Apocynum venetum, a medicinal, economical and ecological plant: a review update

Apocynum venetum L. is an important medicinal perennial rhizome plant with good ecological and economic value. Its leaves have many pharmacological effects such as anti-inflammatory, anti-depression, anti-anxiolytic, etc., while its fibers have the title of “king of wild fibers”. Furthermore, it was suitable for the restoration of degraded saline soil in arid areas. An increasing studies have been published in the past years. A scientometric analysis was used to analyze the publications of Apocynum venetum L. to clearly review the pharmacology, fiber application of Apocynum venetum L. and the potential value with its similar species (Apocynum pictum Schrenk) to the environment.


INTRODUCTION
, commonly known as ''Luobuma'' in Chinese and ''Rafuma'' in Japanese is a perennial herbaceous shrub ( Fig. 1) widely distributed in the temperate regions of Asia, Europe and North America, especially in saline-alkali land, river-banks, fluvial plains and sandy soils (Grundmann et al., 2007;Jiang et al., 2021b;Xie et al., 2012). The species Apocynum venetum L. (Apocynaceae) currently includes 9 subspecies documented on World Flora Plant List (Table S1) (World Flora Online, 2022) A. venetum can adapt to extreme conditions where the surface salinity is up to 20% and the annual average precipitation is more than 250 mm, making the plant of high ecological value for the transformation of coastal saline and barren lands (Thevs et al., 2012;Yuan, Li & Jia, 2020a). A. venetum leaves has been used to produce herbal drugs and tea (Chinese Pharmacopoeia, 2020). Furthermore, since 2002 luobuma tea has been included in the list of health-care food in China (National Health Commission of the People's Republic China, 2002). Lau et al. (2012) confirmed that A.venetum leaf extract could stimulate vascular receptor (alpha-adrenergic and angiotensin II receptors) and inhibit vasoconstriction, suggesting antihypertensive properties of the plant. Modern pharmacological investigations confirmed that A. venetum has, among other effects, anti-inflammatory, anti-depression, anti-anxiolytic, anti-ageing, antioxidants, cardiotonic and hepatoprotective effects (Du et al., 2020;Grundmann et al., 2007;Xie et al., 2015;Xie et al., 2012). A. venetum fiber, known as the ''king of wild fibers'', is receiving increasing attentions in the apparel industry owing to its additional advantage of possessing antibacterial properties (Han et al., 2008;Wang, Han & Zhang, 2007;Xu et al., 2020a). Alongside the rapid increase in A. venetum-related studies, systematic and comprehensive analyses on A. venetum publications is essential. We have previously reviewed the traditional uses, phytochemistry and pharmacology of A. venetum (Xie et al., 2012). As a timely update, this article aims to respond to the rapidly increasing literature on A. venetum studies by: (i) conducting scientometric analysis of the publications on A. venetum and (ii) reviewing the progress recorded on the exploration of the medicinal, economical and ecological benefits of the plant from 2012 to date. For the scientometric analysis, we used Citespace, which is a specifically designed to facilitate the detection of emerging trends and mutations in the scientific literature (Chen et al., 2012). Web of Science Core Collection (WoSCC; Clarivate Analytics, London, UYK) is the premier resource on the Web of Science platform. It is considered as the most trusted citation index on many research topics (Wu, Yakhkeshi & Zhang, 2022). This work can provide researchers and readers with a comprehensive information on A. venetum, covering the areas of phytopharmacy and pharmacology, functional food, ecology, and applications in textile and fiber industry.

SURVEY METHODOLOGY
Data were collected from WoSCC with the following search strategy: Topical Subject The searched time spans 1987-2022, the type of literature was article and review, and the language was English. Our search strategy did not limit the impact factor of journals and the affiliation of authors. A total of 200 publications were obtained, including 190 articles and 10 reviews, and their full record with the cited references was exported in plain text format. CiteSpace 6.1.3 was used to analyze keywords of the literatures, with the time partition set to 1987-2022, the time slice set to 1, the node types set to keyword, G-index set to 25, and the pathfinder, pruning sliced networks and pruning the merged network were used to trim the atlas. Based on the result of keywords analysis of CiteSpace, the chapter topics were divided into the pharmacological effects and related components of A. venetum, A. venetum fiber, other Apocynum species similar to A. venetum: Apocynum pictum Schrenk, and the ecological value of A. venetum and A. pictum, and the topics were discussed. The discussion on the bioactive components cover the period 2012-2022.

