Rumex Species | Encyclopedia MDPI

Rumex Species: Comparison

Please note this is a comparison between Version 4 by Vivi Li and Version 3 by Vivi Li.

Today, more than ever, the search for non-trivial sources of biologically active substances is critical. Plants of the genus Rumex are noteworthy. Plants of this genus stand out for a number of advantages from the dominant plant core of meadow phytocenoses of the temperate climatic zone: a short growing season, an intensive increase in biomass, and undemanding growth conditions. In addition, this plant genus is known as a super-producer of secondary phenolic compounds.

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1. Introduction

In Europe, the consumption of wild edible plants has been an integral part of human nutrition and traditional medicine since ancient times ^[1][2]. However, despite the long history of research on wild-growing plants, scientific interest in them has not weakened for multiple reasons. First, edible wild plants are known to be a good source of primary nutritional compounds (proteins, fats, sugars, vitamins, and minerals) ^[3]. Second, edible wild plants contain various biologically active components that demonstrated health benefits effects (flavonoids, phenolic acids, anthocyanins, tannins, terpenoids, steroidal saponins, glucosinolates, and so on) ^[2]. This shows their potential as nutritional supplements, feed additives, and medicinal agents ^[2][4]. Third, wild plants provide a colossal genetic resource that can be used in breeding programs to increase the resistance of cultivated plants and to improve their nutritional and pharmacological value ^[5].

Among wild plants, Rumex plants have a great potential. They are already widely used as food, fodder, melliferous, and medicinal plants ^[6][7][8]. The Rumex L. genus, from the Polygonaceae Juss. family, has about 200 species. Plants of the Rumex genus are common in Europe, Asia, Africa, and North America, but more widely spread in the temperate zone of the northern hemisphere ^[7].

In some regions, the leaves of Rumex plants (such as R. acetosa, R. acetosella, R. abyssinicus, R. crispus, R. induratus, R. obtusifolius, R. sanguineus, R. tuberosus, R. thyrsiflorus, and R. vesicarius) are used for food, mainly as salads ^[7][9]. The consumption of the Rumex species can be restricted owing to large amounts of oxalic acid and hydroxyanthracene derivatives present, which can cause serious health problems when consumed in high doses ^[9]. However, the latter accumulate mainly in the roots of Rumex plants, and not in the leaves ^[10].

Several Rumex species are included in the pharmacopoeias of various countries. For example, R. crispus is listed in the American Herbal Pharmacopoeia as a general detoxifier and an agent for skin treatment ^[11]. The State Pharmacopoeia of the Russian Federation includes the roots of R. confertus as a herbal medicine, which is used in the treatment of liver diseases, dysentery, pulmonary, and uterine bleeding, as well as a laxative ^[12]. In Nigerian, Indian, Chinese, and Indonesian medicine, the leaves of R. nepalensis are traditionally used for their diuretic, astringent, laxative, and sedative properties ^[13].

Plants of the Rumex genus are rich in secondary metabolites, in particular phenylpropanoids and anthraquinones, which are likely to be responsible for the medicinal properties attributed to these species ^[14]. The list of anthraquinones particularly common in Rumex plants includes, but is not limited to, chrysophanol, physcion, emodin and their glycosides, rhein, nepodin, and so on ^[10]. Despite the possible toxic effect mentioned above, these compounds also show anticarcinogenic, anti-inflammatory, antiarthritic, antifungal, antibacterial, antioxidant, and diuretic activity ^[15][16]. Flavonoids are another important class of compounds that determine the therapeutic effect of Rumex plants. Derivatives of kaempferol, quercetin, apigenin, luteolin, and catechins, as well as derivatives of benzoic and cinnamic acids, lignans, coumarins, and proanthocyanidins, have been isolated from various Rumex species ^[17]. Phenolic compounds are known to have strong antioxidant as well as cardioprotective, immune system promoting, antibacterial, anti-cancer, and anti-inflammatory effects ^[18].

2. Variation in the Content of Some Groups of Phenolic Compounds

In the phenolic composition study, R. acetosella, R. crispus, R. maritimus, R. obtusifolius, and R. sanguineus demonstrated the highest values (Table 1). The total phenolics content in their leaves ranged from 111 to 131 mgg⁻¹. The leaves of R. confertus showed an even lower TPC (about 76 mg g⁻¹), whereas R. acetosa was characterized by the lowest value (about 23 mg g⁻¹).

