Allelopathy and Identification of Volatile Components from the Roots and Aerial Parts of Astragalus mongholicus Bunge

The volatile compounds produced by plants play an important role in plant growth, plant communication, and resistance to biological and abiotic stresses.Astragalus membranaceusvar.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 byFusariumspp. often occurs inAMplanting with increasing severity in continuous monoculture. It is currently still unclear what are the effects of the volatile compounds produced by freshAMon itself, other crops cultivated on the same field afterAM, pathogen, and rhizobia. In this study, we found that seed germination and seedling growth ofAM, lettuce (Lactuca sativaL.), and wheat (Triticum aestivumL.) could be affected if they were in an enclosed space with freshAMtissue. Additionally, 90 volatile compounds were identified by SPME-GC-MS from wholeAMplant during the vegetative growth, 36 of which were specific to aerial parts ofAM(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 inAMwere 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 ofAMand 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 pathogenFusarium solaniby as high as 100%. Meanwhile, all five compounds tested suppressed the rhizobiaSinorhizobium fredii. In summary, this study furthered our understanding of the comprehensive allelopathic effects of the main volatile components ofAM.