Keywords analysis of CiteSpace
Keywords represent the core content of an article and provide information on the topic or the important category to which an article belongs. The keywords with high frequency and highly mediated centrality were analyzed and presented in the form of a visual mapping through the Citespace software (Fig. 2). The most frequent keywords from 1987-2022 were Apocynum venetum L. (111), Apocynum venetum leaves (52), Apocynum venetum leaf extract (31). The keywords with the highest centrality before 2018 include Apocynum venetum L. (0.49), component (0.28), hepatoprotective activity (0.27), identification (0.25) and antioxidant (0.23) ( Fig. 2A, Table 1). However, after 2018, among the top ten keywords showing the highest centrality, two words that are poorly correlated with Apocynum venetum leaves appeared: Apocynum venetum fiber (0.28) and Apocynum pictum schrenk (0.27). These results implied that the studies before 2018 mainly focused on the components and the pharmacological effects of Apocynum venetum leaves while Apocynum venetum fiber and Apocynum pictum Schrenk have also attracted the attention of researchers in recent years.
Based on the keyword co-linear graph ( Fig. 2A), the parameter of ''burstiness'' was set to γ = 0.5, minimum duration = 1. Sixteen burst entries were generated. Among them, the words that have kept the outbreak status were oxidative stress (3.93), Apocynum venetum fiber (3.26), identification (2.68) and tolerance (2.0) (Fig. 2B). These data confirmed that apart from the further in-depth pharmacological investigations, the fiber of this plant has recieved attention in recent years. In addition, the ecological value of Apocynum venetum L and Apocynum pictum Schrenk L has attracted increasing attentions.

The bioactive components of A.venetum Flavonoids
With the deepening of research and the technological improvement in high performance liquid chromatography, mass spectrometry etc., many phytochemicals of A.venetum have been identified and isolated. Some of these phytochemicals were flavonoids such as hyperoside and isoquercetin, which bioactivities have been comprehensively reviewed previously (Xie et al., 2016a;Xie et al., 2016b;Xie et al., 2012). Since then, more studies have reported on the isolation and bioactivities of known and novel flavonoids from A.venetum. The flavonoids isolated from A. venetum since 2012 are listed in Table 2 and their structures shown in Fig. 3. Kaempferol, quercetin, isoquercitrin (quercetin-3-O-β-D-glucose) and astragalin (kaempferol-3-O-β-D-glucose) isolated from A. venetum leaves have significant anti-depressant activities in mice (Yan et al., 2016). Hyperoside isolated from the leaves of A. venetum showed antidepressant-like effect in P12 cell lines which could improve neuronal viability by protecting neurons from corticosterone damage (Zheng et al., 2012). Hyperoside had protective effect on H 2 O 2 -induced apoptosis of human umbilical vein endothelial cells (Hao et al., 2016). For acetaminophen-induced liver injury, both hyperoside and isoquercetin exerted hepatoprotective effect by upregulating the expression and activity of detoxifying enzymes such as sulfotransferases (hyperoside could also increase activities of UDP-glucuronosyltransferase) in liver microsomes and inhibited the activity of cytochrome P450 2E1, accelerating the harmless metabolism of acetaminophen. Additionally, isoquercetin could significantly inhibit acetaminophen induced oxidative stress and nitrosative stress (Xie et al., 2016a;Xie et al., 2016b). Isoquercitrin, isolated from the A. venetum leaf aqueous extract exerted anti-obesity effect in high fat diet induced obese mice by inhibiting adenosine 5 -monophosphate-activated protein kinase (AMPK)/sterol regulatory-element binding protein (SREBP-1c) signaling pathway, glucose uptake, and glycolysis flux. C-1-tetrahydrofolate synthase, carbonyl reductase, and glutathione Stransferase P are potential target proteins of isoquercitrin (Manzoor et al., 2022). 8-Omethylretusin ( Fig. 3) isolated from A venetum leaves showed antifouling activity (Kong et al., 2014). On the other hand, 4 ,7-dihydroxy-8-formyl-6-methoxyflavone isolated from A venetum leaves showed high anti-inflammatory activity via significant inhibitory effect on the production of nitric oxide (NO) and tumor necrosis factor-α (TNF-α) (IC 50 values were 9.0 ± 0.7 and 42.1 ± 0.8 µM, respectively) in lipopolysaccharide-induced mouse peritoneal macrophages (RAW 264.7) (Fu et al., 2022). Wang et al. (2020) investigated the absorption and metabolism of quercetin-3-Osophoroside, isolated from the leaves of A. venetum, in rats. The results indicated that quercetin-3-O-sophoroside was completely absorbed in the small intestine and metabolized in the jejunum to sulfated quercetin-3-O-sophoroside, methylated quercetin-3-O-sophoroside, and methylated quercetin-3-O-sophoroside sulfate. Quercetin-3-Osophoroside was deglycosylated to aglycones by the cecal microbiota to form derivatives of benzoic, phenylacetic and phenylpropionic acids (Wang et al., 2020).
To obtain larger amounts of flavonoids, A. venetum hairy roots were induced with Agrobacterium rhizogenes strain Ar.1193, and 117 kinds of flavonoids were detected in the roots. The flavonoid content and antioxidant activity of the roots were significantly increased as compared to field-planted roots, therefore, this technique could be used for large-scale production of flavonoids from A. venetum (Zhang et al., 2021).

