Phenolic Compounds in Trees and Shrubs of Central Europe

: Plants produce speciﬁc structures constituting barriers, hindering the penetration of pathogens, while they also produce substances inhibiting pathogen growth. These compounds are secondary metabolites, such as phenolics, terpenoids, sesquiterpenoids, resins, tannins and alkaloids. Bioactive compounds are secondary metabolites from trees and shrubs and are used in medicine, herbal medicine and cosmetology. To date, fruits and ﬂowers of exotic trees and shrubs have been primarily used as sources of bioactive compounds. In turn, the search for new sources of bioactive compounds is currently focused on native plant species due to their availability. The application of such raw materials needs to be based on knowledge of their chemical composition, particularly health-promoting or therapeutic compounds. Research conducted to date on European trees and shrubs has been scarce. This paper presents the results of literature studies conducted to systematise the knowledge on phenolic compounds found in trees and shrubs native to central Europe. The aim of this review is to provide available information on the subject and to indicate gaps in the present knowledge.


Introduction
Tree stands are exposed to the action of stress factors, both abiotic and biotic. The former include weather anomalies, UV radiation, intensive lighting, water deficits, substrate salinity, high temperature amplitudes and the presence of heavy metals. In turn, biotic factors include pest insects, pathogenic fungi, bacteria and viruses. Trees counter stressors by initiating defence mechanisms to minimise or eliminate disturbances in growth and development. They are related to the consumption of energy and assimilates, the limited production of biomass, its disadvantageous allocation, as well as reduced reproduction. The action of biotic stressors is mainly connected with trees and woody plants entering into symbiosis with antagonists of pathogens and insects, etc.
Plants produce specific structures constituting barriers, hindering the penetration of pathogens, e.g., resin canals and the presence of waxes and resins on their surface, while they also produce substances inhibiting pathogen growth and reducing the attractiveness of needles, etc. These compounds are secondary metabolites, such as phenolics, terpenoids, sesquiterpenoids, resins, tannins and alkaloids. A considerable number of secondary metabolites protect against the adverse effect of herbivorous insects [1,2], pathogenic fungi [3-6] and bacteria [7,8]. These compounds differ in their chemical  Appl. Sci. 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/applsci Figure 2. The shikimate pathway [27,28]. Figure 2. The shikimate pathway [27,28]. Phenolic compounds, in terms of the structure of the basic carbon skeleton, may be divided into phenolic acids, flavonoids, tannins (hydrolysable and non-hydrolysable tannins-proanthocyanidins) and stilbenes [29].

Phenolic Acids
Phenolic acids, in their structure, contain a hydroxyl and a carboxyl group. Hydroxyl derivatives of benzoic and cinnamic acids are common in the plant world (Table 1). Phenolic compounds, in terms of the structure of the basic carbon skeleton, may be divided into phenolic acids, flavonoids, tannins (hydrolysable and non-hydrolysable tannins-proanthocyanidins) and stilbenes [29].

Phenolic Acids
Phenolic acids, in their structure, contain a hydroxyl and a carboxyl group. Hydroxyl derivatives of benzoic and cinnamic acids are common in the plant world (Table 1). In plant tissues, other complexes of phenolic acids were also identified, e.g., complexes with flavonoids, fatty acids, sterols and polymers of cell walls. Phenolic acids may also be components of anthocyanins or flavones [30,31]. A separate group is composed of depsides, being a complex of two or more molecules of phenolic acids. In plant organisms, including trees, they are formed mainly in the reaction of the so-called shikimate pathway or malate acid.
Tyrosine and phenylalanine are precursors of most phenolic acids, from which, as a result of deamination, cinnamic acid and its hydroxy derivatives are formed [32].
