| Summary: | The volatile compounds produced by plants play an important role in plant growth, plant communication, and resistance to biological and abiotic stresses. <i>Astragalus membranaceus</i> var. <i>mongholicus</i> (<i>AM</i>) is a perennial herbaceous plant (Leguminosae) that is widely cultivated in northwest China. The bioactive compounds in its root have shown various pharmacological activities. Root rot disease caused by <i>Fusarium</i> spp. often occurs in <i>AM</i> planting with increasing severity in continuous monoculture. It is currently still unclear what are the effects of the volatile compounds produced by fresh <i>AM</i> on itself, other crops cultivated on the same field after <i>AM</i>, pathogen, and rhizobia. In this study, we found that seed germination and seedling growth of <i>AM</i>, lettuce (<i>Lactuca sativa</i> L.), and wheat (<i>Triticum aestivum</i> L.) could be affected if they were in an enclosed space with fresh <i>AM</i> tissue. Additionally, 90 volatile compounds were identified by SPME-GC-MS from whole <i>AM</i> plant during the vegetative growth, 36 of which were specific to aerial parts of <i>AM</i> (stems and leaves, AMA), 17 to roots (AMR), and 37 were found in both AMA and AMR. To further identify the allelopathic effects of these volatile compounds, five compounds (1-hexanol, (<i>E</i>)-2-hexenal, (<i>E</i>,<i>E</i>)-2,4-decadienal, hexanal, and eugenol) with relatively high content in <i>AM</i> were tested on three receptor plants and two microorganisms. We found that (<i>E</i>,<i>E</i>)-2,4-decadienal and (<i>E</i>)-2-hexenal showed significant inhibitory effects on the growth of <i>AM</i> and lettuce. One-hexanol and hexanal suppressed the growth of wheat, while eugenol showed a similar effect on all three plant species. Moreover, the activities of these compounds were dose dependent. Notably, we discovered that (<i>E</i>)-2-hexenal and eugenol also inhibited the growth of the pathogen <i>Fusarium solani</i> by as high as 100%. Meanwhile, all five compounds tested suppressed the rhizobia <i>Sinorhizobium fredii</i>. In summary, this study furthered our understanding of the comprehensive allelopathic effects of the main volatile components of <i>AM</i>.
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