Under controlled conditions, lower concentrations of aqueous extracts of Acroptilon repens are needed to inhibit seed germination and seedling growth of weeds than crops.

Preventing weeds from hindering agricultural production often relies on chemical inputs; however, these inputs can be ineffective, increase costs, or harm the environment. Natural products, particularly plant extracts, are being explored as potential sources for developing eco-friendly bioherbicides that replace or reduce the use of chemical herbicides. In this study, we examined to what degree aqueous extract from the allelopathic weed Acroptilon repens L. (Russian knapweed) was able to influence seed germination and seedling growth metrics in vitro. Uniquely, our study compared the impacts of A. repens extract on three weedy species (Sinapis arvensis L., Amaranthus retroflexus L., and Avena fatua L.), a multipurpose species (Secale cereale L.), and two crop species (Triticum aestivum L., and Vicia faba L.) in side-by-side comparisons. As expected, a significant (P ≤ 0.01), linear, and concentration-dependent decline in germination metrics and seedling growth was observed after application of the extract, while mean germination time (P > 0.05) was not changed. Interestingly, the weedy species were affected at lower concentrations compared to the crops. Germination was fully inhibited for S. arvensis ≥ 40 g L-1, A. retroflexus ≥ 80 g L-1, A. fatua ≥ 120 g L-1, T. aestivum ≥ 140 g L-1, and S. cereale ≥ 160 g L-1 of A. repens extract. These findings are based on in vitro assays and do not directly demonstrate allelopathic activity under field or soil conditions. In addition to existing data on the phytotoxic compounds produced by A. repens, our untargeted GC-MS profiling putatively identified several compounds that warrant further exploration, including phenolic compounds, a sesquiterpene (caryophyllene oxide), and lipid-derived constituents such as fatty acids and sterols. Although comprehensive ecological and agronomic assessments are needed prior to field application and further research is needed to isolate and confirm the bioactivity of these putatively identified secondary metabolites, these data suggest that A. repens aqueous extract can inhibit germination and early seedling development of problematic weeds more strongly than crops.