Table 1. Content of some groups of phenolic compounds in the leaves of different Rumex species.

Species	TPC ¹, mg GAE g^–1	TFC, mg RE g^–1	THA, mg CAE g^–1	TCC, mg CE g^–1	PAs, mg CyE g^–1	TTC, mg GAE g^–1
R. acetosa	23 ± 2	18 ± 1	12.7 ± 0.6	0.90 ± 0.05	0.24 ± 0.02	0.46 ± 0.05
R. acetosella	117 ± 7	106 ± 4	18 ± 1	1.3 ± 0.1	2.2 ± 0.2	11 ± 1
R. confertus	76 ± 7	38± 2	4.8 ± 0.3	5.0 ± 0.3	4.0 ± 0.3	6.4 ± 0.3
R. crispus	131 ± 10	92± 5	8.9 ± 0.6	5.2 ± 0.3	6.4 ± 0.3	14 ± 1
R. maritimus	111 ± 6	120 ± 9	5.8 ± 0.6	4.8 ± 0.3	5.0 ± 0.4	7.1 ± 0.6
R. obtusifolius	129 ± 9	92 ± 4	1.9 ± 0.1	6.0 ± 0.4	7.2 ± 0.5	17 ± 1
R. sanguineus	126 ± 5	99 ± 6	1.9 ± 0.1	10.9 ± 0.6	6.6 ± 0.4	12.9 ± 0.7

¹ TPC, total phenolics content; TFC, total flavonoids content; THA, total hydroxycinnamic acids; TCC, total catechins content; PAs, total proanthocyanidins content; TTC, total tannins content.

A high content of flavonoids was characteristic of the leaves of R. maritimus and R. acetosella (Table 1). The total flavonoids content in the leaves of R. crispus, R. obtusifolius, and R. sanguineus varied from 92 to 98 mg g⁻¹. A notably lower content of flavonoids was found in the leaves of R. confertus (about 38 mg g⁻¹) and R. acetosa (about 18 mg g⁻¹).

The hydroxycinnamic acids’ accumulation in the leaves of the studied species showed a somewhat different tendency (Table 1). The highest content was found in the leaves of R. acetosella (about 18 mg g⁻¹). However, as opposed to the TPC and TFC values, the leaves of R. acetosa were characterized by a high total content of hydroxycinnamic acids as well (up to 13 mg g⁻¹). Whereas the leaves of R. obtusifolius and R. sanguineus did not show THA values higher than 2 mg g⁻¹ (Table 1).

The highest total catechins content was found in the leaves of R. sanguineus—about 11 mg g⁻¹ (Table 1). The TCC values of R. obtusifolius, R. confertus, R. crispus, and R. maritimus leaves were almost twice as low (from 4.8 to 6 mg g⁻¹). The R. acetosa and R. acetosella leaves demonstrated the lowest catechin content (from 0.9 to 1.3 mg g⁻¹).

The leaves of R. sanguineus, R. obtusifolius, and R. crispus were shown to have a high amount of proanthocyanids (from 6.4 to 7.2 mg g⁻¹). The lowest PA content was found in the leaves of R. acetosa (0.24 mgg⁻¹) (Table 1). The leaves of R. acetosa were also characterized by a very low content of tannins (less than 0.5 mg g⁻¹), while the highest level of TTC was found in the leaves of R. obtusifolius (about 17 mg g⁻¹) (Table 1).

Thus, various species of Rumex were associated with their own maxima of individual phenolic group levels: R. maritimus—flavonoids, R. acetosella—hydroxycinnamic acids, R. sanguineus—catechins, R. sanguineus, R. obtusifolius, R. crispus—proanthocyanidins, R. obtusifolius —tannins. The leaves of R. acetosa were characterized by the lowest contents of all analyzed phenolic compounds, except for the THA level (Table 1).

