The volatile compounds produced by plants play an important role in plant growth, plant communication, and resistance to biological and abiotic stresses. Astragalus membranaceus var. mongholicus (AM) 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 Fusarium spp. often occurs in AM planting with increasing severity in continuous monoculture. It is currently still unclear what are the effects of the volatile compounds produced by fresh AM on itself, other crops cultivated on the same field after AM, pathogen, and rhizobia. In this study, we found that seed germination and seedling growth of AM, lettuce (Lactuca sativa L.), and wheat (Triticum aestivum L.) could be affected if they were in an enclosed space with fresh AM tissue. Additionally, 90 volatile compounds were identified by SPME-GC-MS from whole AM plant during the vegetative growth, 36 of which were specific to aerial parts of AM (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, (E)-2-hexenal, (E,E)-2,4-decadienal, hexanal, and eugenol) with relatively high content in AM were tested on three receptor plants and two microorganisms. We found that (E,E)-2,4-decadienal and (E)-2-hexenal showed significant inhibitory effects on the growth of AM 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 (E)-2-hexenal and eugenol also inhibited the growth of the pathogen Fusarium solani by as high as 100%. Meanwhile, all five compounds tested suppressed the rhizobia Sinorhizobium fredii. In summary, this study furthered our understanding of the comprehensive allelopathic effects of the main volatile components of AM.

Astragalus membranaceus var. mongholicus (AMM) is a widely used medicinal plant in China, primarily cultivated in the northern regions; however, the selection and breeding of superior varieties remains underdeveloped. To scientifically assess the germplasm resources of AMM and facilitate its breeding, this study collected 33 germplasm samples from five provinces and conducted a comprehensive evaluation of their botanical traits, agronomic traits, quality traits, and powdery mildew resistance. The results indicated that, among the 18 botanical and agronomic traits, the Shannon–Wiener diversity index (H′) for quantitative traits was higher than that for qualitative traits, with the coefficient of variation ranging from 6.82% to 34.14%. The characterization of five flavonoids and eight saponins based on the established UPLC-Q-TOF/MS technique revealed that 13 compounds showed significant abundance variations across germplasms. Correlation analysis revealed that plants with robust above-ground growth usually have higher yields. Moreover, the number of compound leaflets was positively correlated with flavonoid content in the roots, whereas germplasms with smaller above-ground biomass and thinner roots exhibited higher saponin content. Based on the comprehensive scores (F-value), the top three germplasms were An-31, An-26, and An-28, which may serve as promising breeding materials. Cluster analysis grouped the 33 germplasms into five categories, including high-content and high-yield groups. Five germplasms exhibiting strong disease resistance were identified, with An-26 demonstrating the best performance in yield, quality, and resistance. Furthermore, a negative correlation was observed between powdery mildew resistance and flavonoid content in roots. This study will provide a foundation for the AMM breeding and selection.