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Article

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

by
Daniil N. Olennikov
1,
Nina I. Kashchenko
1,* and
Nadezhda K. Chirikova
2
1
Laboratory of Biomedical Research, Institute of General and Experimental Biology, Siberian Division, Russian Academy of Science, 6 Sakhyanovoy Street, 670047 Ulan-Ude, Russia
2
Department of Biochemistry and Biotechnology, North-Eastern Federal University, 58 Belinsky Street, 677027 Yakutsk, Russia
*
Author to whom correspondence should be addressed.
Horticulturae 2026, 12(4), 484; https://doi.org/10.3390/horticulturae12040484
Submission received: 27 February 2026 / Revised: 12 April 2026 / Accepted: 14 April 2026 / Published: 15 April 2026
(This article belongs to the Special Issue Characterization of Bioactive Compounds and Antioxidant Activity of Medicinal Plants: 2nd Edition)
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Abstract

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.

1. Introduction

The genus Rhododendron is widely used in various spheres of human activity, including food applications as a tea component [1]. Of the many species of this genus the most popular Siberian tea-based beverage is sagan-da-li tea made on the basis of Adams’ rhododendron {Rhododendron adamsii Rehder, 1921; syn. Azalea fragrans Adams, 1808; Osmothamnus fragrans (Adams) DC., 1839; R. fragrans (Adams) Maxim., 1870)} [2], an evergreen, low-growing shrub blooming with pale pink to pink flowers [3] (Figure 1a).
Ethnopharmacological evidence regarding the medicinal use of R. adamsii is primarily related to the traditions of Tibetan healers (lamas), which likely reflects the species’ distribution across Northern and Eastern Asia, regions historically influenced by Tibetan medicine. In this system of Eastern medicine, the flowers of R. adamsii, known as da-li, were used to treat conditions described as cold [4], diseases of wind, bile, and phlegm, all lung diseases [5], disorders of the spleen and gastrointestinal tract, general fever, shortness of breath, drowsiness, sweating, infectious diseases, baldness [6], and to strengthen the body [7] as a tonic remedy [8]. The flowers of R. adamsii are also used for similar indications beyond Tibet, including in Manchuria (as qarabur), Mongolia (telelzh dal), Buryatia (sagaan dali), Yakutia, and other regions of Siberia, where traditional medicine adopted both therapeutic practices and medicinal plant materials from Tibetan medicine [4]. Despite the broad therapeutic spectrum attributed to R. adamsii in traditional medicine, frequent references to its tonic effects are particularly notable, likely contributing to the popularization of medicinal preparations derived from this plant as stimulant and adaptogenic agents.
The remarkable popularity of R. adamsii flowers as a medicinal and tonic remedy, based on ancient traditions, has further increased in modern times, shifting this plant material from a strictly medicinal resource to the category of a commonly consumed food and/or tea plant. There is a long-standing tradition of harvesting the flowers at the end of May and brewing flowers to prepare a fragrant decoction known as sagan-da-li (or sagan-dailya) tea [9]. The resulting beverage is bright yellow in color, with a pleasant floral aroma and a slightly sweet taste lacking astringency or bitterness (Figure 1b).
Scientific investigation of R. adamsii began in the early twentieth century and continues today; however, these studies have focused only on the leaves, whereas the flowers have not yet been examined. Analyses of R. adamsii leaves have identified more than 300 compounds, including terpenoids, phenolic compounds, lipids, amino acids, and other constituents [2,10,11,12,13]. To address the existing gap in scientific knowledge and substantiate the beneficial properties attributed to R. adamsii flowers, comprehensive investigations of their chemical composition and biological activity are required.
Early evidence indicates that rhododendron flowers contain a variety of phenolic compounds, predominantly flavonoids, which are recognized for their antioxidant and antiradical activities [14,15,16]. The main subclass of flavonoids found in the most Rhododendron flowers are flavonols, both aglycones and glycosides, including the most common quercetin [17], myricetin [18], kaempferol [19], and gossypetin [20] with derivatives and some minor metabolites as isorhamnetin, 5-O-methylmyricetin, and other [1,21]. Flavan-3-ols as monomers [22] and oligomers (procyanidins) [23] are found less frequently in flowers, but are common metabolites of phenolome of the above-ground parts of rhododendrons. Low occurrence as floral phenols was noted for dihydroflavonols [23], flavones [19], and isoflavones [21], which were described only after LC-MS profiling and were not isolated in pure form due to low concentrations. Generating scientific data on R. adamsii flowers will broaden current knowledge of the chemodiversity of the genus Rhododendron and help establish the presence of beneficial properties in sagan-da-li tea.
As part of an ongoing investigation of plants of the genus Rhododendron [2,24,25,26], this study attempts to elucidate (1) the chemical composition of R. adamsii flowers by liquid chromatography–mass spectrometric (LC–MS) profiling of their extractive substances with particular emphasis on the flavonoid group; (2) the flavonoid content in aqueous R. adamsii flowers tea extracts from raw materials with different degrees of grinding; (3) the antioxidant potential of the resulting R. adamsii flowers tea decoctions using in vitro methods.

2. Materials and Methods

2.1. Plant Material

Flowers of R. adamsii were collected from various regions of Siberia, the Far East, and Mongolia during the flowering period (May–June) from 2017 to 2023 (Table 1). At each sampling site (Nos. 1–16), flowers from 50 individual plants were harvested, placed in boxes containing silica gel, and subsequently dried within ten hours in a convection oven at 40 °C. After drying, all flowers from one site were combined into a single sample to give sixteen samples Nos. 1–16 and analyzed.

2.2. Chemical Composition Assays

Essential oil content was determined using the hydrodistillation method [27]. Ready-to-use assay kits were used for the quantification of lipids (Lipid Quantification Kit, fluorometric assay; MyBioSource Inc., San Diego, CA, USA; cat. No. MBS169253), total phenolic compounds (Phenolic Compounds Assay Kit, colorimetric assay; Abcam Inc., Waltham, MA, USA; cat. No. ab273293), flavonols (OxiSelect Flavonoid Assay Kit, spectrophotometric assay; MyBioSource Inc., San Diego, CA, USA; cat. No. MBS169572), catechins/procyanidins (Proanthocyanidin Assay Kit, spectrophotometric assay; Arigo Biolaboratories Corp., Zhubei, Taiwan, ROC; cat. No. ARG83419), and polysaccharides (Plant Soluble Sugar Assay Kit, colorimetric assay; Elabscience, Houston, TX, USA; cat. No. E-BC-K866-M).

2.3. Antioxidant Assays

The radical scavenging activity of R. adamsii flowers was evaluated by spectrophotometric assays using 2,2-diphenyl-1-picrylhydrazyl (DPPH•) radicals [28], 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS•+) radicals [28], N,N-dimethyl-p-phenylenediamine dihydrochloride (DMPD•) radicals [29], hydroxyl (OH•) radicals [30], and superoxide (O2•−) radicals [28], as well as by coulometric determination of chlorine (Cl•) and bromine (Br•) radicals [31]. Nitric oxide (II; NO) scavenging activity [29] and Fe2+-chelating activity (FeCA) were also assessed using spectrophotometric methods [32]. Trolox (Sigma-Aldrich, St. Louis, MO, USA, cat. No. 648471) was used as the reference standard in all assays. Each analysis was performed in quintuplicate, and the results are expressed as mean ± standard deviation (S.D.).