3. Variation in the Content of Individual Phenolic Compounds

Despite the low value of TPC, R. acetosa demonstrated a remarkable diversity of phenolic compounds, especially phenolic acids (Table 2; Appendix A, Figure A1a). The leaves of R. acetosa contained protocatechuic acid, sinapic acid, caftaric acid, chlorogenic acid, p-coumaric acid, ellagic acid, and other hydroxybenzoic acid derivatives. Among them, sinapic acid was the most present (about 5 mgg⁻¹). Moreover, multiple types of flavonoids, such as derivatives of quercetin (rutin, isoquercitrin, and so son) and luteolin (cynaroside), were found in the leaves.

Table 2. Content of phenolic acids and flavonoids in the leaves of different Rumex species.

Compounds (Retention Time, Min)	Content of Individual Phenolic Compounds, mg g^–1
Compounds (Retention Time, Min)	R. acetosa	R. acetosella	R. confertus	R. crispus	R. maritimus	R. obtusifolius	R. sanguineus
Flavonoids
Catechin (9.7)	–	–	–	1.08 ± 0.07	0.17 ± 0.01	1.32 ± 0.07	12.0 ± 0.8
Quercetin 3-O-rutinoside (rutin) (19.3)	3.4 ± 0.2	–	4.3 ± 0.2	10.2 ± 0.7	9.4 ± 0.6	19.0 ± 1.1	8.6 ± 0.5
Quercetin 3-β-D-glucoside (isoquercitrin) (19.9)	0.56 ± 0.03	–	20.2 ± 1.3	31.9 ± 1.8	22.6 ± 1.5	54.8 ± 3.5	49.5 ± 0.3
Quercetin derivative (16.3)¹	–	–	–	–	39.6 ± 2.9	–	–
Quercetin derivative (16.9)	–	–	–	–	27.1 ± 1.5	–	–
Quercetin derivative (18.3)	2.4 ± 0.2	–	0.94 ± 0.06	–	–	–	–
Quercetin derivative (22.73)	1.31 ± 0.07	–
–
2.9 ± 0.2
–
–
Hydroxybenzoic acid
derivative (12.5)
0.54 ± 0.05
3.0 ± 0.2
–
–	–	–	–

¹ The compounds identified based on UV spectra and quantified by standard with the same aglycon are indicated in italics.

A characteristic feature of R. acetosella was the presence of mostly flavones (derivatives of luteolin and apigenin) in the leaves, in contrast to other species, where flavonols (derivatives of quercetin and kaempferol) prevailed (Table 2; Appendix A, Figure A1b). Moreover, R. acetosella was characterized by a diverse composition and a high content of phenolic acids. The leaves are shown to contain protocatechuic acid, sinapic acid, chlorogenic acid, caffeic acid, and other derivatives of hydroxybenzoic acids.

The leaves of R. confertus, R. crispus, R. maritimus, R. obtusifolius, and R. sanguineus showed the presence of rutin and isoquercitrin, the content ratio of which varied in these species from 1:2.5 to 1:5.8, as well as the presence of astragaline and another kaempferol derivative (Table 2; Appendix A, Figure A1c–g). In fact, R. confertus, R. crispus, R. obtusifolius, and R. sanguineus demonstrated a higher level of isoquercitrin compared with other phenolic compounds present in the leaves.

The leaves of R. crispus were characterized by a high content of kaempferol derivatives (about 37 mg g⁻¹ in total) and gallic acid (about 5 mg g⁻¹) compared with other studied species (Table 2; Appendix A, Figure A1d).

The R. maritimus sample showed the highest concentration of quercetin derivatives (Table 2). Moreover, this species was characterized by a rich qualitative composition of phenolic acids. It includes protocatechuic acid, sinapic acid, chlorogenic acid, caffeic acid, and other derivatives of hydroxybenzoic acids.

The leaves of R. obtusifolius and R. sanguineus had a similar metabolic profile with high levels of flavonoids (quercetin derivatives) and very low levels of phenolic acids (Table 2; Appendix A, Figure A1f,g). However, it should be noted that the leaves of R. sanguineus were high in catechin (up to 12 mg g⁻¹), in contrast to R. obtusifolius and other analyzed species.

