<i>Cuscuta</i> parasitism and simulated herbivory reduce allelopathic effects of the invasive plant <i>Solidago canadensis</i>

Abstract The dominance of alien invasive plants may be restricted by enemies, such as insect herbivores and parasitic plants. A fundamental assumption is that plants exposed to such antagonists produce less biomass and have weakened competitive ability. Here we tested experimentally whether the growth, metabolome and allelopathic effects of the invasive plant Solidago canadensis are affected by simulated herbivory (using jasmonic acid, JA) and Cuscuta australis parasitism. We found that biomass production of Solidago was decreased by JA application and by Cuscuta parasitism, but less so when both were applied simultaneously. In line with the latter, metabolome analysis indicated that growth-related pathways were upregulated under simultaneous JA application and Cuscuta parasitism. Solidago had negative allelopathic effects on germination of one native species and seedling root growth of two other native species, and these effects tended to become weaker when Solidago was either subjected to JA or Cuscuta parasitism, but not when both were applied simultaneously. Metabolome analysis indicated that Solidago down-regulated its accumulation of secondary metabolites when there was only a single stress (i.e. simulated herbivory or Cuscuta parasitism). Correlation analysis between allelopathic effects and differently accumulated metabolites indicated that coumarins (Esculin, Daphnin and Esculetin-7-O-glucoside) and phenolic acid (2-O-p-Cumaroylglycerol) may play an important role in Solidago allelopathy. Overall, our results suggest that Solidago regulates its metabolic pathways related to growth and defence chemistry differently in response to single and dual stresses.