Alliaria petiolata (Brassicaceae) is a biennial herb that has widely invaded Eastern North American forests. Alliaria produces glucosinolates in all its tissues, which when released into the soil through active root exudation or tissue decomposition, decrease soil fungal hyphal abundance and fungal spore germination rates. Recently, it has been shown that Alliaria negatively impacts physiology and alters resource allocation of a native perennial herb by disrupting plant-fungal mutualisms. We tested this mutualism disruption hypothesis on the annual herbs Impatiens capensis and I. pallida that are facultatively mycorrhizal. We inoculated seedlings of Impatiens spp. with either live field soil or gamma irradiated (sterilized) field soil. After two weeks, the pots containing live field soil were treated with a) leaf litter of Alliaria placed on the soil surface, b) leaf litter of Hesperis matronalis (a member of the Brassicaceae with no antifungal properties) placed on the soil surface c) application of a non-systemic fungicide soil drench or d) no additional manipulation. Comparisons among the five treatment groups were used to test whether native plants responded to the Alliaria treatment more similarly to the control treatments (i.e., sterile inoculum or fungicide drench), relative to intact mutualism treatments (Hesperis or live inoculum only).
Results/Conclusions
Compared to Impatiens plants in the live soil, sterile soil, and fungicide treatments, plants in the Alliaria and Hesperis treatments had higher mean biomass accumulation but reduced survival over time. Plants in the Alliaria and Hesperis treatments had the highest average seed production and the fungicide treated plants had the lowest average seed production. Unexpectedly, the sterile and live inoculum treatments produced intermediate and similar number of seeds. Our current results provide mixed support for mutualism disruption. We did not find evidence that Alliaria disrupts plant-fungal mutualisms, but seed set was reduced in fungicide control treatments. We also were unable to detect differences between the positive control (live soil) and negative control (sterile soil) in biomass accumulation, survival, or seedset. Future analyses of Impatiens leaf nutrient content, soil nutrient content, gas exchange, root fungal colonization rate, and soil hyphal abundance from this experiment will provide deeper insights to these results.