2.4. High-Performance Chromatography with Photodiode Array and Ion Trap-Time-of-Flight Mass Spectrometry Detection (HPLC-PDA-IT-TOF-MS)

The analysis was performed using an LC-20 Prominence liquid chromatograph (Shimadzu, Columbia, MD, USA) equipped with an SPD-30AM photodiode array detector (PDA; Shimadzu) and coupled to an LCMS-9050 ion trap–time-of-flight (IT–TOF) mass spectrometer (Shimadzu). Chromatographic separation was performed on a Gold-Turbo Basic C18 column (75 mm × 3 mm, 1.8 μm; Dr. Maisch GmbH, Ammerbuch, Germany). The HPLC conditions were as follows: the mobile phase consisted of eluent A (0.1% formic acid (HCOOH) in water) and eluent B (0.1% HCOOH in acetonitrile). The gradient program was set as 0–1 min, 2–11% B; 1–9 min, 11–24% B; 9–17 min, 24–32% B; 17–35 min, 32–76% B; and 35–45 min, 76–2% B. The injection volume was 1 µL, the flow rate was maintained at 1 mL/min, and the column temperature was set at 25 °C; absorption spectra were recorded over the range of 200–600 nm. MS conditions included electrospray ionization in positive mode, with an interface temperature of 300 °C, desolvation line temperature of 250 °C, and heating block temperature of 400 °C. The nebulizer gas (N2) flow rate was 3 L/min, and the heater gas (air) flow rate was 10 L/min. Collision-induced dissociation (CID) was performed using argon (Ar) at a pressure of 270 kPa and a flow rate of 0.3 mL/min. The capillary voltage was set at 3 kV, and the mass scan range was m/z 100–1900. Metabolites were identified based on retention times, ultraviolet spectra, and mass spectral data, in comparison with reference standards, previously isolated compounds, and published literature data [33,34,35,36,37,38,39]. Data acquisition and processing were performed using LabSolution workstation software ver. 2.01 (Shimadzu) equipped with an internal LC–MS library. For the quantification of compounds 1–10, 12, 13, 15, 19–27, 29–33, 35, 39, 40, 44 calibration curves were constructed using a series of commercially available reference standard solutions at concentrations ranging from 1 to 100 μg/mL, each analyzed in triplicate under the chromatographic conditions described above (Table S1). Based on the obtained results, calibration curves plotting concentration (μg/mL) versus peak area were generated using Advanced Grapher 2.2 software (Alentum Software, Inc., Glen Allen, VA, USA). Compounds 11, 14, 16–18, 28, 34, 36–38, 41–43, 45–54 for which commercially available reference standards were not available were quantified using structurally similar reference standards and conversion factors in calculation formulas (Table S1).

2.5. Plant Extract Preparation

LC–MS profiling and quantitative analysis were performed on R. adamsii extracts prepared from 100 mg of powdered flowers extracted with 10 mL of methanol (MeOH) by sonication at 40 °C for 30 min (ultrasound power 400 W, frequency 40 kHz), followed by centrifugation (9000× g, 15 min). The supernatant was passed through a 0.22 μm syringe filter into a 10 mL volumetric flask, and MeOH was added to reach the final volume. Internal standards, cucumoside L and phlojodicarpin, were added as methanolic solutions to achieve a final concentration of 10 μg/mL in each sample. Flower decoctions of R. adamsii were prepared from 1 g of unground, hand-ground, or pulverized flowers by infusion with 100 mL of boiling water for 20 min, followed by filtration through a cellulose filter into a 100 mL volumetric flask and adjustment to volume with water. Each decoction series had fivefold replication.

2.6. Monosaccharide Analysis of the HPLC Eluates

Using the chromatographic separation conditions described above (Section 2.4), 1 mL of the MeOH extract was separated, and the eluate corresponding to retention times of 12.80–12.84 min was collected. The obtained fraction was concentrated to dryness under reduced pressure, redissolved in 500 μL of 2 M trifluoroacetic acid (TFA), and hydrolyzed by heating at 100 °C for 4 h. The monosaccharide composition was subsequently determined after derivatization with 3-methyl-1-phenyl-2-pyrazolin-5-one using HPLC-UV analysis [28].

2.7. Microcolumn Fast HPLC–UV Assay with Precolumn Incubation with DPPH and Fe2+ Ions

A decoction of R. adamsii (pulverized sample, 100 μL) was mixed with a methanolic solution of DPPH• (0.6%, 50 μL), incubated for 15 min at 20 °C, and centrifuged at 9000× g for 15 min. The resulting supernatant was subjected to chromatographic separation by microcolumn fast HPLC-UV using a MiliChrom A-02 microcolumn liquid chromatograph (EcoNova, Novosibirsk, Russia) coupled with a UV detector 190–360 EcoNova (EcoNova, Novosibirsk, Russia) and a ProntoSIL-120-5-C18 AQ column (50 × 1 mm, 1 μm; Metrohm AG, Herisau, Switzerland) maintained at 30 °C. Separation was performed using a two-phase gradient elution system consisting of eluent A (0.2 M LiClO4 in 0.01 M HClO4) and eluent B (0.01 M HClO4 in acetonitrile), programmed as follows: 0.0–26.6 min, 5–100% B; 26.6–28.6 min, 100% B. The injection volume was 1 μL, and the flow rate was set at 150 μL/min. Chromatograms were recorded at 270 nm. The control sample without DPPH• treatment was diluted with MeOH (1:1) prior to separation. Chromatograms obtained before and after DPPH• treatment were overlaid and compared; a decrease in the chromatographic peak area was interpreted as evidence of the radical-scavenging activity of the compounds corresponding to the affected peaks. Precolumn incubation with Fe2+ ions was performed using a similar procedure with a 0.2% aqueous FeSO4 solution.

2.8. Statistical and Multivariate Analysis

Statistical analyses were performed using one-way analysis of variance (ANOVA) with Statistica software (ver. 12.6, Dell, Round Rock, TX, USA), and mean differences were evaluated using Fisher’s least significant difference (LSD) test at α = 0.05. Statistical significance was established at p < 0.05. Data are presented as means ± standard deviation (S.D.). Linear regression analysis and calibration curve construction were performed using Advanced Grapher 2.2 (Alentum Software, Inc., Ramat-Gan, Israel). Principal component analysis (PCA) based on a data matrix comprising 54 markers and 16 samples was performed using the XLSTAT Software 24.5.1386.0 for Microsoft Excel (Lumivero, Denver, CO, USA) to visualize clustering patterns among sample groups.

3. Results and Discussion

3.1. Chemical Composition of Rhododendron adamsii Flowers and Their Radical Scavenging Activity

Preliminary chemical analysis of R. adamsii flowers collected from 16 regions of Siberia, Asia, and the Far East demonstrated that the principal phytochemical constituents are essential oils, lipids, phenolic compounds, and polysaccharides, with contents ranging from 0.05 to 0.53%, 0.63–2.93%, 2.38–16.83%, and 2.08–9.63%, respectively (Table 2 and Table S2). Within the phenolic fraction, flavonols, dihydroflavonols, catechins, and procyanidins were present at levels of 1.93–11.43%, 0.37–3.81%, 0.01–0.12%, and 0.001–0.031%, respectively, indicating the predominant contribution of flavonols and dihydroflavonols. Antioxidant activity, evaluated by DPPH radical scavenging capacity (IC50 2.39–14.53 μg/mL), revealed that R. adamsii flower extracts possess high antioxidant potential, comparable to or exceeding that of the reference standard Trolox (IC50 11.63 μg/mL).
Further analysis of the results using correlation analysis demonstrated the highest correlation coefficients (r2) and the lowest p-values (<0.05) for the pairs “compound content–DPPH value” were observed for “total phenolic content–DPPH value” (r2 = 0.9956; p-value 6.70 × 10−18), “flavonol content–DPPH value” (r2 = 0.9910; p-value 1.01 × 10−15), and “dihydroflavonol content–DPPH value” (r2 = 0.9621; p-value 2.39 × 10−11), indicating the predominant contribution of these compounds to the observed bioactivity.
High levels of phenolic (PC) and flavonoid (FC) contents have also been reported in other Rhododendron species, such as R. arboreum (extracts: PC 3.6–6.7%, FC 4.9–6.7%) [14] and R. myrtifolium (extract: PC 2.1%, FC 0.9%) [15]. This characteristic accumulation has been associated with pronounced antioxidant activity of the extracts, including significant antiradical potential. Juices and wines prepared from R. arboreum flowers exhibited DPPH scavenging activity with IC50 values of 11.3–27.7 μg/mL [16], whereas water, ethanol, and acetone extracts were less active (IC50 646–926 μg/mL) [14]. The high radical-scavenging activity was reported for R. myrtifolium extract [15], indicating notable antioxidant potential; similar activity has been described for ethanol flower extracts of R. campanulatum (IC50 12.9 μg/mL) and R. anthopogon (IC50 265 μg/mL) [40]. The comparatively higher flavonoid content in R. adamsii flowers appears to contribute to their more efficient radical-scavenging capacity relative to previously studied Rhododendron species.

3.2. LC-MS Profiling of Flavonoids in R. adamsii Flowers

Application of LC–MS profiling to 16 samples of R. adamsii flowers revealed the presence of 23–54 phenolic compounds per sample (Table S3), indicating substantial variation in the phenolic profile depending on the collection site (Figure 2). Based on chromatographic and spectral data, the majority of the identified metabolites were flavonoids (49 compounds), whereas only five non-flavonoid components were detected (Table 3). Among the flavonoids, dihydroflavonols (6 compounds) and flavonols (41 compounds) predominated, while flavones and flavan-3-ols were represented by only a small number of compounds.