In plants, phenolic acids are mostly found in the bound form as esters and glycosides contained in lignins and hydrolysed tannins. Examples in this respect may be provided by hydroxycinnamic acids found in ester complexes with carboxylic acids or with glucose. They appear in ester complexes with the following acids: malonic, tartaric, α-hydroxy-hydrocaffeic, hydroxycinnamic, tartronic, shikimic, galacturonic, glucaric (as caffeic acid glucuronide), gluconic (as feruloylgluconic acid, which main isomer is 2-O-feruloyl gluconic acid) and 4-methoxyaldaric (as 2-O-feruloyl-4-methoxyaldaric acid). In turn, hydroxybenzoic acids are primarily found as glycosides. In plant tissues, other complexes of phenolic acids were also identified, e.g., complexes with flavonoids, fatty acids, sterols and polymers of cell walls [30,31]. Phenolic acids are found in wood of oak, pine, spruce, fir, walnut, willow, birch (leaves), and in the fruits of bird cherry (Table 2) A particularly interesting active compound from the group of hydroxybenzoic acids is ellagic acid, a dimer of gallic acid, found in plants in the free form and (more frequently) in an ester complex with glucose, forming hydrolysable tannins (ellagotannins) [33,34]. It is found in the wood of oak, walnut and sweet chestnut, as well as in berry fruits, such as in strawberries and raspberries [35,36], as well as in the loosestrife family (Lythraceae), particularly pomegranate [10,18,37], and in certain nut seeds [38] and Muscadine grapes. Ellagic acid exhibits, e.g., anti-cancer properties, thanks to which it may inhibit cell division and induce apoptosis in cancer cells [39,40]. Moreover, its anti-inflammatory and antioxidant action [41,42] were investigated and confirmed. Ellagic acid found in Cornelian cherry fruit exhibits immunostimulatory, immunomodulatory, antimicrobial, antioxidative and anti-cancer action. It inhibits the adverse effect of UVB radiation (Ultraviolet B), protects skin against degradation and exhibits anti-inflammatory action [43][44][45]. Ellagic acid is also found in the ester form, bound with glucose, forming hydrolysable tannins (so-called ellagitannins).
Salicylic acid, i.e., 2-hydrobexybenzoic acid, whose natural source is willow, is another compound of particular interest. Willow bark contains a biologically active substance referred to as salicin [46]. Salicin is a β-glucoside of saligenin [47], which, in vivo, undergoes a two-stage transformation consisting of deglycolisation and oxidation to salicylic acid [48,49]. Thanks to the rapid development of chemical synthesis in the late 19th century, this acid has become a direct precursor of other drugs of similar structure, the so-called salicylates, and non-steroid anti-inflammatory drugs. They include, e.g., non-acetylated derivatives of salicylic acid such as sodium salicylate, methyl salicylate, diflunisal, phenyl salicylate (salol), choline salicylate, ethylene glycol salicylate, salicylamide, salsalate, benorylate and diethylamine salicylate [50,51]. In turn, the In plant tissues, other complexes of phenolic acids were also identified, e.g., complexes with flavonoids, fatty acids, sterols and polymers of cell walls. Phenolic acids may also be components of anthocyanins or flavones [30,31]. A separate group is composed of depsides, being a complex of two or more molecules of phenolic acids. In plant organisms, including trees, they are formed mainly in the reaction of the so-called shikimate pathway or malate acid.
Tyrosine and phenylalanine are precursors of most phenolic acids, from which, as a result of deamination, cinnamic acid and its hydroxy derivatives are formed [32].
In plants, phenolic acids are mostly found in the bound form as esters and glycosides contained in lignins and hydrolysed tannins. Examples in this respect may be provided by hydroxycinnamic acids found in ester complexes with carboxylic acids or with glucose. They appear in ester complexes with the following acids: malonic, tartaric, α-hydroxy-hydrocaffeic, hydroxycinnamic, tartronic, shikimic, galacturonic, glucaric (as caffeic acid glucuronide), gluconic (as feruloylgluconic acid, which main isomer is 2-O-feruloyl gluconic acid) and 4-methoxyaldaric (as 2-O-feruloyl-4-methoxyaldaric acid). In turn, hydroxybenzoic acids are primarily found as glycosides. In plant tissues, other complexes of phenolic acids were also identified, e.g., complexes with flavonoids, fatty acids, sterols and polymers of cell walls [30,31]. Phenolic acids are found in wood of oak, pine, spruce, fir, walnut, willow, birch (leaves), and in the fruits of bird cherry ( Table 2).
