Low-level expression of induced defences and increased compensation in Eupatorium adenophorum relative to its native congeneric plant to mechanical wounding and insect herbivory

Native herbivory may be detrimental to plant fitness and thus can contribute a biotic resistance to exotic plant invasion. Eupatorium adenophorum Sprengel, a non-native species, has formed dense monocultures in many southern Provinces of China. Because several native generalist insects feed on E. adenophorum, we evaluated whether the lower level of induced defences and greater compensatory growth contributed to successful invasion of E. adenophorum by comparison with a native congener E. heterophyllum. Early events in the jasmonic acid (JA) cascade and chlorophyll fluorescence, non-target metabolomic profiles, and recovery in biomass compensation of E. adenophorum and its native congener E. heterophyllum were measured during mechanical wounding and herbivory by the generalist insect Helicoverpa armigera. Mechanical and simulated chewing damage immediately initiated defences in the two species. However, E. heterophyllum was more sensitive to tissue loss, and it produced more metabolic defensive compounds than invasive E. adenophorum. Although H. armigera feeding amplified the effects of mechanical wounding on defence induction, the biomass of E. adenophorum was only weakly affected, as its higher compensatory growth could offset the biomass loss and metabolic cost. Overall, our results suggested that H. armigera herbivory and mechanical wounding could induce different physiological responses, and were often specific to invasive E. adenophorum and native E. heterophyllum. Moreover, lower metabolic defences and higher compensation growth may contribute to the invasion success of E. adenophorum.