Polysaccharides
Natural polysaccharides have been proved to possess, among other effects, immune regulatory, anti-oxidative and anti-inflammatory activities, as well as having the advantages of being safe and non-cytotoxic (Liu et al., 2022). Zhou et al. (2019) used different concentrations and kinds of solvents (HCl, H 2 O, NaOH) to extract polysaccharides from A. venetum leaves. The results showed that the polysaccharide yield was the highest with 21.32% (w/w), 0.5 M NaOH at 90 • C, and the bioactivity of the alkaline extracted polysaccharides was the strongest, which was reflected in the antioxidant capacity (DPPH and ABTS radical scavenging activities) and α-glucosidase and lipase inhibitory activities. The 0.5 M NaOH extracted polysaccharides showed a strong inhibitory activity on αglucosidase (IC50 value of 16.75 µg/mL), which was better than the positive control, acarbose (IC50 value of 1,400 µg/mL). In addition, the alkaline polysaccharide-rich extracts were proved to possess hypoglycemic and hypolipidemic effects on mice with high fat diet induced and streptozotocin-induced type 2 diabetes. Moreover, the extract reversed intestinal dysbiosis by increasing the abundance of Odoribacter, Anaeroplasma, Muribaculum, Parasutterella and decreasing the abundance of Enterococcus, Klebsiella, Aerococcus in diabetic mice (Yuan et al., 2020b).
Some polysaccharides were also isolated from various parts of A. venetum and validated for bioactivity. These are summarized in Table 3. ALRPN-1 and ALRPN-2 exerted a significant anti-inflammatory activity in lipopolysaccharide-induced macrophages by regulating the levels of pro-inflammatory mediators (NO) and cytokines (TNF-α, interleukin-6, interleukin-1 β) and the mechanism may involve, in part, extracellular signalrelated kinase (ERK)/mitogen-activated protein kinases (MAPKs) signaling pathway (Liu et al., 2022). Vp2a-II and Vp3 obtained from the flowers of A. venetum showed anticoagulant activity and immunoregulation. The anticoagulant activities of Vp2a-II and Vp3 were assayed in vitro by plasma coagulation parameters (activated partial thromboplastin time (APTT), thrombin time (TT), prothrombin time (PT), fibrinogen). The results showed  that Vp3 significantly prolonged TT and PT, while Vp2a-II significantly prolonged APTT and TT, indicating that the two polysaccharides could inhibit blood coagulation (Wang et al., 2019b). In addition, the polysaccharides could exert immunomodulatory effects by promoting phagocytic activity, enhancing NO secretion and mRNA expression of inducible nitric oxide (iNO) synthase, interleukin-6 and TNF-α which activate RAW264.7 cells. Vp2a-II might activate the MAPK signaling pathway, which then induce the nuclear translocation of NF-κB p65 .
In addition to the pharmacological effects of A. venetum polysaccharides, researchers have also began exploring their other properties. The polysaccharide conjugates (ATPC-A) extracted from A. venetum tea residues with an alkaline solution (0.10 M NaOH) had emulsifying properties and stabilized the emulsion which comprised of amphipathic polysaccharides covalently bound to proteins. The stability of the neat ATPC-A emulsions with a concentration equal to or greater than 1.00 weight % was higher than 5.00 weight % gum arabic during storage at different temperatures and pH values (Chen et al., 2022b).