A particularly interesting active compound from the group of hydroxybenzoic acids is ellagic acid, a dimer of gallic acid, found in plants in the free form and (more frequently) in an ester complex with glucose, forming hydrolysable tannins (ellagotannins) [33,34]. It is found in the wood of oak, walnut and sweet chestnut, as well as in berry fruits, such as in strawberries and raspberries [35,36], as well as in the loosestrife family (Lythraceae), particularly pomegranate [10,18,37], and in certain nut seeds [38] and Muscadine grapes. Ellagic acid exhibits, e.g., anti-cancer properties, thanks to which it may inhibit cell division and induce apoptosis in cancer cells [39,40]. Moreover, its anti-inflammatory and antioxidant action [41,42] were investigated and confirmed. Ellagic acid found in Cornelian cherry fruit exhibits immunostimulatory, immunomodulatory, antimicrobial, antioxidative and anti-cancer action. It inhibits the adverse effect of UVB radiation (Ultraviolet B), protects skin against degradation and exhibits anti-inflammatory action [43][44][45]. Ellagic acid is also found in the ester form, bound with glucose, forming hydrolysable tannins (so-called ellagitannins).
Salicylic acid, i.e., 2-hydrobexybenzoic acid, whose natural source is willow, is another compound of particular interest. Willow bark contains a biologically active substance referred to as salicin [46]. Salicin is a β-glucoside of saligenin [47], which, in vivo, undergoes a two-stage transformation consisting of deglycolisation and oxidation to salicylic acid [48,49]. Thanks to the rapid development of chemical synthesis in the late 19th century, this acid has become a direct precursor of other drugs of similar structure, the so-called salicylates, and non-steroid anti-inflammatory drugs. They include, e.g., non-acetylated derivatives of salicylic acid such as sodium salicylate, methyl salicylate, diflunisal, phenyl salicylate (salol), choline salicylate, ethylene glycol salicylate, salicylamide, salsalate, benorylate and diethylamine salicylate [50,51]. In turn, the acetylated derivative of this acid, i.e., aspirin, is an anti-inflammatory, analgesic, antipyretic and antirheumatic drug. Oak Quercus robus L.
Based on our own conducted research on a representative number of plant material samples of wild-growing trees and shrubs in north-western Poland, it was found that spruce needles had the highest content of phenolic acids, and pine needles the lowest (Table 3). Among the analysed acids, we found in fir benzoic and vanillic, in larch caffeic and coumaric acids, in pine caffeic and ferulic, and, in spruce, 4 hydroxybenzoic, caffeic and chlorogenic, which were found in the highest concentration in the needles tested. Salicylic acid and rozmaric acid, on the other hand, were present at a very low concentrations in all tested samples, except for larch for the latter acid. The samples varied considerably in terms of the content of phenolic acids (Table 3). Elderberry, bird cherry and dogwood fruits and dogwood leaves were characterised by high phenolic acid content, and the samples were particularly rich in benzoic acid (especially dogwood), p-coumaric acid and chlorogenic acid. The highest variation was found in the case of benzoic acid, where, in the dogwood fruit sample, it was present in concentrations of 655 µg/g d.w. and 17 µg/g d.w. for bird cherry bark. Protocatechuic, 4-hydroxybenzoic, vanillic, caffeic, salicylic and rozmaric acids were present in low concentrations in almost all samples (Table 4).

Flavonoids
The chemical structure of all flavonoids is based on the hydrocarbon skeleton of flavone (Figure 1). They differ in the number and type of substituents, while differences between these compounds result primarily from a different structure in only one basic ring. Chalcone formed via biosynthesis from phenylalanine is a precursor of flavonoids. Its synthesis starts with shikimic acid. Flavonoids are found not only as free molecules (aglycones), but also much more frequently in the bound form with sugars (glycosides). To date, over 7000 various flavonoids have been identified, which, in terms of their chemical structure, are divided into flavones, flavonols (3-hydroxyflavones), flavanones, flavanols (flavan-3-oles), flavanonoles, anthocyanidins, isoflavones and neoflavonoids (Figure 1, Table 5). Thanks to their unique structure, flavonoids may protect the cell against reactive oxygen species (ROS) generated in the organism [81,82]. Flavonoids are phytoalexins, i.e., substances serving protective functions, formed as a result of the plant's contact with a pathogen, frequently inducing the expression of several genes encoding enzymes of the phenolic biosynthesis pathway [84]. Isoflavonoids are highly toxic towards fungal pathogens, which is particularly evident in such compounds as pterocarpanes, isoflavanes, isoflavones and isoflavonones. The mechanism of their action consists of the inhibition of spore development and mycelium growth as well as the damage of fungal cell membrane structure [85][86][87][88]. Flavonoids are compounds that are commonly found in plants; therefore, they constitute an everyday part of the average human diet (approx. 1 g/day). They are, among others, found in fruits (chokeberry, citrus fruits, blueberries, blueberries, grapes, cherries) and vegetables (onions, tomatoes, peppers, soybeans, broccoli) and in trees and shrubs (Table 6).  Catechin, naringenin, quercetin, [79] Horse chestnut quercetin, kaempferol, rutin [111] Bird cherry Prunus padus L. fruits Catechin, epicatechin, hyperoside, quercitrin quercetin, rutin [112] Based on our own conducted research on plant material samples, it was found that fir needles and elderberry fruit had the highest content in flavonoids, and spruce needles and bird cherry leaves the lowest (Tables 7 and 8). Catechin and naringenin were present in the highest concentration in the needles tested.