3.2.1. Dihydroflavonols: Taxifolin and Glycosides

In the dihydroflavonol group, only taxifolin (dihydroquercetin, Tax) derivatives were detected in R. adamsii flowers, occurring both in the free aglycone form (22) and as glycosides (3, 11, 13, 14, 15). Comparison with reference standards enabled the identification of Tax 3-O-glucoside (glucodistylin, 3), Tax 3′-O-glucoside (13), and Tax 3-O-arabinopyranoside (15), whereas 11 and 14 were tentatively assigned as Tax O-hexoside (m/z 465 → 303) and Tax O-pentoside (m/z 435 → 303), respectively [41]. With the exception of the aglycone, none of the identified Tax derivatives have previously been reported in R. adamsii flowers; however, 15 has been identified in R. ferrugineum [42], Rhododendron mucronulatum [43], and in 22 Chinese Rhododendron species [44]. Tax derivatives were previously detected in R. adamsii leaves, predominantly as rhamnosides, glucuronides, and various non-acylated and acylated oligoglycosides [2], suggesting distinct metabolic pathways of Tax in the leaves and flowers of R. adamsii.

3.2.2. Flavonols: Myricetin and Glycosides

Myricetin (Myr) was identified in R. adamsii flowers both as the aglycone (31) and as glycosides of non-acylated and acylated types. The compounds Myr 3-O-rutinoside (7), Myr 3-O-glucoside (isomyricitrin, 9), Myr 3-O-galactoside (gmelinoside I, 12), Myr 3-O-arabinopyranoside (19), and Myr 3′-O-glucoside (cannabiscitrin, 23) were identified through comparison of chromatographic and spectral characteristics with reference standards. In previous investigations of leaf constituents, compounds 7, 9, and 12 were also detected in R. adamsii [2]. Compound 18 exhibited identification data closely resembling those of 19, suggesting it may correspond to either Myr 3-O-arabinofuranoside or Myr 3-O-xyloside, both known plant flavonoids [45] that have not previously been reported in Rhododendron species.
Compound 28 exhibited mass spectral fragmentation consistent with a Myr O-pentoside (m/z 449 → 317); however, its UV spectrum displayed a pronounced long-wavelength band I at approximately 370 nm, characteristic of Myr 3′-O-glucosides [46]. Substitution at ring B was further supported by the longer retention time of 28 (t = 12.81 min) compared with 19 (t = 11.15 min), despite their similar mass spectral features, indicating greater lipophilicity of 28. Preparative HPLC isolation of fraction 28 followed by acid hydrolysis of the eluate revealed xylose as the only monosaccharide. These data allow the preliminary assignment of 28 as Myr 3′-O-xyloside, although further structural elucidation is required to confirm the sugar ring configuration. A structurally related compound, Myr 3′-O-β-D-xylopyranoside, has been previously reported from two Rhododendrons as R. ferrugineum [47] and R. tomentosum [45]. The remaining myricetin derivatives were identified as acetylated O-pentosides based on characteristic mass spectral losses of acetyl groups, corresponding to the elimination of one acetyl moiety (m/z 491 → 449; 47, 48) or two acetyl moieties (m/z 533 → 491, 449; 50, 52, 53) [48]. Accordingly, 47 and 48 were tentatively assigned as Myr O-pentoside O-acetates, while 50, 52, and 53 were characterized as Myr O-pentoside di-O-acetates. Compound 28 is considered the most probable parent structure owing to its relatively high abundance compared with other Myr O-pentosides; however, definitive structural confirmation requires further investigation. While simple Myr 3-O-glycosides have previously been reported in other Rhododendron species [1], the occurrence of 3′-O-glycosides and their acylated derivatives is described here for the first time in the genus.

3.2.3. Flavonols: Quercetin and Glycosides

Quercetin (Que, 40), the most common flavonol among studied Rhododendron species [49], has been consistently reported in R. adamsii in previous studies [2,10,11,12] and was also detected in this profiling analysis. In addition to the aglycone, the plant contains several glycosides previously described, including Que 3-O-glucoside (isoquercitrin) [2], Que 3-O-galactoside (hyperoside, 21) [2,50], Que 3-O-arabinofuranoside (avicularin, 24) [2,10], and Que 3-O-rhamnoside (quercitrin, 27) [2,10,50]. Furthermore, several glycosides were identified for the first time in R. adamsii flowers, including Que 3-O-arabinopyranoside (guaijaverin, 25), Que 3-O-xyloside (reynoutrin, 26), Que 4′-O-glucoside (spiraeoside, 33), and Que 3′-O-glucoside (35). Notably, some of these compounds have previously been reported in other Rhododendron species, such as 25 in R. luteum [51] and 26 in R. anthopogonoide [52].
Compounds 16 and 17 were identified as Que O-pentosides (m/z 433 → 301); however, their shorter retention times (t = 10.53–10.77 min) compared with those of the knpwn 3-O-pentosides 24–26 (t = 12.24–12.53 min) suggest that the sugar moiety is likely attached at C-5 or C-7 [53]. In contrast, Que O-pentosides 36 and 37 exhibited longer retention times (t = 15.79–15.98 min), indicating increased lipophilicity, which is consistent with substitution on ring B at positions C-3′ or C-4′ [54]. Compounds 34 (Que O-hexoside; m/z 463 → 301; t = 15.51 min) and 38 (Que O-deoxyhexoside; m/z 447 → 301; t = 16.47 min) also eluted later than their respective 3-O-analogs. Considering the chromatographic proximity of 34 to Que 4′-O-glucoside (33) and the previously established position of Que 3′-O-glucoside (35), 34 can reasonably be proposed as Que 4′-O-galactoside [45] because alternative positional isomers appear unlikely.
The group of acetylated quercetin monoglycoside derivatives was detected in the chromatographic region characterized by high retention times. This group included esters of Que O-deoxyhexoside bearing one acetyl group (m/z 489 → 447, 301; 45, 46) and two acetyl groups (m/z 531 → 489, 447, 301; 49, 51), as well as one Que O-pentoside di-O-acetate (54; m/z 517 → 475, 433, 301). Compounds with such structural characteristics have not previously been reported in the genus Rhododendron, highlighting the importance of detailed structural elucidation of these acylated glycosides for a more comprehensive understanding of the genus chemodiversity.

3.2.4. Flavonols: Kaempferol and Glycosides

The presence of kaempferol (Kae, 44) and seven of its glycosides was established in R. adamsii flowers, including known plant flavonoids such as Kae 3-O-arabinofuranoside (juglanin, 29), Kae 3-O-arabinopyranoside (30), Kae 3-O-rhamnoside (afzelin, 32), and Kae 4′-O-glucoside (39). Some of these flavonols, including 29 and 32, were previously detected in R. adamsii leaves [2], and free kaempferol is considered a characteristic phenolic constituent of the genus Rhododendron [1]. Three additional compounds were tentatively identified as Kae O-pentosides (41, 42) and a Kae O-deoxyhexoside (43). Given their longer retention times compared with the corresponding 3-O-glycosides, it is reasonable to assume that the carbohydrate moiety is attached to ring B, which in the case of kaempferol suggests substitution at the 4′-O position [54]. Although kaempferol pentosides (arabinosides and xylosides) with this substitution pattern have not yet been reported from plants, a deoxyhexoside in the form of Kae 4′-O-rhamnoside has previously been isolated from Phyllanthus niruri [55].

3.2.5. Single (Non-Diverse) Flavonoids

One flavone, vitexin 2″-O-rhamnoside (6), and one flavan-3-ol, (+)-catechin (8), were identified in their respective categories in R. adamsii flowers. Although flavone C-glycosides are generally uncommon in the genus Rhododendron compared with flavones and dihydroflavones, they may occur sporadically in flowers and leaves, as previously reported for vitexin and isovitexin in R. luteum [56] and vitexin in R. amesiae [57]. Catechins, in contrast, are widely distributed constituents in the genus Rhododendron as a whole [1].

3.2.6. Non-Flavonoid Components

Five non-flavonoid compounds were detected in R. adamsii flower samples and identified as five hydroxycinnamate derivatives, namely 4-O-caffeoylquinic acid (1), 5-O-caffeoylquinic acid (5), 1-O-caffeoylglucose (2), and 1-O-feruloylglucose (4), and 5-O-p-coumaroyl-quinic acid (10). Compounds 1 and 5 have previously been reported in R. adamsii leaves [2], whereas cinnamoyl glucoses (2, 4) and acid 10 are described here for the first time in this species.

