As one of the most noxious plant invaders in China, Eupatorium adenophorum Sprengel not only severely threatens the biodiversity security but also leads to huge economic costs to the country. E. adenophorum has allelochemicals, which can inhibit seed germination of some species, such as pea, sensitive to allelochemicals. However, The effects of E. adenopherum′s allelochemicals on wild species that compete directly with it during the early stage of invasion are not very clear. Furthermore, the relationship between allelopathy and invasiveness is not known in E. adenophorum although it had been hypothesized that allelopathy plays an important role in biological invasion for other invasive species. In this study, the effects were determined of aqueous extract of leaves collected from 6 growth and developmental stages (from 60 d to 925d old plants) on seed germination and seedling growth of 4 herbaceous species. The main purpose of this study was to know whether allelopathy was development dependent in E. adenopherum, and the relationship between invasiveness and allelopathy.
Allelopathy of E. adenophorum was concentration dependent. At high concentration, aqueous leaf extract of E. adenophorum could significantly inhibit seed germination rate and speed of the four studied native species, whereas its effect was not significant occasionally at lower concentration or even increased seed germination. At both high and low concentration, E. adenophorum leaf extract could inhibit radicle and hypocotyl growth of the four natives except hypocotyl growth of Dipliptera roxburghiana, but occasionally its effect was not significant or even increased hypocotyl growth. The sensitivities to E. adenophorum′s allelochemicals were different among different native species and among different variables of the same species. Variables related to seed germination were more susceptible for Sida szechuensis and D. roxburghiana than for Isodon ternuifolia and Mariscus cyperinus, while growth-related variables more susceptible for M. cyperinus than for the others. Seed germination speed was more susceptible than germination rate; and radicle than hypocotyls. In general, with the increase of development stage, the inhibitory effects on natives of E. adenopherum leaf extract at the same concentration increased and the lowest inhibitory concentration decreased. The results indicated that allelopathy increased with development in E. adenopherum, and the role of allelopathy may be different at different stage of invasion.