Abstract:Exogenous calcium ions play an important role in the response of plants to environmental stresses, and parasitic plants cause significant damage to the growth of invasive plants. However, there has been no empirical study on the interactive effect of exogenous calcium ions and parasitic plants on the growth of invasive plants. Alternanthera philoxeroides (Amaranthaceae), native to South America, is a globally noxious invasive weed that has expanded into China and is now widespread throughout the country. Cusucta australis (Convolvulaceae), a native holoparasitic plant, is found to parasitize A. philoxeroides in the field. Here, we conducted a greenhouse experiment to test the interactive effect of parasitic C. australis and exogenous calcium ions on the morphology and structure of invasive Alternanthera philoxeroides stems, with the aim of elucidating the role played by calcium ions during the response of invasive plants to native parasitic plants. Parasitic C. australis significantly decreased the total length, branch number, internode number, diameter and pulp cavity diameter of A. philoxeroides stems, but significantly increased the collenchyma and cortex thickness of A. philoxeroides stems. The addition of exogenous calcium ions had no significant effect on the total length, branch number, internode number, diameter, pulp cavity diameter and vascular bundles diameter of A. philoxeroides stems, but increased the number of vascular bundles, and decreased the internode length, collenchyma thickness and cortex thickness of A. philoxeroides stems. The interaction between parasitism and exogenous calcium ions was found to have an effect on the collenchyma and cortex thickness of A. philoxeroides stems, with the result of increased thickness in both parts. This indicated an antagonistic interactive effect between parasitism and exogenous calcium ions on the collenchyma and cortex thickness of A. philoxeroides stems. This antagonistic interactive effect suggests that exogenous calcium ions could significantly improve the defense of host plants to parasitic plants and reduce the damage caused by parasitic plants on their hosts.