3.2.7. Occurrence of Compounds in R. adamsii Populations

All identified compounds exhibited variable occurrence across the analyzed R. adamsii flower samples (Table S3). Twenty compounds were detected in all samples (100% occurrence), 19 occurred in 50–100% of samples, and 15 were present in fewer than half of the analyzed samples. The most consistently detected phenolic constituents of R. adamsii flowers included 5-O-caffeoylquinic acid, the majority of 3-O-glycosides of Tax, myricetin, and quercetin, as well as flavonol aglycones. In contrast, the rarest compounds (12.5–18.8% occurrence) were 4-O-caffeoylquinic acid, Que O-pentoside 16, Kae O-pentoside 42, Kae O-deoxyhexoside 43, and free kaempferol. Given the significant geographic and environmental variability among the collection sites, the observed differences in compound occurrence are likely influenced by ecological factors. Overall, HPLC profiling of R. adamsii flowers revealed 54 compounds, 37 of which are reported for the first time in this species, indicating the presence of notable (bio)chemical differences between the flowers and the previously studied leaves, which have been profiled in several earlier investigations [2,10,11,12].

3.3. Quantitative Analysis of Flavonoids in Rhododendron adamsii Flowers

Quantitative analysis demonstrated substantial variation among R. adamsii flower samples in the content of individual compounds and compound groups (Table 4 and Table S3). The highest levels of total phenolic compounds (TPCs) and total flavonoids (TFCs) were observed in samples collected from Buryatia (TPC 139.60–155.82 mg/g DW; TFC 131.31–147.54 mg/g DW). Comparable values were recorded for samples from Siberia (TPC 109.05–127.16 mg/g DW; TFC 105.67–120.14 mg/g DW), whereas the lowest concentrations were detected in flowers collected from Mongolia (TPC 18.74–28.83 mg/g DW; TFC 16.53–26.52 mg/g DW) (Table S3).
The total Tax content in R. adamsii flowers ranged from 2.51 to 35.17 mg/g DW, including 2.40–33.90 mg/g DW of total Tax glycosides, of which Tax 3-O-arabinopyranoside accounted for 2.17–29.23 mg/g DW. These levels in samples No 1–13 (11.18–35.17 mg/g DW) exceed those reported for the well-known Tax accumulator Larix sibirica, in which the compound concentration in various tissues ranges from 10 to 25 mg/g DW [58].
The total Myr content in R. adamsii flowers ranged from 4.68 to 42.65 mg/g DW, largely attributable to the high levels of non-acylated Myr glucosides (4.08–31.87 mg/g DW), with lower contributions from acylated Myr glucosides (0.10–13.92 mg/g DW) and free Myr (0.52–3.82 mg/g DW). The predominant derivative was Myr 3′-O-xyloside, present at 3.54–27.77 mg/g DW and accounting 51.9–75.6% of the total Myr content in the flower samples No 1–16. Common dietary sources of Myr include black tea (0.30 mg/g DW), guava (0.55 mg/g DW), garlic (0.70 mg/g DW), and semambu leaves (0.85 mg/g DW) [59]. Even the samples with the lowest Myr content in this study exceeded these values, underscoring the potential dietary significance of R. adamsii flowers.
Que derivatives represent one of the most prominent flavonoid groups in R. adamsii flowers, with a total concentration ranging from 9.08 to 64.66 mg/g DW, comprising predominantly non-acylated glucosides (8.89–60.72 mg/g DW), smaller amounts of acylated glucosides (0–5.08 mg/g DW), and minor levels of the aglycone (0.10–1.73 mg/g DW). Four major quercetin derivatives—isoquercitrin (2.11–10.51 mg/g DW), hyperoside (1.02–4.73 mg/g DW), reynoutrin (1.72–9.21 mg/g DW), and quercitrin (2.84–28.52 mg/g DW)—collectively account for approximately 82% of the total Que content. High Que derivatives levels have been reported in buckwheat herb (0.2–0.4% DW) [60,61], a raw material for the industrial production of flavonoids, as well as in Allium fistulosum leaves (1.5 mg/g DW), black tea (1.1 mg/g DW) [59] and strawberry (8.08 mg/g DW) [62], supporting R. adamsii flowers characterization as a rich natural source of Que.
Kae derivatives, unlike other flavonoid classes, are present in R. adamsii flowers in relatively low amounts, ranging from 0.22 to 4.24 mg/g DW, predominantly as non-acylated Kae glucosides (0.22–4.23 mg/g DW). Although dietary sources rich in Kae are limited, notable examples include onion leaves (0.8 mg/g DW), papaya shoots (0.5 mg/g DW), and lemongrass (0.2 mg/g DW) [59], suggesting that R. adamsii flowers may serve as a valuable alternative resource of this flavonoid.
Catechins, apigenins, and non-flavonoid components, together accounted for less than 10 mg/g, representing no more than 7% of the total phenolic content. Overall, when considering R. adamsii flowers as a source of dietary flavonoids, they can be characterized as Que-, Myr-, and Tax-rich plant material.
Application of PCA to the quantitative data revealed significant differences in the distribution of compounds among the various populations of R. adamsii (Figure 3, Table S4). The results indicate the presence of at least three distinct clusters, each characterized by a specific floral phenolic profile.
The largest cluster (Cluster I) comprises samples characterized by the greatest diversity of compounds and the highest concentrations of individual constituents. This cluster includes plants collected in Eastern Siberia (Republic of Buryatia), the central part of the Republic of Sakha (Yakutia), Krasnoyarsk Krai, as well as the Magadan Region and Zabaikalsky Krai in the latitude/longitudes range 51–63° N/101–152° E, respectively. In contrast, four samples, whose markers are positioned close to the F1 axis, form a compact Cluster II originating from areas closer to the northern limits of the Eurasian distribution range of R. adamsii (lat./long. 70–72° N/101–128° E) exhibited considerable changes in their floral phenolic profiles. These changes were characterized by an increased proportion of lipophilic compounds, primarily acylated flavonol O-glycosides, along with a decreased concentration of non-acylated flavonol and dihydroflavonol monoglycosides. The lower left region of the PCA biplot corresponds to Cluster III, which comprises populations from Mongolia, Tuva, and southern Krasnoyarsk Krai (lat./long. 46–52° N/93–101° E). These populations are characterized by a low content or complete absence of lipophilic components and an overall reduced level of flavonoids.
The pronounced differences in chemical composition among the samples are likely attributable to variations in the natural environments where the plants grow. The climate corresponding to Cluster II can be characterized as harsh and Arctic, typical of the far northern regions. Permafrost predominates, shaping the annual weather pattern: winters are prolonged and extremely cold, whereas summers are brief and cool. Seasonal dynamics are strongly skewed toward winter, with a very short warm period during which the average temperature reaches only approximately +9 °C. Precipitation is low, which is typical for the tundra zone. The relatively high content of lipophilic flavonoids reflects the plant’s adaptation to withstand extreme conditions during growth, flowering, and fruiting, enhancing its survival [63]. Cluster III, located inland, experiences low precipitation and high insolation, which impacts the overall phenolic profile and reduces the production of these compounds [64]. In contrast, cluster I exhibits optimal growth conditions, characterized by a continental climate with moderate climatic parameters. The absence of extreme environmental stress in these areas may favor the accumulation of higher concentrations of phenolic compounds. Although additional data are required to make definitive conclusions, the preliminary analysis supports the trends described above.

