Involvement of Allelopathy in the Invasive Potential of Tithonia diversifolia

Tithonia diversifolia (Hemsl.) A. Gray (Asteraceae) is native to Mexico and Central America. The species is spreading quickly and has naturalized in more than 70 countries. It has often been recorded as a harmful invasive plant that disturbs native plant communities. Phytotoxic chemical interactions such as allelopathy between invasive plants and native plants have been reported to play an important role in the invasion. Evidence for allelopathy of T. diversifolia has accumulated in the literature over 30 years. Thus, the objective of this review was to discuss the possible involvement of allelopathy in the invasive potential of T. diversifolia. The extracts, root exudates, and plant residues of T. diversifolia inhibited the germination and growth of other plant species. The soil water and soil collected from T. diversifolia fields also showed inhibitory growth effects. The decomposition rate of T. diversifolia residues in soil was reported to be high. Phytotoxic substances such as sesquiterpene lactones were isolated and identified in the extracts of T. diversifolia. Some phytotoxic substances in T. diversifolia may be released into the soil through the decomposition of the plant residues and the exudation from living tissues of T. diversifolia, including its root exudates, which act as allelopathic substances. Those allelopathic substances can inhibit the germination and growth of neighboring plants and may enhance the competitive ability of the plants, make them invasive.