Quality Evaluation and Antioxidant Activity of Cultivated Gentiana siphonantha: An Ethnic Medicine from the Tibetan Plateau

Gentiana species are widely used in traditional and modern medicine, and Gentiana siphonantha is an important medicinal representative. To evaluate the quality characteristics of cultivated G. siphonantha roots, the accumulation patterns of iridoid glycosides and antioxidant activities across different cultivation ages and harvest months were investigated. Five major iridoid glycosides were quantified, and antioxidant capacity was assessed through DPPH, ABTS, and FRAP assays. Quality was subsequently multidimensionally evaluated using principal component analysis (PCA), orthogonal partial least squares–discriminant analysis (OPLS-DA), membership function analysis, and entropy weight–TOPSIS analysis, and the relationship between iridoid glycoside content and antioxidant activity was analyzed. Results showed that 3-year-old cultivated roots had the highest total iridoid glycoside content (134.60 mg·g−1 DW), indicating the optimal cultivation age. Peak glycoside accumulation occurred in 4-year-old plants harvested in June–July, identifying this period as the optimal harvest time, as supported by multivariate statistical and comprehensive evaluation. Antioxidant activity increased with cultivation age, with samples collected in June or August showing higher capacities, and it was positively correlated with total iridoid glycoside content, particularly with FRAP (p < 0.05). In conclusion, cultivated G. siphonantha exhibits stable quality and favorable antioxidant activity, providing a basis for standardized cultivation, quality evaluation, and rational utilization.