3.4. Effect of Grinding Rhododendron adamsii Flowers on Flavonoid Content and Antioxidant Activity of Tea Extracts

The primary method of consuming R. adamsii flowers is in the form of a decoction, a practice recommended by ancient physicians and preserved to this day as the simplest and most accessible preparation method. Despite its apparent simplicity, the manner in which the plant material is processed prior to brewing may influence the final product. In this study, three forms of raw material were examined: unground (UG), hand-ground with a particle size of 1–3 mm (HG), and pulverized with a particle size of <0.1 mm (PV). The final decoctions are expected to vary in chemical composition owing to differences in the degree of grinding of the raw materials, which will, in turn, influence their biological activity. Therefore, we investigated decoctions prepared from R. adamsii flowers with three different types of grinding and subsequently performed a comparative evaluation of their antioxidant properties using in vitro methods. This type of activity was chosen to assess beneficial properties because of the high flavonoid content, known for their antioxidant effects [14,15,16], and the established use of flower teas as a source of antioxidants [65,66].
The data on the content of 16 flavonoids in decoctions prepared from differently ground flowers show different levels of the target compounds. The total flavonoid content in tea decoctions prepared from UG, HG, and PV R. adamsii flowers was 88.37, 95.01, and 140.01 mg/100 mL, respectively (Table 5). These results confirm previous findings regarding the positive effect of reduced particle size on the yield of extractive substances into the extraction medium [67].
The main compounds identified in R. adamsii tea decoctions were Tax 3-O-arabinopyranoside (mg/100 mL: 29.27 UG, 30.42 HG, 39.95 PV), Que 3-O-rhamnoside (19.08 UG, 19.74 HG, 28.25 PV), Myr 3′-O-xyloside (16.69 UG, 17.53 HG, 25.54 PV), Que 3-O-glucoside (6.04 UG, 6.53 HG, 12.63 PV), and Que 3-O-xyloside (5.36 UG, 6.34 HG, 9.37 PV). A notable feature of the aqueous decoctions was the limited transfer of acylated flavonol glycosides (41, 52–54) into the extract, which can be attributed to the high polarity of water and the relatively low solubility of lipophilic flavonoids in this solvent. It is also noteworthy that decoctions prepared from UG and HG flowers exhibited comparable quantitative profiles, likely reflecting the presence of flavonoids not only in cellular structures but also on the flower surface. Although decoctions prepared from PV material contained the highest flavonoid concentrations, UG tea still demonstrated substantial flavonoid levels compared with other flower-based teas [65,66] and may therefore be considered suitable for regular consumption as a dietary source of phenolic compounds.
The antioxidant potential of R. adamsii flower decoctions was evaluated using antiradical assays against DPPH•, ABTS•+, DMPD•, OH•, O2•−, Cl•, and Br• radicals, as well as NO scavenging and FeCA assays (Table 6). The IC50 values obtained for artificial radicals ranged from 3.05 to 5.12 μg/mL for DPPH•, 2.11 to 3.90 μg/mL for ABTS•+, and 38.11 to 59.34 μg/mL for DMPD•. Assays involving naturally occurring radicals showed IC50 values of 9.63–14.08 μg/mL for OH• and 52.69–83.35 μg/mL for O2•−. The inactivation capacity toward inorganic radicals was 450.09–537.14 mg Trolox equivalents/g for Cl• and 408.20–509.28 mg Trolox equivalents/g for Br•. NO scavenging activity ranged from 0.52 to 0.81 mg/mL, while FeCA was 1.87–2.59 mM Fe2+/g. Compared with the standard antioxidant Trolox, all measured antioxidant parameters except chlorine radical (Cl•) and bromine radical (Br•) scavenging capacity were similar to or higher than those of the reference. Among the decoctions, PV exhibited the highest activity, while UG showed the lowest, likely owing to the higher flavonoid content in the PV decoction.
Separation of R. adamsii flower decoction by microcolumn fast HPLC-UV following precolumn incubation with Fe2+ ions and DPPH• radical solution enabled on-line monitoring of antioxidant interactions and revealed a rapid decrease in peak areas for most phenolic compounds, particularly flavonoids, compared with the untreated sample (Figure 4).
The compounds exhibiting the greatest reduction in peak area (>90% within 5 min) included quercetin 3-O-rhamnoside (27), myricetin 3′-O-xyloside (28), Tax 3-O-arabinopyranoside (15), quercetin 3-O-glucoside (20), quercetin 3-O-galactoside (21), myricetin 3′-O-glucoside (23), quercetin 3-O-arabinosides (24, 25), quercetin 3-O-xyloside (26), and the non-flavonoid 5-O-caffeoylquinic acid (5). These findings indicate that 3-O- and 3′-O-glycosides of quercetin, myricetin, and Tax are primarily responsible for metal ion chelation and free radical inactivation, thereby making the predominant contribution to the metal-binding and antiradical activity of the decoction.
Available information on the antioxidant properties of Rhododendron flowers remains limited; however, existing data indicate a direct relationship between phenolic compounds, particularly flavonoids, and the expression of biological activity. Early studies on R. arboreum flower extracts reported DPPH IC50 values ranging from 11 to >900 μg/mL [14,16,68]. Flavonoids isolated from R. yedoense flowers demonstrated pronounced antiradical activity against DPPH• and O2•— radicals, with quercetin and myricetin derivatives identified as the most active constituents [17]. Similarly, extracts from R. ungernii flowers exhibited strong DPPH• and ABTS•+ scavenging activity, primarily attributed to myricetin and quercetin 3-O-glycosides [22]. Antioxidant flavonoids of R. mucronulatum, detected using an on-line HPLC–ABTS•+ assay, were isolated and identified as myricetin, quercetin, and kaempferol [69]. The incorporation of R. myrtifolium flowers into yogurt has been shown to enhance the product’s antiradical activity in the DPPH assay, thereby improving its shelf life during 28 days of storage [15]. Overall, the findings obtained for R. adamsii flowers are consistent with the general understanding that phenolic compounds make a substantial contribution to antioxidant capacity.

3.5. Rhododendron adamsii Flower Tea: Outlook and Recommendations

Studies have demonstrated that R. adamsii flowers are capable of accumulating high levels of flavonoids, reaching up to 14.7%, including various derivatives of quercetin, myricetin, Tax, and kaempferol, which may be consumed as dietary constituents in tea prepared from this plant. More broadly, species of the genus Rhododendron are used in the food industry and in domestic practice for the preparation of flower-based teas, including those from R. arboreum [14,16,68], R. mucronulatum [69], R. myrtifolium [15], R. yedoense [17], and R. ungernii [22], which are reported to possess various beneficial properties such as antioxidant and anti-inflammatory activities. Given the wide distribution of Rhododendron species and the low toxicity of their flower aqueous extracts (compared to leaves and shoots) [1], flower teas from many species are likely used in various cuisines worldwide. However, information on their use is limited, highlighting the need for further documentation and study of their beneficial properties. The flowers of R. adamsii provide a representative example of locally valued plant material: widely recognized and traditionally consumed as a tea in Siberia and Mongolia, yet largely unknown to populations in other regions of the world.
The high flavonoid content of the prepared flower decoction supports the consideration of R. adamsii as a valuable additional source of phenolic compounds. Although no official guidelines exist for flavonoid intake, an epidemiologically associated beneficial intake level of at least 500 mg is generally considered beneficial [70,71], which can be achieved by drinking four cups of R. adamsii flower tea per day. This places R. adamsii tea on a similar level to traditional black and green teas, which contain an average of 50–150 mg of flavonoids per 100 mL [72].
Currently, the primary source of R. adamsii flowers is wild natural populations distributed in the subalpine zone and, less frequently, in alpine areas and the upper forest belt. The species predominantly grows on carbonate substrates, rocky slopes, scree lichen tundra, and exposed rocks of Siberia and Mongolia. Such habitats complicate large-scale harvesting and increase the cost of the final product. The ontogenetic cycle of R. adamsii is characterized by the development of perennial orthotropic shoots and plagiotropic branches arising from the main axes, with annual shoot increments of approximately 7–10 cm. These shoots grow over a period of 5–6 years until reaching a height of 25–30 cm. Subsequently, shoot growth ends with the formation of a terminal inflorescence, followed by branching and crown development. The main growth cycle comprises three stages (up to 5 years, up to 15 years, and up to 25 years) during which the shoot system becomes more complex. This development leads to the lodging of older shoots, which spread along the slope, take root, and contribute to the formation of new shoots [73,74]. Attempts to introduce R. adamsii into cultivation have demonstrated that the species exhibits very slow growth [75]; however, cultivated plants are capable of reaching the fruiting stage and may serve as a potential source of raw material for tea production [76]. Considering the slow regeneration of natural populations of R. adamsii, the uncontrolled harvesting of raw material, often resulting in severe degradation or complete destruction of natural stands, raises considerable conservation concerns and underscores the need for more effective protective measures to preserve wild populations of this species. Accordingly, it is essential to advance research focused on the development of cultivated forms of R. adamsii, which would reduce pressure on natural habitats while enabling the production of a more accessible and economically feasible raw material for tea products.

4. Conclusions

Flowers of R. adamsii were investigated by LC–MS profiling in combination with antioxidant activity assays in the first study aimed at characterizing the flavonoid composition and bioactivity of this species. Among the fifty-four compounds identified in sixteen samples, forty-nine were flavonoids, predominantly glycosides of quercetin, myricetin, Tax, and kaempferol. Habitat conditions considerably influenced both the qualitative composition and quantitative levels of flavonoids, with the greatest diversity and highest concentrations observed in samples from Siberia. The principal flavonoids detected in R. adamsii flowers were Tax 3-O-arabinopyranoside, quercetin 3-O-rhamnoside, myricetin 3′-O-arabinoside, quercetin 3-O-glucoside, and quercetin 3-O-xyloside. The composition of the aqueous decoction was affected by the degree of grinding of the raw material, with PV samples yielding the highest flavonoid content compared with UG and HG material. The increased flavonoid concentration in the decoctions corresponded to increased antioxidant activity, as demonstrated by in vitro antiradical assays, NO scavenging, and ferrous ion chelation tests. Overall, the results indicate that R. adamsii flowers represent a promising new source of diverse flavonoids with antioxidant properties that may contribute to meeting the body’s demand for beneficial bioactive compounds.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/horticulturae12040484/s1: Table S1: Regression equations, correlation coefficients, standard deviation, limits of detection, limits of quantification, and linear ranges for reference standards; Table S2: Content of essential oil, lipids, total phenolics, flavonols, dihydroflavonols, catechins, procyanidins, polysaccharides and radical-scavenging activity of Rhododendron adamsii flowers from 16 sites; Table S3: Content of compounds 1–54 in Rhododendron adamsii flowers from 16 sites and compounds occurrence; Table S4: PCA raw data.

