Chemical responses of an invasive plant to herbivory and abiotic environments reveal a novel invasion mechanism

Plasticity in resource allocation of the invasive Phytolacca americana: Balancing growth, reproduction, and defense along urban-rural gradients

In response to varying environments along urban and rural gradients, invasive plants may strategically allocate resources to enhance their invasiveness. However, how invasive plants balance their resources for growth, reproduction, and defense as responses to biotic and abiotic factors across these gradients remain unclear. We conducted field surveys on the growth, reproduction, and herbivory of the invasive species Phytolacca americana across diverse urban and rural habitats. Leaf samples were collected to analyze the nutritional content, primary and secondary metabolites. We found that plant growth rates, specific leaf area, leaf nitrogen content, and concentrations of flavonoids and saponins were higher in urban habitats, while reproduction, herbivory, and carbon-to‑nitrogen ratios were lower than those in rural habitats. We also found a trade-off between growth rate and herbivory, as well as trade-offs among defense traits associated with herbivory (e.g., leaf mass per area, the inverse of leaf nitrogen content, and carbon‑nitrogen ratio) and the production of metabolites associated with abiotic stress tolerance (e.g., soluble sugars, flavonoids, and saponins). As earlier studies showed low levels of genetic diversity within and between populations, our findings suggest that the urban-rural gradient patterns of resource allocation are primarily phenotypic plasticity in response to herbivory in rural areas and abiotic factors in urban areas. Our study sheds light on the mechanisms by which urbanization affects plant invasions and offers insights for the implementation of their management strategies.

Behavior of higher trophic levels associated with an invasive plant varies among populations

Invasive plants from their native and introduced ranges differ in their interactions with herbivores but it is not known whether they also vary in their interactions with herbivore natural enemies. Here, we used olfactometer bioassays and cage experiments to investigate how foraging behaviors of 2 parasitoid and 1 hyperparasitoid species depended on plant population origin. Triadica sebifera (Euphorbiaceae) is native to China but invasive in the United States. In China, it is fed on by a specialist noctuid Gadirtha fusca (Lepidoptera: Nolidae), which hosts a parasitoid Apanteles sp. (Hymenoptera: Microgastinae) and hyperparasitoid (Hymenoptera: Eurytomidae) plus a generalist aphid Toxoptera odinae (Homoptera: Aphidiidae) parasitized by Lysiphlebus confusus (Hymenoptera: Aphidiinae). Both parasitoids preferred plants infested by their host over herbivore-free plants in olfactometer bioassays. Apanteles sp. and Eurytomid wasps preferred G. fusca infested plants from China populations over those from US populations in olfactometer bioassays but L. confusus wasps did not discriminate between T. odinae infested plants from China vs. US populations. Similarly, G. fusca caterpillars on China population plants were more likely to be parasitized than ones on US population plants when they were in the same cage but odds of parasitism for T. odinae did not differ for those on China vs. US population plants. These results suggest that populations from the native and introduced ranges may differ in traits that impact higher trophic levels. This may have implications for successful control of invasive plants as biocontrol agents are introduced or herbivores begin to feed on them in their introduced ranges.

Effects of Warming, Phosphorous Deposition, and Both Treatments on the Growth and Physiology of Invasive Solidago canadensis and Native Artemisia argyi

Anthropogenic climate change and species invasion are two major threats to biodiversity, affecting the survival and distribution of many species around the world. Studying the responses of invasive species under climate change can help better understand the ecological and genetic mechanisms of their invasion. However, the effects of warming and phosphorus deposition on the phenotype of native and invasive plants are unknown. To address the problem, we applied warming (+2.03 °C), phosphorus deposition (4 g m^-2 yr^-1 NaH₂PO₄), and warming × phosphorus deposition to Solidago canadensis and Artemisia argyi to measure the direct effects of environmental changes on growth and physiology at the seedling stage. Our results reveal that the physiology parameters of A. argyi and S. canadensis did not change significantly with the external environment. Under phosphorus deposition, S. canadensis had higher plant height, root length, and total biomass compared to A. argyi. Interestingly, warming has an inhibitory effect on the growth of both A. argyi and S. canadensis, but overall, the reduction in total biomass for S. canadensis (78%) is significantly higher than A. argyi (52%). When the two plants are treated with warming combined with phosphorus deposition, the advantage gained by S. canadensis from phosphorus deposition is offset by the negative effects of warming. Therefore, under elevated phosphorus, warming has a negative effect on the invasive S. canadensis and reduces its growth advantage.

Increasing flavonoid concentrations in root exudates enhance associations between arbuscular mycorrhizal fungi and an invasive plant

Many invasive plants have enhanced mutualistic arbuscular mycorrhizal (AM) fungal associations, however, mechanisms underlying differences in AM fungal associations between introduced and native populations of invasive plants have not been explored. Here we test the hypothesis that variation in root exudate chemicals in invasive populations affects AM fungal colonization and then impacts plant performance. We examined flavonoids (quercetin and quercitrin) in root exudates of native and introduced populations of the invasive plant Triadica sebifera and tested their effects on AM fungi and plant performance. We found that plants from introduced populations had higher concentrations of quercetin in root exudates, greater AM fungal colonization and higher biomass. Applying root exudates more strongly increased AM fungal colonization of target plants and AM fungal spore germination when exudate donors were from introduced populations. The role of root exudate chemicals was further confirmed by decreased AM fungal colonization when activated charcoal was added into soil. Moreover, addition of quercetin into soil increased AM fungal colonization, indicating quercetin might be a key chemical signal stimulating AM fungal associations. Together these results suggest genetic differences in root exudate flavonoids play an important role in enhancing AM fungal associations and invasive plants' performance, thus considering root exudate chemicals is critical to unveiling mechanisms governing shifting plant-soil microbe interactions during plant invasions.