Arbuscular mycorrhizal fungi shift competitive relationships among crop and weed species

Aims: Arbuscular mycorrhizal (AM) symbioses affect plant competitive relationships within and among species and may be involved in the interactions among agricultural weed species and crops, depending on their mycorrhizal status. In this work, the impact of native AM fungi (AMF) on maize-weed(s) and weed-weed competitive relationships was assessed, using Solanum nigrum and Chenopodium album as model host and non-host weeds, respectively. Methods: Growth performance, nutrient use and competitive ability of crop and weed species were assessed in the pure stand and in different model plant communities of host and non-host species. Results: Results showed that maize performance decrease was more severe when grown with C. album than with S. nigrum. Differential responses to AMF occurred in the two weed species tested: mycorrhizal S. nigrum showed reduced biomass and N uptake when grown in competition with C. album. The negative performances observed when mycorrhizal S. nigrum grew in competition with C. album corresponded to C. album larger biomass production and N uptake. Conclusions: Results showed that AMF are able to alter the competitive relationships between co-occurring plant species differing in their mycorrhizal status (host/non-host), thus representing key soil organisms to be taken into account in sustainable weed management strategies

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

Background and aims: 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. Methods: 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 (AMexudates) were directly applied to S. media seeds and seedlings. Results: 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. Conclusions: Results from this study confirm that nonhosts 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