Volatile-mediated allelopathy: Perovskia atriplicifolia Benth. volatile organic compounds trigger phenylpropanoid–lignin remodelling in Amaranthus retroflexus L

This study investigated the allelopathic effects of volatile organic compounds (VOCs) emitted by Perovskia atriplicifolia Benth. on the weed Amaranthus retroflexus L. . Headspace SPME-GC-MS identified 70 VOCs, mainly o-cymene (∼33%), δ-3-carene (∼15%), and 1,8-cineole (∼8%), in P. atriplicifolia . Bioassays revealed strong inhibitory effects: germination was reduced by approximately 40% at 100 g and 62% at 200 g of P. atriplicifolia biomass, while root length decreased by more than 60% at both doses. At doses around the ED50, treated seedlings developed reduced fresh weight (−16%), lower relative water content (−13%), and higher osmotic potential (+16%). Cuticular analyses indicated a shift toward long-chain aliphatics, and metabolomics showed increased accumulation of phenylpropanoid intermediates (e.g., sinapic, ferulic, p -coumaric acids) together with higher PAL activity (+52%), PPO activity (+180%), total phenolic and lignin content (+61%). Overall, P. atriplicifolia VOCs, dominated by o-cymene, δ-3-carene, and 1,8-cineole, significantly impair the germination and early growth of A. retroflexus , triggering metabolic reprogramming toward phenylpropanoid–lignin biosynthesis and cuticular reinforcement. This study advances the understanding of volatile-mediated allelopathy by linking biochemical profiles to integrative physiological and metabolomic responses in A. retroflexus plants.