Abstract: Xanthium italicum is an invasive weed with a widespread global distribution causing serious damage to local agricultural production and the ecological environment in the invaded areas. Although multiple mechanisms have been reported to contribute to the invasive success of X. italicum, the extent of the intraspecific differentiation and phenotypic plasticity of this species in invaded habitats requires further evaluation. In this study, we established three different nitrogen application treatments in a common garden, namely, no nitrogen addition (NN), low nitrogen (LN: 2 g urea per pot), and high nitrogen (HN: 4 g urea per pot). Each treatment contained ten populations of 100 individuals (ten individuals per population) of invasive X. italicum and native Xanthium sibiricum under different nitrogen treatments. Due to seed limitations, native X. sibiricum were not subjected to the NN treatment. Under the NN treatment, we detected significant phenotypic differences among different invasive X. italicum populations with respect to six growth traits (root length, shoot length, crown breadth, base diameter, relative chlorophyll content, and biomass). Furthermore, when subjected to the LN and HN treatments, invasive X. italicum was characterized by significantly higher phenotypic plasticity compared with that of native X. sibiricum in terms of biomass and base diameter. Our findings suggest that phenotypic plasticity and intraspecific differentiation may play important roles in facilitating the invasive success of X. italicum in China, thereby increasing the risk of further biological invasion.

Key words: biological invasion, common garden, nitrogen addition, population differentiation, plasticity response, invasive plant