From spring to autumn: How harvest season and species shape the phytochemical and biological properties of Forsythia leaf extracts

Members of the genus Forsythia spp., well-known ornamental shrubs, are widely recognised for their early spring flowering and ethnopharmacological significance, particularly in traditional Asian medicine. While the phytochemical profile, biological effects, and industrial applications of their fruits and flowers are well documented, the leaves remain largely underexplored. Specifically, limited attention has been given to their seasonal variability and utility in the pharmaceutical and functional food industry. This study aimed to evaluate seasonal fluctuations in bioactive compounds (lignans, phenylethanoids, flavonoids, caffeoylquinic acids) in leaves of two European Forsythia species: F. × intermedia Zabel var. spectabilis and F. suspensa Zabel var. sieboldii. Leaf samples were collected monthly from May to September to determine the optimal harvest period for the highest phytochemical content suitable for herbal processing. Comprehensive qualitative and quantitative analyses, employing UHPLC-PDA-ESI-MS/MSn and HPLC-PDA, were integrated with PCA to explore correlations between seasonal environmental shifts and metabolic profiles. Reference standards were prepared from spring leaf extracts using preparative HPLC. The immunomodulatory potential of selected extracts and marker compounds was assessed in vitro using human PBMC-derived monocytes/macrophages. The results demonstrated distinct species-specific and season-dependent variations in metabolite concentrations, with the highest levels of bioactive constituents observed in spring-harvested leaves. This variability directly correlated with biological potency, as spring samples exhibited the most robust immunomodulatory effects. These findings identify spring as the optimal harvest period for high-quality Forsythia leaf raw material. This study contributes to the knowledge on Forsythia leaves as an efficient source of biomass for phytotherapy and functional food formulations.