Isolation of Allelochemicals from Rhododendron capitatum and Their Allelopathy on Three Perennial Herbaceous Plants

<i>Rhododendron capitatum</i> community expansion is a major threat to alpine meadow. Allelopathy is an important mediator in managing relationships between plants in natural ecosystems. However, allelopathy and specific allelochemicals of <i>R. capitatum</i> have not been studied yet. In this study, the allelopathy of the foliage litter of <i>R. capitatum</i> was explored on <i>Elymus nutans</i>, <i>Poa pratensis</i> and <i>Medicago ruthenica</i>, and the chemical composition and their allelopathy were studied. The results showed that the aqueous extract of the foliage litter of <i>R. capitatum</i> had an allelopathy of "low concentration promotion and high concentration inhibition" on the germination of <i>E. nutans</i>, <i>P. pratensis</i>, and <i>M. ruthenica</i>. Organic acids, fatty acids, terpenes, phenols, and phenolic acid compounds were identified, with Zanamivir (77.81%), alpha-linolenic acid (18%), Kaurenoic acid (23.50%), 4-hydroxyphenylglycolic acid (21.54%), and Quinic acid (28.24%) having the highest relative content, and all five compounds showed significantly inhibitory effects on seed germination and seedling growth of <i>E. nutans</i>, <i>P. pratensis</i>, and <i>M. ruthenica</i>, which further suggests that the five compounds are the critical allelochemicals for negative allelopathy of <i>R. capitatum</i>. These results highlight the crucial role of inhibitory allelopathy produced by <i>R. capitatum</i> in the establishment and expansion of its populations.