Author Contributions

Conceptualization, methodology, software, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, writing—review and editing, visualization, supervision, project administration, funding acquisition, D.N.O., N.I.K. and N.K.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Education and Science of Russia, grant numbers 126021217180-7; FSRG-2026-0010.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors have reviewed and edited the output and take full responsibility for the content of this publication. The authors are very thankful to Igor Pospelov (Institute of Ecology and Evolution Problems of the Russian Academy of Sciences, Federal State Budgetary Institution “Taimyr Nature Reserves”) for kindly providing the photo of R. adamsii (https://www.inaturalist.org/observations/40298478; accessed on 28 February 2026).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
ABTS•+2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) radicals
Br•Bromine radicals
Cl•Chlorine radicals
DMPD•N,N-dimethyl-p-phenylenediamine dihydrochloride radicals
DPPH•2,2-diphenyl-1-picrylhydrazyl radicals
DWDry weight
FeCAFe2+-chelating activity
HGHand-ground
HPLC-PDA-IT-TOF-MSHigh-performance chromatography with photodiode array and ion trap-time-of-flight mass spectrometry detection
HPLC-UVHigh-performance chromatography with ultraviolet detection
KaeKaempferol
LC-MSLiquid chromatography-mass spectrometry
MyrMyricetin
NONitric oxide (II)
OH•Hydroxyl radicals
O2•−Superoxide radicals
PCAPrincipal component analysis
PVPulverized
QueQuercetin
TaxTaxifolin
TFCsTotal flavonoids content
TPCsTotal phenolic compounds content
UGUnground

