Why some invasive plant species transmogrify from weak competitors at home to strong competitors abroad remains one of the most elusive questions in ecology. Some evidence suggests that disproportionately high densities of some invaders are due to the release of biochemicals that are novel, and therefore harmful, to naive organisms in their new range. So far, such evidence has been restricted to the direct phytotoxic effects of plants on other plants. Here we found that one of North America's most aggressive invaders of undisturbed forest understories, Alliaria petiolata (garlic mustard) and a plant that inhibits mycorrhizal fungal mutualists of North American native plants, has far stronger inhibitory effects on mycorrhizas in invaded North American soils than on mycorrhizas in European soils where A. petiolata is native. This antifungal effect appears to be due to specific flavonoid fractions in A. petiolata extracts. Furthermore, we found that suppression of North American mycorrhizal fungi by A. petiolata corresponds with severe inhibition of North American plant species that rely on these fungi, whereas congeneric European plants are weakly affected. These results indicate that phytochemicals, benign to resistant mycorrhizal symbionts in the home range, may be lethal to naive native mutualists in the introduced range and indirectly suppress the plants that rely on them.

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Novel weapons: invasive plant suppresses fungal mutualists in america but not in its native europe

Garlic mustard (Alliaria petiolata, Brassicaceae) is an invasive, nonindigenous species currently invading the understory of North American woodlands where it is a serious threat to the native flora. Part of this success might be due to allelopathic interference by garlic mustard. Two congeneric species, the European Geum urbanum and the North American Geum laciniatum, were tested for allelopathic inhibition of germination by garlic mustard. Seeds were germinated either on substrate contaminated by garlic mustard or on substrate with contamination neutralized by activated carbon. Allelopathic effects of native European and invasive North American garlic mustard populations were also compared. Activated carbon increased germination by 14%, indicating that garlic mustard contaminated the substrate through root exudates. Activated carbon in turn counteracted this effect. The two test species differed in their sensitivity to allelopathic interference. North American G. laciniatum had a much stronger increase in germination when activated carbon was added to the substrate, independent of the origin of garlic mustard. In contrast, the European G. urbanum germinated better in substrate precultivated with North American garlic mustard, whereas activated carbon increased its germination only in substrate precultivated with European garlic mustard.

Allelopathic inhibition of germination by Alliaria petiolata (Brassicaceae).

We examined the effects of garlic mustard (Alliaria petiolata) leachates on germination of arbuscular mycorrhizal (AM) fungal spores, colonization of plant roots by AM fungi and germination and root growth of monocot and dicot plants under laboratory conditions. In the field we examined the effect of garlic mustard on AM inoculum potential (MIP). Water leachates of garlic mustard prevented germination of spores of the AM fungus Gigaspora rosea, inhibited the formation of AM associations with tomato (Lycopesicum esculentum) and significantly reduced germination of tomato seeds. Garlic mustard leachates also reduced root length of tomato and sorghum (Sorghum bicolor) seedlings. Under field conditions we found a significant negative correlation between the density of garlic mustard and the mycorrhizal inoculum potential of the soil in which the plants grew. These results suggest that garlic mustard may reduce the competitive abilities of native plants by interfering with the formation of mycorrhizal associations and root growth.

Effect of Garlic Mustard [Alliaria petiolata (Beib. Cavara & Grande)] Extracts on Plants and Arbuscular Mycorrhizal (AM) Fungi

Alliaria petiolata (Bieb.) Cavara & Grande (garlic mustard, Brassicaceae) is a Eurasian herb that is currently invading forested areas throughout portions of the northern United States and southern Canada. Alliaria petiolata enters plant communities in an aggressive fashion, often displacing resident understory species. One possible mechanism that may enable garlic mustard to successfully invade these areas is hypothesized superiority as a competitor. In order to determine the competitive potential of A. petiolata, a multiple deWit replacement series was conducted in the greenhouse between A. petiolata and three native species: Impatiens capensis, an herbaceous annual, and Acer negundo and Quercus prinus, two woody perennials. Each target species was grown in mixture with A. petiolata, and all species were grown in monoculture. After 21 wk, plants were harvested and dried. Aboveground dry‐weight biomass (yield) was then used to calculate the relative yield per plant, relative yield total, and mean aggress...

https://www.jstor.org/stable/10.1086/314156

Competitive Ability of Alliaria petiolata (Garlic Mustard, Brassicaceae), an Invasive, Nonindigenous Forest Herb

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.

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

Alliaria petiolata (M. Bieb.) Cavara & Grande (garlic mustard) is a non-indigenous member of the Brassicaceae that is invading woodlands throughout eastern North America. Previous work has demonstrated that this species is having a negative effect on the diversity of understory communities and is actively displacing native species. The purpose of this study was to evaluate the extent to which allelopathy might be acting as the mechanism of interference. Extracts of garlic mustard root and shoot tissues were applied to seeds and seedlings of four target species used as bioassays: radish, winter rye, hairy vetch, and lettuce. While seed germination rates varied by species and extract concentration, total germination after 5-7 d was largely unaffected by any extract concentration. Only radish seeds treated with the most concentrated root solution exhibited a depressed germination relative to the water control. Likewise, seedling biomass was generally unaffected by any extract treatment. Only shoot biomass for rye was significantly depressed with the highest concentration of leaf extract. Our data provide little evidence that allelopathy is involved in the invasive success or community interference of this non-indigenous species, even though the Brassicaceae are well known to possess potentially biologically active compounds with allelopathic effects.

Sheryl L. Hanson

Papers

An Assessment of the Allelopathic Potential of the Invasive Weed Alliaria petiolata (Brassicaceae)