Does the salt stress intensify the independent allelopathy and the co-allelopathy of Solidago canadensis L. and Conyza canadensis (L.) Cronq.?

The allelopathy of invasive plants is crucial to their successful invasion. The co-invasion of two invasive plants is common. Salt stress may affect the allelopathy of invasive plants, particularly the co-allelopathy of two invasive plants. This study aimed to analyze the independent allelopathy and the co-allelopathy of two Asteraceae invasive plants, Solidago canadensis L. and Conyza canadensis (L.) Cronq., on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. under a gradient of salt stress. The germination bioassay of L. sativa was conducted via the incubation in Petri dishes (diameter: 9 cm). The two Asteraceae invasive plants caused significant allelopathy on L. sativa. The independent allelopathy of S. canadensis was greater than that of C. canadensis and the co-allelopathy of the two Asteraceae invasive plants. Salt stress regardless of the concentration strengthened the independent allelopathy and the co-allelopathy of the two Asteraceae invasive plants. Heavy salt stress triggered a stronger impact on the independent allelopathy and the co-allelopathy of the two Asteraceae invasive plants than light salt stress. The degree of enhancement for the independent allelopathy of S. canadensis on L. sativa was clearly greater than that of C. canadensis and the co-allelopathy of the two Asteraceae invasive plants under light salt stress, but it was similar in the degree of enhancement for the independent allelopathy and the co-allelopathy of the two Asteraceae invasive plants under heavy salt stress.