Shanshan Zhong

One of the main factors in the successful invasion of invasive plants is their allelopathy on the growth performance (especially seed germination and seedling growth) of neighboring plants. Salt stress, mainly mediated by soil salinization, may affect or even facilitate the process of invasion of invasive plants via their allelopathy. The aim of this study was to evaluate the allelopathy effect of four Asteraceae invasive plants, including Canada goldenrod (Solidago canadensis L.), horseweed (Conyza canadensis (L.) Cronq.), rallway beggaricks (Bidens pilosa L.), and daisy fleabane (Erigeron annuus (L.) Pers.), on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. under a gradient of salt stress in an hydroponic incubation experiment. Salt stress significantly reduced seed germination and seedling growth of L. sativa. These four invasive plants are known to negatively affect seed germination and seedling growth of L. sativa through their allelopathy. The allelopathy of S. canadensis was stronger than that of the three other invasive plants. Salt stress significantly intensified the allelopathy of the four studied invasive plants (especially S. canadensis), and the facilitation of salt stress on the allelopathy of the four invasive plants (especially S. canadensis) significantly increased with increasing intensity of salt stress. Therefore, the increased level of salt stress may facilitate the process of invasion of the four studied invasive plants (especially S. canadensis) via their increased allelopathy negatively affecting the seed germination and seedling growth of neighboring plant species.

Most of the allelopathic studies have focused on the independent allelopathy of one invasive plant, but have ignored the co-allelopathy of the two invasives. The variations in the type of acid rain can modulate the invasiveness of invasives via the changes in the allelopathy. Thus, it is vital to elucidate the allelopathy of invasives, particularly the co-allelopathy of the two invasives, under acid rain with different types, to illuminate the mechanisms driving the co-invasion of two invasives under diversified acid rain. However, little progress has been finished in this aspect presently. This study aimed to evaluate the co-allelopathy of two Asteraceae invasives Solidago canadensis L. and Erigeron annuus L. treated with acid rain with different nitrogen-to-sulfur ratios on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. via a hydroponic experiment. Aqueous extracts of the two Asteraceae invasives generated obvious allelopathy on L. sativa. S. canadensis aqueous extracts caused stronger allelopathy. There may be an antagonistic effect for the co-allelopathy of the two Asteraceae invasives. Nitric acid at pH 5.6 weakened the allelopathy of the two Asteraceae invasives, but the other types of acid rain strengthened the allelopathy of the two Asteraceae invasives. The allelopathy of the two Asteraceae invasives increases with the increasing acidity of acid rain, but the allelopathy of the two Asteraceae invasives decreases with the increasing nitrogen-to-sulfur ratio of acid rain. Accordingly, the species number of invasives, and the acidity and type of acid rain modulated the impacts of acid rain on the allelopathy of the two Asteraceae invasives.