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Horticulturae 12 00484 g001
Figure 1. Rhododendron adamsii flowering shrub in its natural habitat (a), sagan-da-li tea decoction with unground, hand-ground, and pulverized dry flowers (b).
Figure 1. Rhododendron adamsii flowering shrub in its natural habitat (a), sagan-da-li tea decoction with unground, hand-ground, and pulverized dry flowers (b).
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Horticulturae 12 00484 g002
Figure 2. HPLC-PDA chromatogram of R. adamsii flower extract (sample No. 12) with the most diverse HPLC profile at 270 nm. Compounds 1–54 description shown in Table 3. IS—internal standards (IS-1—cucumoside L; IS-2—phlojodicarpin).
Figure 2. HPLC-PDA chromatogram of R. adamsii flower extract (sample No. 12) with the most diverse HPLC profile at 270 nm. Compounds 1–54 description shown in Table 3. IS—internal standards (IS-1—cucumoside L; IS-2—phlojodicarpin).
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Horticulturae 12 00484 g003
Figure 3. Results of principal component analysis (PCA) used the content of 54 phenolic compounds in the 16 sites of R. adamsii flowers.
Figure 3. Results of principal component analysis (PCA) used the content of 54 phenolic compounds in the 16 sites of R. adamsii flowers.
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Horticulturae 12 00484 g004
Figure 4. Microcolumn fast HPLC-UV chromatograms of R. adamsii flower decoction (sample No. 12) at 270 nm before (A) and after precolumn incubation with FeSO4 (B) and DPPH• (C). Compounds description shown in Table 2.
Figure 4. Microcolumn fast HPLC-UV chromatograms of R. adamsii flower decoction (sample No. 12) at 270 nm before (A) and after precolumn incubation with FeSO4 (B) and DPPH• (C). Compounds description shown in Table 2.
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Table 1. R. adamsii sample description.
Table 1. R. adamsii sample description.
Site No.CoordinatesRegionCollection Place (Voucher No.)Height, a.m.s.l.Collection Year
No. 172°14′32.9″ N 126°57′26.6″ ESiberiaRepublic Sakha (Yakutia), Bulunskii ulus (YAK-ERI-0620-073/062)352020
No. 271°37′58.7″ N 128°53′13.3″ ESiberiaRepublic Sakha (Yakutia), Bulunskii ulus, Tiksi (YAK-ERI-0620-073/065)402020
No. 371°00′45.2″ N 127°27′34.6″ ESiberiaRepublic Sakha (Yakutia), Bulunskii ulus (YAK-ERI-0621-064/081)2802021
No. 472°07′18.1″ N 110°33′16.0″ ESiberiaKrasnoyarskii Krai, Dolgano-Nenetskii Region (KRA-ERI-0623-021/015)402023
No. 570°32′06.2″ N 101°19′16.8″ ESiberiaKrasnoyarskii Krai, Dolgano-Nenetskii Region (KRA-ERI-0618-021/003)2502018
No. 663°02′23.6″ N 138°23′40.4″ ESiberiaRepublic Sakha (Yakutia), Tomponskii ulus (YAK-ERI-0619-005/051)8202019
No. 763°21′18.2″ N 152°37′47.4″ EFar EastMagadan Oblast, Srednekanskii Region (MAG-ERI-0520-034/032)4702020
No. 856°31′02.8″ N 118°38′22.2″ ESiberiaZabaykalsky Krai, Kalarskii Region (ZAB-ERI-0523-071/064)16502023
No. 955°34′28.2″ N 113°38′10.5″ ESiberiaRepublic Buryatia, Bauntovskii Region (BUR-ERI-0522-031/016)15802022
No. 1051°17′18.3″ N 105°15′38.3″ ESiberiaRepublic Buryatia, Dzhidinskii Region (BUR-ERI-0517-037/054)20802017
No. 1151°55′55.8″ N 102°26′07.6″ ESiberiaRepublic Buryatia, Tunkinskii Region (BUR-ERI-0521-028/018)13702021
No. 1251°43′25.2″ N 101°00′03.9″ ESiberiaRepublic Buryatia, Tunkinskii Region (BUR-ERI-0519-018/023)20202019
No. 1351°58′14.0″ N 95°29′20.8″ ESiberiaRepublic Tyva, Kaa-Khemskii Region (TYV-ERI-0522-017/091)22502022
No. 1452°50′35.6″ N 93°22′38.1″ ESiberiaKrasnoyarskii Krai, Ermakovskii Region (KRA-ERI-0621-067/011)18002021
No. 1550°39′26.1″ N 99°19′08.3″ EMongoliaHovstgol, Ulaan-Uul (MON-ERI-0519-015/033)18502019
No. 1646°58′24.6″ N 101°16′54.9″ EMongoliaBulgan, Arkhangai (MON-ERI-0522-029/008)29502022
Table 2. Compound group content and antiradical potential (DPPH) of R. adamsii flowers, linear correlation data in pares “compound content-DPPH value” and p-values.
Table 2. Compound group content and antiradical potential (DPPH) of R. adamsii flowers, linear correlation data in pares “compound content-DPPH value” and p-values.
Compound GroupValue RangeLinear Correlation Data 1p-Value
Essential oil, %0.05–0.53y = −0.0171x + 0.4357; r2 = 0.21800.07
Lipids, %0.63–2.93y = −0.0894x + 2.3414; r2 = 0.28970.03
Total phenolics, %13.53–16.83y = −1.2377x + 19.595; r2 = 0.99566.70 × 10−18
Flavonols, %9.30–11.43y = −0.8478x + 13.482; r2 = 0.99101.01 × 10−15
Dihydroflavonols, %2.11–3.81y = −0.2737x + 4.2855; r2 = 0.96212.39 × 10−11
Catechins, %0.01–0.12y = −0.0057x + 0.0999; r2 = 0.33910.02
Procyanidins, %0.001–0.031y = −0.0009x + 0.0158; r2 = 0.20860.08
Polysaccharides, %2.08–9.63y = −0.2109x + 7.2613; r2 = 0.18310.10
DPPH, IC50, μg/mL2.39–14.53–
1 Data includes linear regression equation (y = ax + b) and correlation coefficient r2.
Table 3. Retention time (tR), molecular formula (MF), MS data, UV pattern (UVP), identification level (IL) and occurrence of compounds 1–54 found in R. adamsii flowers.
Table 3. Retention time (tR), molecular formula (MF), MS data, UV pattern (UVP), identification level (IL) and occurrence of compounds 1–54 found in R. adamsii flowers.
No.tR, minCompoundMF (Error, ppm)MS, [M–H]−, m/zMS 2,
m/z		UVP 1IL 2Early Found in R. adamsii Leaves [Ref.]Occurrence of Compound 3, %
14.904-O-Caffeoyl-quinic acid, trans-isomerC16H18O9 (1.1)353179C11 (SA, 65969, 98)Yes [23]18.8
25.141-O-Caffeoyl β-glucoseC15H18O9 (0.2)341179C11 (MCE, W416228, 97)No25.0
36.01Taxifolin 3-O-glucoside (glucodistylin)C21H22O12 (1.0)465303F11 (CF, CFN99389, 98)No43.8
46.421-O-Feruloyl β-glucoseC16H20O9 (0.7)355193C11 (Sy, FG509, 98)No56.3
56.655-O-Caffeoyl-quinic acid, trans-isomerC16H18O9 (1.1)353179C11 (SA, 94419, 98)Yes [23]100
67.27Vitexin 2′′-O-rhamnosideC27H30O14 (0.8)577431F21 (CF, CFN98177, 98)No56.3
77.91Myricetin 3-O-rutinosideC27H30O17 (1.0)625479, 317F31 (MN, M38196, 98)Yes [23]62.5
88.28(+)-CatechinC16H18O9 (1.2)289 F41 (SA, PHR1963, 95)Yes [23]62.5
98.33Myricetin 3-O-glucoside (isomyricitrin)C21H20O13 (1.7)479317F31 (SA, 9AD2400F, 98)Yes [23]75.0
108.575-O-p-Coumaroyl-quinic acid, trans-isomerC16H18O8 (0.7)337163C21 (SA, 935573, 95)No100
118.84Taxifolin O-hexosideC21H22O12 (1.2)465303F13 [33]No100
128.91Myricetin 3-O-galactoside (gmelinoside I)C21H20O13 (1.0)479317F31 (CF, CFN97817, 98)Yes [23]81.3
139.66Taxifolin 3′-O-glucoside C21H22O12 (0.5)465303F11 (CF, CFN96494, 98)No100
1410.02Taxifolin O-pentosideC20H20O11 (0.9)435303F13 [33]No100
1510.22Taxifolin 3-O-arabinopyranosideC20H20O11 (0.3)435303F11 (VI, 06344, 94)No100
1610.53Quercetin O-pentosideC20H18O11 (0.9)433301F52 [34]No12.5
1710.77Quercetin O-pentosideC20H18O11 (1.4)433301F52 [34]No31.3
1811.02Myricetin O-pentosideC20H18O12 (1.0)449317F32 [35]No31.3
1911.15Myricetin 3-O-arabinopyranosideC20H18O12 (1.7)449317F31 (MCE, N0581, 98)No100
2011.48Quercetin 3-O-glucoside (isoquercitrin)C21H20O12 (0.4)463301F61 (SA, 16654, 98)Yes [23]100
2111.46Quercetin 3-O-galactoside (hyperoside)C21H20O12 (0.2)463301F61 (SA, 00180585, 98)Yes [18,23,28,29,30]100
2211.78Taxifolin (dihydroquercetin)C15H12O7 (0.9)303 F11 (SA, PHL89284, 95)Yes [18,23,28,29,30]100
2311.94Myricetin 3′-O-glucoside (cannabiscitrin)C21H20O13 (1.1)479317F71 (CF, CFN95278, 98)No100
2412.24Quercetin 3-O-arabinofuranoside (avicularin)C20H18O11 (1.0)433301F61 (CF, CFN98961, 98)Yes [17,23,28,30]100
2512.53Quercetin 3-O-arabinopyranoside (guaijaverin)C20H18O11 (1.4)433301F61 (CF, CFN98211, 98)No100
2612.81Quercetin 3-O-xylopyranoside (reynoutrin)C20H18O11 (1.0)433301F61 (CF, CFN96236, 98)No100
2713.06Quercetin 3-O-rhamnoside (quercitrin)C21H20O11 (0.9)447301F61 (SA, PHL89346, 95)Yes [17,23,29]100
2812.81Myricetin 3′-O-xyloside (tent.)C20H18O12 (1.5)449317F72 [35]No100
2913.28Kaempferol 3-O-arabinofuranoside (juglanin)C20H18O10 (0.5)417285F81 (CF, CFN96238, 98)Yes [23]81.3
3014.05Kaempferol 3-O-arabinopyranosideC20H18O10 (0.9)417285F81 (BOC, NP1830, 95)No62.5
3114.21MyricetinC15H10O8 (1.2)317 1 (CF, CFN98877, 98)Yes [18,23,28,29,30]100
3214.63Kaempferol 3-O-rhamnoside (afzelin)C21H20O10 (0.7)431285F81 (CF, CFN98757, 98)Yes [30]81.3
3315.22Quercetin 4′-O-glucoside (spiraeoside)C21H20O12 (0.7)463301F61 (CF, CFN70300, 98)No62.5
3415.51Quercetin O-hexosideC21H20O12 (0.9)463301F62 [34]No37.5
3515.61Quercetin 3′-O-glucoside C21H20O12 (1.1)463301F61 (CF, CFN95271, 98)No37.5
3615.79Quercetin O-pentosideC20H18O11 (1.5)433301F62 [34]No31.3
3715.98Quercetin O-pentosideC20H18O11 (1.9)433301F62 [34]No37.5
3816.47Quercetin O-desoxyhexosideC21H20O11 (1.8)447301F62 [34]No37.5