Other phytochemical components of A. venetum
Many studies have reported other phytochemicals from A. venetum leaf extracts and their pharmacological effects. The ethanol extract of A. venetum leaf possesses anti-cancer activity. A fraction separated from the extract could inhibit the proliferation of Human PCa cells tumor cells. Lupeol accounted for approximately one-fifth (19.3% w/w) of the components of the fraction and was implicated for the induced cytotoxicity against PCa cells. The fraction and lupeol elicited similar anti-proliferative mechanisms, involving: regulating apoptosis signal molecules (P53, cytochrome c, Bcl-2, and caspase 3 and 8), promoting G2/M arrest through impairing the DNA repair system via downregulating the expression of uracil-DNA glycosylase, as well as downregulating the expression of β-catenin (Huang et al., 2017). In preventing D-galactose-induced oxidative damage in mice, the polyphenol extract of A. venetum was superior to the antioxidant vitamin C (Guo et al., 2020). Within its safe concentration range (0-100 µg/ml), the polyphenol extract of A. venetum inhibited U87 glioma cell proliferation and caused cell apoptosis by affecting NF-κB and genes of other relevant pathways (Zeng et al., 2019). Additionally, A. venetum leaf extract inhibited doxorubicin induced cardiotoxicity through (protein kinase B) Akt/(B-cell lymphoma-2) Bcl-2 signaling pathway . The efficacy and mechanism of action of individual chemical components, as well as their possible synergistic effects, of A. venetum leaf extract need to be further investigated.

