No evidence for allelopathic effects of arbuscular mycorrhizal fungi on the non-host plant Stellaria media

Increasing evidence suggests that several plants, particularly non-mycorrhizal species, are negatively affected by the presence of arbuscular mycorrhizal fungi (AMF). Mechanisms explaining suppressive effects of AMF are, however, still poorly understood. Here we test whether growth suppression of the non-host weed Stellaria media in the presence of AMF can be explained by mycorrhizal alellopathy. We grew S. media in microcosms where an active AM mycelium was supported by neighboring wheat (Triticum aestivum) plants. To test for allelopathy, we added activated carbon (AC) to the soil substrate. In addition, we performed two complementary experiments where extracts from roots extensively colonized by AMF (AM exudates) were directly applied to S. media seeds and seedlings. Stellaria media plants grown in microcosms with AM mycelium showed an 8-fold biomass reduction compared to microcosms where AMF were absent. The addition of AC, which is thought to reduce allelopathic effects by binding organic compounds, did not greatly mitigate the negative effect of AM mycelium on S. media growth. Moreover, AM exudates did not significantly reduce S. media germination and growth. Results from this study confirm that non-hosts like S. media can be highly suppressed in the presence of AMF. However, we found no evidence that mycorrhizal allelopathy was a major mechanism responsible for growth suppression of S. media in the presence of AMF. Other mechanisms might therefore be more significant in explaining suppressive effects of AMF on non-host plant species.