biennial herb invasive in North America that produces a suite of noxious compounds, including glucosinolates and several flavonoid glycosides. These compounds may act allelopathically by inhibiting normal root development and/or symbioses formation in neighboring plants. We explored the effects of a glucosinolate enriched fraction, a flavonoid glycoside enriched fraction, and a whole plant extract during four developmental stages (germination, presymbiosis growth, symbiosis formation, and postsymbiosis growth) of symbioses in pale jewelweed (Impatiens pallida). Plants were grown in glass chambers to allow non-destructive viewing of roots and mycorrhizal hyphae, which were visualized using autofluorescence microscopy after excitation with blue light. Exposure to garlic mustard extracts containing glucosinolates for five months lowered jewelweed germination by almost 50% compared to controls. Uncolonized jewelweed root systems grown in soil amended by activated carbon were longer, larger, and better at exploring the soil than those grown without activated carbon, regardless of the extract they were exposed to. Uncolonized root systems exposed to a whole plant extract grew faster than other treatments. After four weeks of exposure during the symbiosis formation phase of growth, mycorrhizal colonization was lowest in roots exposed to either the whole plant extract or the glucosinolate enriched fraction. In contrast, a whole plant extract stimulated mycorrhizal hyphae production in the soil more than either a glucosinolate or flavonoid glycoside enriched fraction. The secondary metabolites produced by garlic mustard interact in complex ways, even stimulating some phases of growth, to affect surrounding plants and mycorrhizae.