Beneficial effects of cadmium on plant defense of an invasive plant

Soil contamination by heavy metals is widespread as an inevitable consequence of every-increasing industrial and mining activities. In contaminated habitats, many sensitive plant species may extinct, and a few species, specifically the invasive plants with higher metal tolerance, can survive or colonize in such empty niche. The accumulation of heavy metals in plant tissues is one of the defense strategies against pathogens and herbivores. However, little is known regarding whether increased metal content is beneficial to the defense of invasive plants when competing with native plants. In this study, we explored the foliar chemistry of invasive alligator weed, Alternanthera philoxeroides, and its native congener sessile joyweed, A. sessilis, in response to cadmium (Cd) stress, and further examined how Cd amendment affected the performance and host selection of common cutworm Spodoptera litura (generalist), and alligator weed flea beetle Agasicles hygrophila (specialist). We found that the overall defense ability of invasive A. philoxeroides was higher than its native congener, but the defense mechanisms against specialist and generalist herbivores differed under Cd stress. Agasicles hygrophila was susceptible to high Cd stress, and the Cd amendment caused the beetle to shift hosts from A. philoxeroides to A. sessilis. For S. litura, organic rather than inorganic elemental defenses reduced its fitness on invasive plant, and the larvae prefer native plant against invasive plant in most scenarios. Native plant will be more vulnerable to herbivores when competing with invasive plant, especially in metal-polluted habitats. Our study provides a richer view of invasive mechanism in natural systems and appeals for paying more attention to soil contamination in assessing the potential ecological risk of plant invasion.