Rhododendron adamsii Flowers as a Potential Source of Tea-Derived Flavonoid Antioxidants

Rhododendron adamsii Rehder, also known as sagan dali, is one of the most valued northern rhododendron species of Siberia and Mongolia as both a medicinal and food plant. Its flowers are traditionally used by indigenous communities in daily life to prepare teas that are attributed with medicinal properties in local traditional medicine. However, the lack of reliable data on the chemical composition and bioactivity of R. adamsii flowers has limited their broader application and underscores the need for comprehensive studies to verify their beneficial properties. The application of liquid chromatography–mass spectrometry enabled the identification of fifty-four compounds in sixteen samples of different origins, with flavonoids representing the dominant group and belonging to various aglycone types. Among the identified metabolites were dihydroflavonols of the taxifolin series; flavonols of the myricetin, quercetin, and kaempferol series; as well as several minor flavonoid and non-flavonoid compounds. Thirty-seven of these compounds are reported for the first time in this species. The total phenolic content in R. adamsii flowers can reach 155.82 mg/g, of which up to 147.54 mg/g are flavonoids. The analysis revealed variation in both the qualitative profile and quantitative levels of individual compounds among different populations, suggesting the presence of distinct R. adamsii chemotypes. The preparation of flower tea was associated with high rates of flavonoid transfer into the decoction, particularly when pulverized raw material was used compared with unground or hand-ground samples. This was reflected in the enhanced antioxidant activity of the decoctions, which was maximal for pulverized flowers in in vitro assays against artificial and natural free radicals, as well as in nitric oxide scavenging and Fe2+-chelating tests. These results suggest that R. adamsii flowers and their tea represent a new possible source of flavonoids and after additional clinical evidence may serve as valuable antioxidant ingredients for the development of functional foods.