Phytotoxic Effects of the Aqueous Extracts of Magnolia biondii Pamp. Flower Litter and the Joint Action of Allelochemicals

<i>Magnolia biondii</i> Pamp., an elegant ornamental tree that graces early spring landscapes, has flower buds that are widely used as Traditional Chinese Medicine 'Xin Yi'. In this study, the phytotoxic effects of aqueous extracts derived from tepal litter (EMT) and bract litter (EMB) of <i>M. biondii</i> flower were evaluated on six target plant species: <i>Triticum aestivum</i> L., <i>Lactuca sativa</i> L., <i>Zoysia pacifica</i>, <i>Agrostis canina</i> L., <i>Trifolium pratense</i> L. and <i>Axonopus compressus</i>. Secondary metabolites in EMT and EMB were tentatively characterized by liquid chromatography high-resolution mass spectrometry (LC-HRMS), and the joint action of allelochemicals was examined. Our findings revealed that both EMT and EMB significantly inhibited the seed germination and seedling growth of all target plants in a concentration-dependent manner. There were 120 and 98 secondary metabolites annotated by LC-HRMS in EMT and EMB, respectively. Among them, malic acid (MA) and citric acid (CA) with high relative contents showed synergistic phytotoxicities on seed germination and seedling growth of <i>L. sativa</i> and <i>Z. pacifica</i>. In summary, the flower litter of <i>M. biondii</i> displayed significant allelopathic effects, and the synergistic effects of MA and CA probably played an important role.