The highest concentration of flavonoids found in elderberry fruits turned out to be quercetin. A high concentration of catechins was observed in samples of the bark and fruit of bird cherry. Among the analysed flavonoids, vitexin, rutin, quercetin, apigenin, kaempferol, and luteolin were present in very low concentrations in all sample needles. Samples of the fruit, bark and leaves of the examined trees and shrubs contained low concentrations of luteolin, vitexin and kaempferol (Table 8). Experiments in vitro and in vivo show the varied attributes of these compounds, including their antioxidant, anti-inflammatory, anticancer, antiatherosclerotic and anti-aggregational properties, as well as their capacity for plugging vessels and detoxification. The multidirectional spectrum of the functions of flavonoids suggests a wide range of prospective applications for these compounds, not only in the prevention of many diseases, but also in their therapy (e.g., cancers, cardiovascular disease, atherosclerosis, diabetes, etc.) [111][112][113][114].

Tannins
Tannins are a group of organic chemical compounds, derivatives of phenols, which are naturally produced by plants. Tannins are usually divided into two basic groups: hydrolysable and non-hydrolysable (condensed) tannins [115][116][117].
In the centre of the hydrolysable tannin molecule is a monosaccharide (glucose or other polyols, e.g., branched sugar-hamamelose, shikimic, quinic acid, and even pectin), whose hydroxyl groups, partially or completely, are esterified with gallic acid residues, e.g., m-digalus acid. These tannins are easily hydrolysed by weak acids and bases or enzymes to monomeric products. Depending on the type of resulting products, gallotannins and ellagitannins are distinguished. Gallotanins are the simplest tannins, containing, in their molecule, glucose and ester in association with gallic acid. Another example of such compounds is tannic acid (C 76
In the centre of the hydrolysable tannin molecule is a monosaccharide (glucose or other polyols, e.g., branched sugar-hamamelose, shikimic, quinic acid, and even pectin), whose hydroxyl groups, partially or completely, are esterified with gallic acid residues, e.g., m-digalus acid. These tannins are easily hydrolysed by weak acids and bases or enzymes to monomeric products. Depending on the type of resulting products, gallotannins and ellagitannins are distinguished. Gallotanins are the simplest tannins, containing, in their molecule, glucose and ester in association with gallic acid. Another example of such compounds is tannic acid (C76H52O46); see Figure 4.   (Figure 4), but the C4→C6 bond also exists (both called B-type) . The flavan-3-ol units can also be doubly linked by an additional ether bond at C2→O7 (A-type). The size of PA molecules can be described by their degree of polymerisation (114). Three common flavan-3-ols, which differ in their hydroxylation patterns, are found in PAs. Proanthocyanidins, consisting exclusively of (epi)catechin, are called procyanidins (PCs). Proanthocyanidins, containing (epi) afzelechin or  (Figure 4), but the C4→C6 bond also exists (both called B-type). The flavan-3-ol units can also be doubly linked by an additional ether bond at C2→O7 (A-type). The size of PA molecules can be described by their degree of polymerisation (114). Three common flavan-3-ols, which differ in their hydroxylation patterns, are found in PAs. Proanthocyanidins, consisting exclusively of (epi)catechin, are called procyanidins (PCs). Proanthocyanidins, containing (epi) afzelechin or (epi)gallocatechin as subunits, are named propelargonidins (PPs) or prodelphinidins (PDs), respectively. Propelargonidins and PDs are less common in nature than procyanidins [115].