A. venetum fiber
The fiber of A. venetum has been used in textile and paper industries with superior properties compared to other commonly used fibers. Fiber from Apocynum species has a higher average length to diameter ratio (up to 1219) compared to kenaf (209), another natural plant fiber (Liu et al., 2020;Wang, Han & Zhang, 2007;Xie et al., 2012). Another reason for the popularity of A. venetum fabric is the antibacterial effect that A. venetum fiber naturally possesses (Li et al., 2012;Song et al., 2019). Such antibacterial activity might be because: (i) A. venetum fiber has small openings between microstructures, which improve the breathability of the A. venetum fabric, which subsequently destroy the environment for bacterial growth (Han et al., 2008); (ii) the A. venetum stem cells contain tanning agents, which is resistant to microbial decomposition (Thevs et al., 2012); (iii) the presence of water-insoluble polyphenol derivatives confers antimicrobial properties to the fabric (Xu et al., 2020a). A. venetum is rich in cellulose, but impurities such as pectin, lignin, and waxes must be removed to produce clean fibers (Lou et al., 2019). In the direction of environmental safety and high efficiency, various degumming methods have been proposed, including chemical degumming, biological degumming and microwave-assisted ultrasonic degumming. A study revealed that microwave-assisted ultrasonic degumming showed the advantages of requiring less chemical reagents during degumming (1 kg raw A. venetum bast needed 0.6 kg of reagents while the chemical degumming treatment required 1.34 kg) and shorter time, as well as higher quality (low residual gum content of 5.15%; lignin content less than 3%; whiteness more than 80% in the refined A. venetum fibers) (Li et al., 2020). Degumming methods and the fiber quality of A. venetum reported from 2012 to 2022 are listed in Table  4.
In addition to the textile industry, A. venetum fiber also has many potential applications in medicine as well as in the construction industry. Microcrystalline cellulose (MCC-N) from A. venetum fibers was shown to have a rougher structure and less macrostructure than commercially available microcrystalline cellulose (MCC-C). MCC-N had a crystallinity of up to 78.63% and a thermal stability comparable to that of MCC-C, which made it suitable as a load-bearing material for composite structures, and could be used in polymer composites with high temperature resistance (Halim, 2021). Furthermore, cellulose nanofibers (CNFs) from A. venetum straw were added into poly lactic acid (PLA), and the prepared PLA/CNFs film did not only improve the wettability and permeability of PLA, but also had superior antibacterial properties (the antibacterial growth inhibition rate on Escherichia coli and Staphylococcus aureus were 96.31% and 92.83% at PLA/6% (w/w) CNFs film, respectively). Then, polyvinyl pyrrolidone was added to this film to form a sustained-release nanofiber membrane (PLA/drug-loaded PVP nanofiber membranes), and a purified sea buckthorn was embedded in the drug-loaded film to evaluate its performance. The nanofiber membrane extended and sustained the release of purified The method uses metal salts of aluminum for pretreatment, which is more sustainable. Chemical degumming Pre-acid treatment (2% H2SO4, temperature: 60 • C, M: L-1:15, time: 1 h) >> washing >> firstcooking (5% NaOH, 3% Na2SiO3, 2.5% Na2SO3, temperature: 100 • C, M: L-1:10, time: 2.5 h) >> washing >> second-cooking (15% NaOH, 3% Na2SiO3, 2% sodium tripolyphosphate, temperature: 100 • C, M: L-1:10, time: 2.5 h) >> washing >> acid rinsing (1 g/L H2SO4) >> washing >> dewatering >> shaking >> drying sea buckthorn, and the cumulative release reached a maximum of 75.41%. It showed the advantage of a profile with a high initial release followed by a slow diffusion phase (Wang et al., 2021b;Wang et al., 2019a). In addition, when the hydrogel was prepared with chitosan as the matrix, the addition of CNFs improved the mechanical properties and swelling rate of the chitosan-based hydrogel. As the CNFs was 1.5%, the compressive strength of the hydrogel increased by nearly 20%, the swelling capacity reached 140%. In this form, the antibacterial efficacy against Escherichia coli and Staphylococcus aureus were 98.54% and 96.15%, respectively (Wang et al., 2021a). See Abubakar, Gao & Zhu (2021) for further details on the composition, properties and degumming methods of A. venetum fiber.

Other Apocynum species similar to A. venetum: Apocynum pictum Schrenk
Due to excessive exploitation, wild A. venetum has declined in recent years. A similar species, Apocynum pictum Schrenk (Apocynum hendersonii Hook) is often used in the market as a substitute for A. venetum due to their similarity in morphological characteristics and geographical distribution. The incorporation of A. pictum may affect the safety and effectiveness of A. venetum (An et al., 2013;Chan et al., 2015;Zheng et al., 2022). Although A. pictum has not been included in the Chinese Pharmacopoeia (Chinese Pharmacopoeia, 2020), some studies have reported that it is an important medicinal plant (Gao et al., 2021;Jiang et al., 2021a). For the quality control of A. venetum and to explore the potential application of A. pictum, some studies compared the similarities and differences between the two species in terms of genome size, flavonoid content, chemical composition and biological activity. The whole genomes of the two species were both small and similar, with 232.80 megabase (A. venetum) and 233.74 megabase (A.pictum). The contents of quercetin, hyperoside and total anthocyanin in A. venetum were much higher than those of A. pictum, which was considered to be the reason for the difference in color between the two species (Gao et al., 2019). Hyperoside could be a suitable chemical marker to distinguish between the two species (Gao et al., 2019). In addition, A. venetum has a better antioxidant activity than A. pictum (Chan et al., 2015). However, recent studies have shown that the flavonoids from A. pictum (quercetin-3 Apart from the pharmacological value, in recent years, A. pictum is often studied together with A. venetum because of its high ecological value.

