Drought stress intensifies the phytotoxicity of five Asteraceae exotic invasive plants

The phytotoxicity of exotic invasive plants (EIP) is vital for their successful invasion. At the family level, Asteraceae contains the largest number of EIP species in China. Drought stress may affect the phytotoxicity of EIP by altering the production and secretion of allelochemicals. Therefore, it is important to estimate the phytotoxicity of multiple EIP under drought stress. In this study, the phytotoxicity of five Asteraceae EIP, i.e., Solidago canadensis L., Aster subulatus Michx., Bidens pilosa L., Erigeron annuus (L.) Pers., and Conyza canadensis (L.) Cronq., were estimated using leaf extracts. Their effects on seed germination and seedling growth of the horticultural Asteraceae species Lactuca sativa L. under drought stress (simulated by polyethylene glycol-6000) were assessed. These five Asteraceae EIP, particularly S. canadensis, inhibited seed germination of L. sativa. Aster subulatus, B. pilosa, E. annuus, and C. canadensis increased seedling growth of L. sativa. The phytotoxicity of these five Asteraceae EIP decreased in the following order: S. canadensis > E. annuus > A. subulatus > B. pilosa ≈ C. canadensis. Drought stress intensified the phytotoxicity of all five Asteraceae EIP. The level of stress intensity mediated by these five Asteraceae EIP, particularly by S. canadensis, A. subulatus, and B. pilosa, increased with increasing degree of drought stress. Thus, an increasing growing degree of drought stress may accelerate the invasion process of these five Asteraceae EIP via the effects of intensified phytotoxicity on growth performance of adjacent species, particularly on seed germination and seedling growth.