3916.92Kaempferol 4′-O-glucosideC21H20O11 (1.0)447285F81 (FA, 004347, 95)No25.0
4017.09QuercetinC15H10O7 (0.2)301 F61 (SA, Q4951, 95)Yes [17,18,23,28,29,30]100
4117.33Kaempferol O-pentosideC20H18O10 (0.5)417285F82 [36]No100
4218.11Kaempferol O-pentosideC20H18O10 (1.7)417285F82 [36]No12.5
4319.20Kaempferol O-desoxyhexosideC21H20O10 (0.8)431285F82 [36]No18.8
4419.74KaempferolC15H10O6 (0.3)285 F81 (SA, 96353, 99)Yes [17,18,23,29]12.5
4526.53Quercetin O-desoxyhexoside O-acetateC23H22O12 (0.5)489447, 301F62 [37]No75.0
4627.89Quercetin O-desoxyhexoside O-acetateC23H22O12 (0.7)489447, 301F62 [37]No81.3
4727.93Myricetin O-pentoside O-acetateC22H20O13 (2.1)491449, 317F72 [35]No75.0
4828.20Myricetin O-pentoside O-acetateC22H20O13 (1.5)491449, 317F72 [35]No75.0
4928.69Quercetin O-desoxyhexoside di-O-acetateC25H24O13 (1.4)531489, 447, 301F62 [37]No75.0
5029.47Myricetin O-pentoside di-O-acetateC24H22O14 (1.1)533491, 449, 317F72 [35]No75.0
5129.63Quercetin O-desoxyhexoside di-O-acetateC25H24O13 (1.1)531489, 447, 301F62 [37]No75.0
5229.87Myricetin O-pentoside di-O-acetateC24H22O14 (1.7)533491, 449, 317F72 [35]No100
5330.02Myricetin O-pentoside di-O-acetateC24H22O14 (1.2)533491, 449, 317F72 [35]No81.3
5430.11Quercetin O-pentoside di-O-acetateC24H22O13 (1.9)517475, 433, 301F62 [37]No75.0
IS-12.52Cucumoside L (internal standard 1)C34H42O21 (2.1)785623, 461F11 (LC, [38], 92)––
IS-231.92Phlojodicarpin (internal standard 2)C15H16O5 (0.9)275 Co1 (LC, [39], 93)––
1 UV patterns. Cinnamates: C1—caffeic/ferulic acid derivatives, trans-form (λmax 322–324 nm), C2—p-coumaric acid derivatives, trans-form (λmax 310–312 nm), Co—simple coumarins (λmax 320–322 nm), F1—taxifolin derivatives (λmax 290–292 nm), F2—apigenin glycosides (λmax 266–268, 330–332 nm), F3—myricetin 3-O-glycosides (λmax 250–252, 260–262, 349–352 nm), F4—catechins (λmax 270–272 nm), F5—quercetin 5/7-O-glycosides (λmax 252–254, 370–372 nm), F6—quercetin 3/3′/4′-O-glycosides (λmax 254–255, 266–267, 355–357 nm), F7—myricetin 3-O-glycosides (λmax 250–252, 370–372 nm), F8—kaempferol glycosides (λmax 264–266, 345–348 nm). 2 Identification levels: (1) identified compounds after comparison of retention times, UV and mass-spectral data with previously isolated compounds or commercial reference standard (In brackets—manufacturer: BOC—BOC Sciences (Frankfurt am Main, Germany); CF—ChemFaces (Wuhan, China); FA—Pharmaffiliates (Panchkula, India); LC—Laboratory collection (ref., purity percentage); MCE—MedChemExpress (Monmouth Junction, NJ, USA); MN—MolNova (Ann Arbor, MI, USA); Sy—Synthose (Concord, NA, Canada); SA—SigmaAldrich (St. Louis, MO, USA); VI—VILAR (Moscow, Russia); catalog number; purity percentage); (2) putatively annotated compounds after comparison of retention times, UV and mass-spectral data with literature data (In brackets—ref.). 3 Occurence of compound calculated as [(number of cites positive for the compound)/(total number of cites)] × 100%.
Table 4. Variation in phenolic compounds content in R. adamsii flowers.
Table 4. Variation in phenolic compounds content in R. adamsii flowers.
Phenolic Compoundsmin, mg/g DWmax, mg/g DWMedian, mg/g DWIQR *
Taxifolin glycosides2.4033.9022.8711.53
Taxifolin aglycone0.091.791.010.40
Total taxifolins (dihydroflavonols)2.5135.1723.8911.86
Myricetin non-acylated glucosides4.0631.8721.537.85
Myricetin acylated glucosides0.1013.926.214.09
Myricetin glycosides4.1638.8327.759.17
Myricetin aglycone0.523.822.081.31
Total myricetins4.6842.6529.849.69
Quercetin non-acylated glucosides8.8960.7239.2816.13
Quercetin acylated glucosides0.005.082.032.06
Quercetin glucosides8.8963.0241.3114.13
Quercetin aglycone0.101.730.900.71
Total quercetins9.0864.6642.2214.72
Kaempferol non-acylated glucosides0.224.231.822.11
Kaempferol aglycone0.000.010.000.00
Total kaempferols0.224.241.822.11
Total flavonol glycosides13.31106.0870.8917.95
Total flavonol aglycones0.715.472.991.34
Total flavonols14.02111.5573.8919.59
Total catechins (flavan-3-ols)0.000.810.070.02
Total apigenins (flavones)0.000.010.000.01
Total flavonoids16.53147.5497.8631.80
Benzoic acid derivatives0.101.620.780.95
Hydroxycinnamates2.046.984.502.95
Total non-flavonoids2.218.545.283.92
Total phenolic compounds identified18.74155.82103.1435.34
* IQR―Interquartile Range (IQR = Q3 − Q1; for Q1, Q3 value see Table S3).
Table 5. Content of 16 selected flavonoids in tea decoctions prepared from unground, hand-ground, and pulverized R. adamsii flowers (sample No. 12), mg/100 mL ± SD (n = 5).
Table 5. Content of 16 selected flavonoids in tea decoctions prepared from unground, hand-ground, and pulverized R. adamsii flowers (sample No. 12), mg/100 mL ± SD (n = 5).
CompoundGrinding Type
UngroundHand-GroundPulverized
Taxifolin 3-O-arabinopyranoside29.27 ± 0.5830.42 ± 0.6139.95 ± 0.80 *
Quercetin 3-O-glucoside (isoquercitrin)6.04 ± 0.126.53 ± 0.14 *12.63 ± 0.26 *
Quercetin 3-O-galactoside (hyperoside)3.35 ± 0.063.52 ± 0.065.66 ± 0.11 *
Myricetin 3′-O-glucoside (cannabiscitrin)2.05 ± 0.042.28 ± 0.04 *3.28 ± 0.07 *
Quercetin 3-O-arabinofuranoside (avicularin)1.72 ± 0.032.03 ± 0.04 *3.17 ± 0.06 *
Quercetin 3-O-arabinopyranoside (guaijaverin)2.48 ± 0.053.22 ± 0.06 *4.50 ± 0.09 *
Quercetin 3-O-xyloside (reynoutrin)5.36 ± 0.106.34 ± 0.12 *9.37 ± 0.18 *
Quercetin 3-O-rhamnoside (quercitrin)19.08 ± 0.3819.74 ± 0.40 *28.25 ± 0.57 *
Myricetin 3′-O-xyloside16.69 ± 0.3317.53 ± 0.32 *25.54 ± 0.50 *
Myricetin1.89 ± 0.042.24 ± 0.04 *3.23 ± 0.06 *
Kaempferol 3-O-rhamnoside (afzelin)0.24 ± 0.000.64 ± 0.02 *1.18 ± 0.02 *
Quercetin0.20 ± 0.000.40 ± 0.01 *1.04 ± 0.02 *
Kaempferol O-pentoside 41traces0.12 ± 0.000.93 ± 0.02 *
Myricetin O-pentoside di-O-acetate 52tracestraces0.63 ± 0.02
Myricetin O-pentoside di-O-acetate 53tracestraces0.40 ± 0.01
Quercetin O-pentoside di-O-acetate 54tracestraces0.25 ± 0.01
Total flavonoid content88.3795.01140.01
Asterisk (*) indicates significant difference (p < 0.05) vs. the unground group.
Table 6. Antioxidant parameters of tea extracts from unground, hand-ground, and pulverized R. adamsii flowers (sample No. 12; n = 4).
Table 6. Antioxidant parameters of tea extracts from unground, hand-ground, and pulverized R. adamsii flowers (sample No. 12; n = 4).
AssayGrinding Value
UngroundHand-GroundPulverizedTrolox
DPPH•, IC50, μg/mL5.12 ± 0.15 *4.97 ± 0.14 *3.05 ± 0.09 *11.63 ± 0.22
ABTS•+, IC50, μg/mL3.90 ± 0.10 *3.67 ± 0.10 *2.11 ± 0.05 *4.21 ± 0.07
DMPD•, IC50, μg/mL59.34 ± 2.37 *54.11 ± 2.1938.11 ± 1.50 *53.16 ± 1.59
OH•, IC50, μg/mL14.08 ± 0.61 *12.73 ± 0.52 *9.63 ± 0.41 *16.08 ± 0.64
O2•−, IC50, μg/mL83.35 ± 3.32 *79.16 ± 2.69 *52.69 ± 2.10 *93.67 ± 3.74
Cl•, mg trolox/g450.09 ± 9.02 *486.25 ± 9.70 *537.14 ± 11.28 *1000
Br•, mg trolox/g408.20 ± 8.16 *421.37 ± 10.11 *509.28 ± 10.61 *1000
NO, IC50, mg/mL0.81 ± 0.040.78 ± 0.03 *0.52 ± 0.02 *0.85 ± 0.04
FeCA, mM Fe2+-ions/g1.87 ± 0.05 *2.01 ± 0.05 *2.59 ± 0.07 *1.51 ± 0.04
Asterisk (*) indicates significant difference (p < 0.05) vs. the Trolox group. Abbreviation used for 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) scavenging activity, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) cation radical (ABTS•+) scavenging activity, N,N-dimethyl-p-phenylenediamine dihydrochloride radical (DMPD•) scavenging activity, hydroxyl radical (OH•) scavenging activity, superoxide radical (O2•−) scavenging activity, chlorine radical (Cl•) scavenging activity, bromine radical (Br•) scavenging activity, nitric oxide (NO) scavenging activity, Fe2+-chelating activity (FeCA).
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Olennikov, D.N.; Kashchenko, N.I.; Chirikova, N.K. Rhododendron adamsii Flowers as a Potential Source of Tea-Derived Flavonoid Antioxidants. Horticulturae 2026, 12, 484. https://doi.org/10.3390/horticulturae12040484

AMA Style

Olennikov DN, Kashchenko NI, Chirikova NK. Rhododendron adamsii Flowers as a Potential Source of Tea-Derived Flavonoid Antioxidants. Horticulturae. 2026; 12(4):484. https://doi.org/10.3390/horticulturae12040484

Chicago/Turabian Style

Olennikov, Daniil N., Nina I. Kashchenko, and Nadezhda K. Chirikova. 2026. "Rhododendron adamsii Flowers as a Potential Source of Tea-Derived Flavonoid Antioxidants" Horticulturae 12, no. 4: 484. https://doi.org/10.3390/horticulturae12040484

APA Style

Olennikov, D. N., Kashchenko, N. I., & Chirikova, N. K. (2026). Rhododendron adamsii Flowers as a Potential Source of Tea-Derived Flavonoid Antioxidants. Horticulturae, 12(4), 484. https://doi.org/10.3390/horticulturae12040484

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