In this study, the black fertile (BSs) and the red unfertile seeds (RSs) of the Greek endemic Paeonia clusii subsp. rhodia (Stearn) Tzanoud were studied for the first time. Nine phenolic derivatives, trans-resveratol, trans-resveratrol-4'-O-β-d-glucopyranoside, trans-ε-viniferin, trans-gnetin H, luteolin, luteolin 3'-O-β-d-glucoside, luteolin 3',4'-di-O-β-d-glucopyranoside, and benzoic acid, along with the monoterpene glycoside paeoniflorin, have been isolated and structurally elucidated. Furthermore, 33 metabolites have been identified from BSs through UHPLC-HRMS, including 6 monoterpene glycosides of the paeoniflorin type with the characteristic cage-like terpenic skeleton found only in plants of the genus Paeonia, 6 gallic acid derivatives, 10 oligostilbene compounds, and 11 flavonoid derivatives. From the RSs, through HS-SPME and GC-MS, 19 metabolites were identified, among which nopinone, myrtanal, and cis-myrtanol have been reported only in peonies' roots and flowers to date. The total phenolic content of both seed extracts (BS and RS) was extremely high (up to 289.97 mg GAE/g) and, moreover, they showed interesting antioxidative activity and anti-tyrosinase properties. The isolated compounds were also biologically evaluated. Especially in the case of trans-gnetin H, the expressed anti-tyrosinase activity was higher than that of kojic acid, which is a well-known whitening agent standard.
Keywords: Paeonia clusii subsp. rhodia (Stearn) Tzanoud; Paeoniaceae; anti-tyrosinase assay; antioxidant activity; flavonoids; monoterpenic glucosides; seeds; stilbenes; total phenolic content.