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Phragmites australis is one of the most widespread and invasive plants on earth. Allelopathic interference has been considered as a possible way associated with its invasiveness in wetlands. A series of ecologically realistic experiments was conducted to explore allelochemical phytotoxicity of Phragmites. Germination bioassays using aqueous extracts of different organs (leaf, stem, root and rhizome) of Phragmites were tested with model seeds (Lactuca sativa and Raphanus sativus) and associated plant species (Juncus pallidus and Rumex conglomeratus). These studies showed that leaf and rhizome extracts exhibited strong inhibition on germination, biometric and physiological parameters (all P ≤ 0.001). Dose–response studies confirmed LC50 (4.68% and 11.25%) of Lactuca for leaf and rhizome extracts respectively. Root growth of Juncus and Rumex was inhibited by 75% and 30%, respectively, in leaf leachate-incorporated soil. Chlorophyll content and maximum quantum yield (Fv/Fm) were significantly reduced with leaf and rhizome leachates. The stability and quantity of water-soluble phenolics in anaerobic versus aerobic condition may influence phytotoxic effects to other species. Phragmites organs can be ranked in order of allelopathic potentiality as follows: leaf > rhizome > root > stem. The present study highlighted the potential impacts of allelochemicals on plant recruitment in wetlands invaded by Phragmites.

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This study investigated the possible phytotoxicity induced by Phargmites australis on phenotypic and physiological parameters of recipient plants with identification of major inhibitors in the donor plant. This was achieved using aqueous extracts of different organs and root exudates of P. australis in laboratory and greenhouse experiments with Lactuca sativa as the model test plant. The observed reduced liquid imbibition and altered resource mobilization in seeds of L. sativa, in particular an insufficient carbohydrate supply, demonstrated that the onset of germination might be negatively affected by phytotoxicity. Dose-response studies pointed out that oxidative stress through reactive oxygen species production could potentially cause the observed germination and seedling growth reductions. The osmotic effects by mannitol solution on germination as well as growth and physiology at a level of (0.57 and (0.45 bar, respectively, demonstrated that the results from aqueous plant extracts were partially induced by the osmotic potential on and above those levels. Overall, the relative strength of inhibition on measured parameters was the highest in leaf extract, followed by rhizome, root, stem, and inflorescence. Root exudates of P. australis also had negative impacts by reducing germination and growth of plant. High-performance liquid chromatography (HPLC) analysis revealed gallic acid, a potent phytotoxin, as a major compound with an order of leaf ! inflorescence ! rhizome !root ! stem.