Abstract: Arbuscular mycorrhizal fungi (AMF) can facilitate plant invasions, particularly in nutrient-depleted soils. Establishment of this mutualism often depends on both soil nutrients and secondary chemicals released by plants. Yet, it remains unclear how invasive plants maintain or enhance AMF colonization in nutrient-deficient soil. We investigated the effects of different levels of nitrogen (N: 0, 2 or 10 ppm) and phosphorus (P: 0, 6 or 25 ppm) added to sand in a factorial design on three flavonoids (isoquercitrin, quercitrin, quercetin) associated with AMF colonization in root exudates of Triadica sebifera from introduced and native populations. We assessed effects of soil nutrients alone on secondary chemical metabolism with sterilized growth substrate treatments. We found that plants from introduced populations had higher root exudate concentrations of quercitrin and quercetin plus greater AMF colonization than those from native populations under every N and P treatment combination. The effects of P availability on flavonoids depended on N availability. Flavonoid concentrations and AMF colonization decreased with P addition under no N treatment but increased with P addition under low N treatment. P addition had smaller effects on AMF colonization under the high N treatment. In sterilized growth substrate, P addition had stronger effects on quercitrin than quercetin. These results suggest that soil nutrients may influence AMF colonization by modifying flavonoids in root exudates. Our findings provide evidence that the responses of flavonoid metabolism to soil nutrients are an important driver of AMF colonization and the ability of invasive plants to sustain performance under nutrient stress.

Key words: Triadica sebifera, quercetin, quercitrin, soil nutrient starvation, arbuscular mycorrhizal fungi, combination effect