Abstract: Aster subulatus is a common weed of wheat (Triticum aestivum) and Chinese cabbage (Brassica chinensis) in farmland. To explore its invasion mechanism, a Petri dish method was used to evaluate the potential allelopathic effect of aqueous extracts from the stem and leaf of A. subulatus on T. aestivum and B. chinensis. In general, the germination rates and germination indexes of T. aestivum and B. chinensis decreased with increasing concentrations of the A. subulatus aqueous extracts. T. aestivum was not sensitive to lower concentrations of the aqueous extracts but was more strongly inhibited than B. chinensis by higher concentrations (20, 40 g·L^-1) of the aqueous extracts. The fresh weight, shoot length, and root length of T. aestivum and B. chinensis were more strongly inhibited as the concentration of the aqueous extracts increased. The aqueous extracts at higher concentrations (20, 40 g·L^-1) had significant negative effects on the fresh weight and shoot length of B. chinensis, and significant negative effects on those of T. aestivum at lower concentrations (5, 10 g·L^-1). In all treatments, higher concentrations of the aqueous extracts had stronger negative effects than lower concentrations. The aqueous extracts at the lowest concentration (5 g·L^-1) negatively affected the root length of T. aestivum, but not that of B. chinensis. As the concentration of the aqueous extracts increased, the activities of protective enzymes (catalase, peroxidase, superoxide dismutase) gradually decreased, except that the catalase activity fluctuated in B. chinensis. During treatment with the aqueous extracts, the malondialdehyde content and membrane permeability gradually increased in B. chinensis, while the malondialdehyde content of T. aestivum decreased first and then increased, and its root activity and chlorophyll content showed downward trends. Overall, the aqueous extracts had stronger negative effects on the physiological indexes of B. chinensis than on those of T. aestivum. The results of this study showed that aqueous extracts of A. subulatus inhibited the germination and growth of two crop plants by reducing protective enzyme activities, disrupting the membrane system, and affecting respiration and photosynthesis. For all measured indexes, the standard error values were higher for B. chinensis than for T. aestivum, indicating that B. chinensis was more sensitive than T. aestivum to the aqueous extracts of A. subulatus.

Key words: Aster subulatus, allelopathic effect, Triticum aestivum, Brassica chinensis