Identification and comparison of allelopathic effects from leaf and flower volatiles of the invasive plants Mikania micrantha

Volatilization, one of the most important mechanisms of the allelopathic effects of an exotic noxious weed Mikania micrantha, has not been adequately investigated to date. In this study, laboratory bioassays showed that the effects of volatiles from the leaves and flowers of M. micrantha on seed germination and seedling growth were negative for all four tested plants (Lactuca sativa, Chrysanthemum coronarium, Bidens pilosa, Abutilon theophrasti). Moreover, the inhibitory effect of the leaf volatiles was generally greater than that of the flower volatiles. To assess the reason for the above differences and further explore which compounds played the most crucial roles, the volatiles from the two tissues were absorbed by solid phase microextraction (SPME) and identified by gas chromatography and mass spectrometry (GC–MS). Then, 19 and 10 terpenes were determined respectively. α-Terpineol, β-ocimene, β-myrcene, α-pinene and caryophyllene had the maximum differences in content and concentration, which were selected for further bioassays with B. pilosa. The results indicated that morphological indices and SOD activity decreased with increasing concentrations of chemicals, whereas the contents of chlorophyll, soluble protein and MDA represented adverse changes. In addition, significant responses were observed in the treatments with α-terpineol at 1.0 μL·L−1 and lower concentrations, while similar trends were observed in the treatments with β-ocimene, β-myrcene, α-pinene and caryophyllene at 10 μL·L−1 and higher concentrations. It was concluded that terpenoids released through volatilization have an important role in the allelopathic effect of M. micrantha, and the oxygenated monoterpene α-terpineol played a crucial role in these effects.