The ecological value of A. venetum and A. pictum
Phytoremediation is one of the appropriate ways to deal with land problems such as drought, salinity and metal pollution (Pilon-Smits, 2005). Apocynum spp. were selected to stabilize sands and restore the degraded saline lands due to their advantages of easy propagation, resistance to harsh environment, and high economic value (Jiang et al., 2021a; Table 5. Different PEG-6000 concentrations (0%-35%) was used to simulate natural drought conditions to study the effect of drought stress on the germination of Apocynum spp. seeds. The results showed that low concentrations PEG (0-20%) had no significant impact on the germination rate of Apocynum spp. seeds. However, when the concentration was more than 20%, the germination rates of the seeds were reduced, and the negative impact on A. pictum seeds was higher than that on A.venetum. In addition, after the drought stress was alleviated, the seeds were able to germinate under appropriate conditions (Han et al., 2021;Jiang et al., 2021a). Moreover, the membership function (A mathematical tool for representing fuzzy sets) was used to comprehensively evaluate the drought resistance of A. venetum and another desert economic plant, Lycium ruthenicum, by analyzing the physiological and biochemical indices (the content of chlorophyll a, chlorophyll b, proline and soluble sugar, antioxidant enzyme activity, etc.). The results showed that when the soil moisture content was 9.70%, 6.89% and 5.54%, the drought resistance of A. venetum was stronger than that of Lycium ruthenicum (Wang, 2017). Low concentration of salt solution (0-200 mM NaCl) had no significant effect on the germination rate of current season mature seeds the two species (Jiang et al., 2021a;Shi et al., 2014). However, another study showed that under 200 mM NaCl stress, the growth and development of A. venetum seedlings were inhibited, the phenotypic characteristics (plant height, root length, leaf length, leaf width) were damaged, and the total flavonoid content decreased. However, salt stress increased the content of quercetin and kaempferol in seedlings (Xu et al., 2020b). In addition, the seeds of Apocynum spp. both exhibited high tolerance to lithium salts during germination, particularly LiCl (Table 5) (Gao et al., 2020;Jiang, Wang & Tian, 2018a;Jiang et al., 2018b). The simulated critical value of A. venetum was as high as 196 mM (Jiang, Wang & Tian, 2018a). To put the salt tolerance of A. venetum into perspective, Brassica carinata, another heavy metal tolerant plant with phytoremediation potential, has a germination rate of less than 50% at LiCl concentration above 120 mM (Li et al., 2009). Notably, the addition of lithium in soil did not reduce the concentrations and antioxidant capacity of total flavonoids, rutin and hyperoside in A. venetum leaves (Jiang et al., 2019). Therefore, Apocynum spp. are suitable for the restoration of degraded saline soil in arid areas, and are promising species in the remediation of lithium pollution in the environment (Jiang et al., 2021a;Jiang et al., 2021b;Rouzi et al., 2018).

CONCLUSIONS
Looking back on the research history of A. venetum, the research focuses mainly on the components and pharmacological effects of A. venetum leaves. At present, many of the pharmacological effects are attributable to flavonoids, however these active components and their synergistic mechanism need to be further studied. In addition to flavonoids, some polysaccharides (Vp2a-II, Vp3) and triterpenoid (uvaol) from A. venetum have also shown pharmacological effects. However, the current research in this area is still lacking. In recent trends, the fiber of A. venetum have attracted attention. Apart from its textile value, the potential application of the fiber in other industries needs further exploration in future studies. The ecological value of Apocynum spp. is gradually being revealed by multiple research.
This study provided rich and rigorous CiteSpace analysis on A. venetum. However, as a limitation, we analyzed only the papers written in English, and within the WoS database, therefore it may not be comprehensive enough to reflect the entire research status. For example, we searched a major Chinese scientific literature database, the China National Knowledge Infrastructure (CNKI), and more than 2,000 Apocynum related publications were retrieved, although these were not within the analysis scope of the current study. This further attests to the interest Apocynum species have received from the scientific community over the past decades.