Agronomic Practices Shape Tissue-Specific Antioxidant Capacity and Metabolic Profiles in Achillea millefolium L.

This study investigates the influence of agronomic management on the accumulation of bioactive compounds and the antioxidant capacity of Achillea millefolium L., a medicinal species of increasing relevance for pharmaceutical and nutraceutical applications. Different cultivation strategies were applied, including controlled drought stress, foliar fertilization, and inoculation with plant growth–promoting rhizobacteria (PGPR), in order to evaluate their impact on tissue-specific metabolic responses. The total antioxidant capacity (TAC) of flowers and roots was determined using FRAP, DPPH, and ABTS spectrophotometric assays, while metabolite profiling was performed by UHPLC–MS/MS analysis. Clear differences in antioxidant activity were observed among plant organs and cultivation treatments. Flower extracts showed intermediate antioxidant capacity, with FRAP values ranging from 55.86 to 66.55 mg TE g−1 extract and the highest activity consistently recorded for treatment F_010 (addition of K, P fertilizers under water stress conditions and PGPR absence) across all assays. Root extracts exhibited substantially lower antioxidant values (FRAP 19.40–33.69 mg TE g−1), although samples R_000 (no foliar fertilization, under water stress conditions and PGPR absence) and R_100 (no foliar fertilization, under water stress conditions and presence of PGPR) displayed comparatively higher radical scavenging activity. Metabolic profiling revealed a shared presence of caffeic acid derivatives and flavonoids, including mono- and di-caffeoylquinic acids and apigenin-related compounds, with marked quantitative differences among tissues. Overall, the results demonstrate that agronomic practices significantly influence the accumulation and distribution of antioxidant metabolites in A. millefolium L., highlighting the importance of cultivation strategies for optimizing the production of bioactive phytochemicals.