4. Antioxidant Activity of the Rumex Extracts

Extracts from R. crispus demonstrated high antioxidant activity based on all three methods (Table 3). A high level of antioxidant activity was also found in R. maritimus extracts (according to the ABTS and FRAP methods). The lowest antioxidant activity was shown by the extracts of R. acetosa (Table 3).

Table 3. Antioxidant activity of extracts from the leaves of different Rumex species.

Species	AOA (DPPH) ¹, mg TE g^–1	AOA (ABTS), mg TE g^–1	AOA (FRAP), mg TE g^–1
R. acetosa

Antioxidant activity is caused by the presence of certain components in plant samples, usually compounds of phenolic nature. Correlation analysis carried out during this research proved a positive relationship between antioxidant activity and the total content of phenolic compounds (r = 0.785–0.921, p ≤ 0.05), flavonoids (r = 0.602–0.918, p ≤ 0.05), proanthocyanidins (r = 0.721–0.842, p ≤ 0.05), and tannins (r = 0.591–0.776, p ≤ 0.05) (Table 4). However, the results related to the content of hydroxycinnamic acids were unexpected. Either there was no significant correlation between the antioxidant activity level (according to the DPPH and ABTS methods) and THA, or there was an inverse correlation of moderate strength (when based on the FRAP method).

Table 4. Correlation matrix with the Pearson coefficient values for phenolic compounds and antioxidant activity of Rumex extracts.

Variables	TPC ¹	TFC	THA	TCC	PAs	TTC	DPPH	ABTS	FRAP
3.1 ± 0.3	5.1 ± 0.5	3.9 ± 0.3
TPC	1	0.881 **	–0.317 ^ns	0.567 *	0.822 **	0.915 **	0.806 **	0.921 **	0.785 **
R. acetosella	TFC31 ± 3	48 ± 3	27 ± 3
–
1	–0.114	^ns	0.368	^ns	0.586 *	0.664 *	0.602 *	0.918 **	0.714 **	R. confertus	22 ± 1.3
THA	37 ± 3		39 ± 4
1	–0.820 **	–0.768 **	–0.354	^ns	–0.174	^ns	–0.322	^ns	–0.563 *	R. crispus	69 ± 4	56 ± 4	57 ± 3
TCC				1	0.809 **	0.537 *	0.389 ^ns	0.513 *	0.614 *	R. maritimus	31 ± 2	63 ± 4	61 ± 4
PAs					1	0.826 **	0.721 **	0.751 **	0.842 **	R. obtusifolius
TTC	37 ± 2	48 ± 4	43 ± 2
		R. sanguineus	35 ± 3	52 ± 4	47 ± 4	–	–	–	–
Quercetin derivative (23.1)	2.0 ± 0.1	–	–	–	–	–	–
			1	0.776 **	0.701 **	0.591 *
DPPH							1	0.728 **	0.742 **
ABTS			Quercetin derivative (24.1)	3.4 ± 0.2	–	–	–	–	–	–
Kaempferol 3-O-glucoside (astragalin) (24.7)	–	–	1.82 ± 0.09	24.4 ± 1.6	4.4 ± 0.3	5.1 ± 0.3	8.6 ± 0.6
Kaempferolderivative (22.8)	–	–	0.75 ± 0.04	12.9 ± 1.0	1.4 ± 0.1	2.6 ± 0.2	3.2 ± 0.2
					1	0.909 **
FRAP									1	Kaempferolderivative (20.9)	–	–	–	–	4.9 ± 0.3	–	–
Luteolin 7-O-glucoside (cynaroside) (20.7)	0.51 ± 0.03	4.3 ± 0.3	–	–	–	–	–
Luteolinderivative (15.5)	–	89.5 ± 4.7	–	–	–	–	–
Apigeninderivative (19.4)	–	5.1 ± 0.3
Phenolic acids
Gallic acid (3.8)	–	–	–	5.3 ± 0.3	–	0.34 ± 0.02	0.33 ± 0.02
3,4-Dihydroxybenzoic acid (protocatechuic acid) (5.8)	0.12 ± 0.01	0.58 ± 0.03	–	0.56 ± 0.03	0.21 ± 0.01	–	0.21 ± 0.01
Sinapic acid (8.2)	4.9 ± 0.4	1.22 ± 0.08	1.5 ±0.1	–	1.8 ± 0.1	–	–
Caftaric acid (9.2)	1.7 ± 0.1	–	–	–	–	–	–
Chlorogenic acid (10.2)	1.21 ± 0.09	3.04 ± 0.17	1.8 ± 0.1	–	0.19 ± 0.01	–	–
Caffeic acid (10.5)	–	0.93 ± 0.05	–	0.10 ± 0.01	0.29 ± 0.03	–	–
p-Coumaric acid (14.2)	0.15 ± 0.02	–	–	–	–	–	–
Ellagic acid (17.9)	0.28 ± 0.02	–	–	0.83 ± 0.05	–	–	–
Hydroxybenzoic acid derivative (11.2)	0.97 ± 0.05	4.0 ± 0.2	–

