While mutualisms are common, the effectiveness of these interactions is strongly influenced by environmental conditions. The interaction between plants and arbuscular mycorrhizal fungi (AMF) is typically mutualistic - plants provide AMF with carbon while AMF supply plants with soil nutrients/water. Native forest understory herbs commonly associate with AMF and depend heavily upon AMF-derived resources for physiological function. However, invasion of garlic mustard (Alliaria petiolata), a species with allelopathic effects on AMF, threatens this mutualism. If garlic mustard allelochemicals kill AMF nutrient-providing hyphae, native plant physiology could be impacted. Our previous work demonstrated that short-term exposure to garlic mustard reduced photosynthesis in a native mycorrhizal forest herb, Maianthemum racemosum (MARA). To determine if these effects extend throughout the growing season and impact long-term carbohydrate storage, we conducted a greenhouse experiment where MARA plants were treated with either: fungicide (mimicking the effects of garlic mustard), or leaves from garlic mustard or Hesperis matronalis, an allelochemical-free relative of garlic mustard. Each treatment was crossed with a drought treatment to assess the impact of additional stress on the MARA-AMF interaction. Plant physiology was measured weekly from June-September, and plants were harvested periodically to assess rhizome carbohydrate storage.
Results/Conclusions
We found a significant decline in photosynthetic rate across the first 4 weeks of the experiment in the garlic mustard drought treatment compared to the H. matronalis drought treatment (mixed effects model, p < 0.05 for 4 weeks). During 3 of the 4 weeks, photosynthetic rates in the garlic mustard drought treatment did not differ from the fungicide drought treatment. For well-watered plants, we found that fungicide treated plants had consistently lower photosynthetic rates than H. matronalis treated plants, and this effect was significant for 5 weeks out of the growing season (p < 0.05 for 5 weeks). While determining if observed reductions in photosynthesis translated into decreased carbohydrate storage, we discovered MARA does not store starch like other Maianthemum species, but stores carbohydrates as fructans. The only soluble sugar in the rhizome is sucrose, the pre-cursor to AMF-destined sugars. Future analyses will determine if our treatments affected the storage of these unique carbohydrates. Our results to date suggest that under drought conditions, garlic mustard is as effective as a fungicide and depresses MARA physiology. This effect is pronounced early in the growing season, when AMF-derived resources may be critical. We propose that the disruption of native plant mutualisms may be a widespread mechanism behind ecosystem invasion.