Functional phenotyping of Agastache germplasm reveals antioxidant–neuroprotective linkages

Agastache spp. are medicinal and aromatic plants rich in phenolic acids and flavonoid-derived compounds. Among these, representative metabolites such as rosmarinic acid (RA), tilianin (Til), and acacetin (AC) have been widely associated with antioxidant and neuroprotective activities. Despite extensive phytochemical studies, integrated evaluations linking antioxidant indices, key metabolites, developmental traits, and biological activities across diverse germplasm remain scarce. In particular, quantitative evidence connecting phenolic-based antioxidant capacity with flowering behavior and readily observable morphological traits is lacking, constraining functional trait-based selection and utilization of Agastache accessions. Total phenolic content (TPC), DPPH, and ABTS radical-scavenging activities exhibited strong positive co-variation (r ≥ 0.8), confirming that expansion of the phenolic pool is associated with antioxidant capacity. Although RA, Til, and AC share common upstream precursors, their downstream correlation patterns diverged, reflecting differential regulation through enzymatic activity and glycosylation processes. Korean accessions showed significantly higher RA accumulation, suggesting signals of local adaptation or usage-driven selection. Days to flowering exerted broad pleiotropic effects, positively influencing antioxidant indices, RA–Til–AC accumulation, and multiple vegetative growth traits, while showing a negative association with inflorescence length, highlighting its role as an integrative regulator of development, biomass allocation, and secondary metabolism. Inflorescence color further shaped functional trait profiles, with purple accessions displaying stronger radical-scavenging activity but reduced flavonoid accumulation. In an MPP⁺-induced neurotoxicity model using SH-SY5Y cells, several accessions, including Namcheon and K000899, reproducibly restored neuronal cell viability. These findings demonstrate tight interactions among antioxidant capacity, secondary metabolism, flowering time, and morphological traits in Agastache germplasm. The results provide a quantitative framework for functional trait-based selection and support the development of Agastache cultivars and value-added functional ingredients.