¹ AOA (DPPH), antioxidant activity determined by the DPPH (2,2-diphenyl-1-picrylhydrazyl) assay; AOA (ABTS), antioxidant activity determined by the ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assay; AOA (FRAP), ferric reducing antioxidant power.

5. Correlation between Phenolic Compounds Content and Antioxidant Activity

¹TPC, total phenolics content; TFC, total flavonoids content; THA, total hydroxycinnamic acids; TCC, total catechins content; PAs, total proanthocyanidins content; TTC, total tannins content; DPPH, antioxidant activity determined by the DPPH (2,2-diphenyl-1-picrylhydrazyl) assay; ABTS, antioxidant activity determined by the ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assay; FRAP, ferric reducing antioxidant power. ** Correlation is significant at p ≤ 0.01; * correlation is significant at p ≤ 0.05; ^ns, correlation is not significant (p > 0.05).

6. Heat Map and Cluster Analysis of Studied Rumex Species Based on the Content of Phenolic Compounds and Antioxidant Activity of Their Extracts

Based on the normalized values of the studied parameters, a heat map with cluster analysis was built (Figure 1). The dendrogram presented in Figure 1 (top) demonstrates that all the studied parameters can be divided into four clusters. The first cluster includes total phenolic content, antioxidant activity according to the ABTS method, and the total flavonoid content. The second cluster consists of total tannin content and antioxidant activity based on the DPPH method. The third cluster includes the total content of catechins, proanthocyanidins, and antioxidant activity based on the FRAP method. A separate cluster is formed by hydroxycinnamic acids.

Figure 1. Heat map with clusters for studied variables (at the top) and Rumex species (at the left). TPC, total phenolics content; TFC, total flavonoids content; THA, total hydroxycinnamic acids; TCC, total catechins content; PAs, total proanthocyanidins content; TTC, total tannins content; DPPH, antioxidant activity determined by the DPPH (2,2-diphenyl-1-picrylhydrazyl) assay; ABTS, antioxidant activity determined by the ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) assay; FRAP, ferric reducing antioxidant power.

The dendrogram on the left shows that the analyzed Rumex species can be divided into two large clusters (Figure 1). The first of them consists of only R. acetosa, and the second of all the other studied species. The second cluster includes multiple groups. One of them has only R. acetosella, whereas the other group includes R. sanguineus, R. obtusifolius, R. crispus, and R. confertus. The dendrogram shows that, in the latter group, R. sanguineus and R. obtusifolius in turn form a micro-group characterized by very similar composition.

Appendix A

Figure A1. Chromatographic profile of the phenolic acids and flavonoids at 340 nm for (a) R. acetosa, (b) R.acetosella, (c) R. confertus, (d) R. crispus, (e) R. maritimus, (f) R. obtusifolius, (g) R. sanguineus: 1—protocatechuic acid; 2—sinapic acid; 3—caftaric acid; 4—chlorogenic acid; 5, 6—hydroxybenzoic acid derivatives; 7—p-coumaric acid; 8—ellagic acid; 9—quercetin derivative; 10—rutin; 11—isoquercitrin; 12 –cymaroside; 13, 14, 15—quercetin derivatives; 16—caffeic acid; 17—luteolin derivative; 18—apigenin derivative; 19—kaempferol derivative; 20—astragalin; 21—gallic acid; 22, 23—quercetin derivatives; 24—kaempferol derivative.

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