The basic route for the synthesis of all tannins is the pathway associated with sugar catabolism, leading to shikimic acid. Gallic acid is formed from shikimic or quinic acid, which is a substrate in various types of condensation, resulting in the synthesis of tannins [115].
Proanthocyanidins (also called condensed or non-hydrolysable tannins) are also found in leaves, lignified parts of plants, as well as flowers and fruits [118]. Inflorescences of hawthorn (Crataegi inflorescentia, Crataegus sp. Rosaceae) are well-known sources of proanthocyanidin that have been used in herbal medicine for years [119,120]. Recently, intensive studies have been conducted on an extract from the bark of maritime pine Pinus pinaster (Pinaceae), patented as Pycnogenol, whose proanthocyanidin content is 85%. This preparation exhibits, e.g., strong antioxidant properties (Table 9) [121][122][123]. Table 9. Proanthocyanidins (condensed tannins) found in trees and shrubs.

Stilbenes
Stilbenes are metabolites of the phenylpropanoid pathway, activated under biotic and abiotic stress. They are compounds with a 1,2-diphenylethylene skeleton. Only some unrelated plant species are capable of synthesising and accumulating stilbenes. The enzyme facilitating this synthesis is stilbene synthase (STS). In plants, stilbenes serve several functions, among which the most significant is related to strong antimicrobial properties; thus, they are classified as phytoalexins [142]. Other known functions also include their repellent action against herbivores, as well as their allelopathic and antioxidant properties. Stilbenes are produced in small amounts; however, biosynthesis is activated primarily post infection, while it is also triggered by wounding, UV radiation, ozone and aluminium ions. Resveratrol (3,5,4 -trihydroxy-trans-stilbene) is one of the most extensively described stilbenes [142][143][144].
Resveratrol is found in the form of two isomeric forms: cis and trans ( Figure 1). Transresveratrol is a phenol stilbene found in many plants, e.g., the grape family (Vitaceae), and is particularly common in grape vines (Vitis vinifera). The other form of resveratrol, i.e., cis, is formed as a result of the isomerisation of trans-resveratrol and after the decomposition of resveratrol polymer molecules during the fermentation of grape skins, due to the action of UV radiation, and at high pH [144]. Resveratrol was first isolated in 1940 from the roots of Veratum gradiflorum [145]. Its highest concentration was recorded in the roots of Japanese knotweed (Polygonum cuspidatum). In folk medicine, this plant was successfully used to treat pyoderma, mycoses and venereal diseases [146]. Moreover, resveratrol has been applied in cancer prevention and treatment thanks to its ability to effectively inhibit each stage of neoplasia, i.e., the initiation, promotion and progression of the disease [147,148].
Stilbenes are secondary metabolites that are relatively rarely found in nature (Table 10). To date, they have been reported in almost 70 unrelated plant species belonging to approx. 30 genera and 12 families. The greatest stilbene contents are detected in plants from the pine family (Pinaceae), the grape family (Vitaceae), the beech family (Fagaceae), the mulberry family (Moraceae) and the grass family (Poaceae) [141]. Table 10. Stilbenes found in trees and shrubs.
Piceatannol and resveratrol [156] Morus spp. Mulberry Resveratrol [157] The presented literature sources indicate that phenolic substances of plant origin, particularly those obtained from trees and shrubs growing in a temperate climate zone, exhibit a beneficial effect on human health. Thanks to the presence of bioactive compounds in those plants, they have found applications as detoxicants, vitamin supplements, as well as preparations boosting immunity and adjunctive medication in the treatment of various diseases. There is a considerable body of data indicating that a diet rich in bioactive compounds plays a significant role in the prevention of cardiovascular diseases and cancer. In view of the fact that the treatment of chronic pain, cancer, cardiovascular disease and a number of other diseases requires a combination of several therapeutic methods, alternative therapies using plant origin preparations are gaining popularity. It also needs to be stressed that molecular mechanisms of action, in the case of active substances contained in plant preparations, have not been fully elucidated and require further research.
The review of the literature presented in this paper presents the potential of trees and shrubs native to temperate zones as sources of phenolic substances.