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Introduction
Methodology
The UK Forest Resource
Secondary Metabolites From Trees

Phenolics
Nitrogenous compounds
Terpenes
References

Non-Timber Markets For Trees
Extraction Technologies For Tree Metabolites
Adding Value To Tree Metabolites
Further Research
Modelling Tools
Secondary (Special) Metabolites From Trees
Flavanoids, terpenes phenols, alkaloids, sterols, waxes, fats, tannins, sugars, gums, suberins, resin acids and carotenoids are among the many classes of compounds known as secondary or special metabolites (Gottlieb 1990). In the realm of wood processing they are everything that is not a structural polysaccharide or lignin. The array of compounds is daunting, with wide ranging chemical, physical and biological activities. The concentrations of these metabolites vary between species (they may contain as little as 1% or as much as one-third of their dry weight as secondary metabolites), between tissues (higher concentrations occur in bark, heartwood, roots, branch bases and wound tissues), among species from tree to tree and from season to season. They thus present numerous challenges to the utilisation of trees (e.g. contamination of cellulose fibres during paper production) and numerous opportunities for adding value to forest products (e.g. pharmaceuticals, adhesives and preservatives). The database reviews the structure, tissue specificity and properties of the many compounds isolated and identified in UK forest tree species. This section provides a brief overview of the breadth and variety of secondary metabolites from wood. Information on the primary structural compounds in trees (e.g. celluloses) is discussed elsewhere in this report.
The production and accumulation of a wide variety of organic chemicals is one of the major mechanisms by which plants defend themselves against herbivory, and attacks by microbial pathogens and invertebrate pests. Most of these chemicals are products of secondary metabolism, originally thought to be the waste products not needed by plants for primary metabolic functions. It is, however, well known now that their presence in different parts of the plant (root, leaves, bark etc) deters feeding by slugs, snails, insects and vertebrates, as well as attacks by viruses, bacteria and fungi (Winks & Schimmer 1999). For example, pine needles are known to contain a number of secondary metabolites (SMs) that have been shown to depress forage digestibility in mammals (Adams et al. 1992), and essential oil from Pinus sylvestris has been shown to possess a strong antibacterial activity (Schales, Gerlach, & Koster 1993). Besides their role in the chemical defence of a plant, SMs (e.g. volatile fragrant monoterpenes, coloured anthocyanins and carotenoids) may also act as chemical signals to attract beneficial animals for pollination and seed dispersal. Some SMs are known to exhibit both of these functions; for example, anthocyanins and monoterpenes act as insect attractants in flowers, but may be insecticidal and antimicrobial when present in leaves (Winks 1999). Plants are also known to produce many volatile chemicals (aldehydes, esters, amines) in response to damage by pests and diseases and also to alert other plants, and in some cases to attract predators to combat attacking pests.
The content of SMs varies hugely among plant species; some may contain up to a third of their dry weights as SMs. Generally, tropical and sub-tropical plant species contain much greater amounts of extractives than the ones in the temperate regions. Furthermore, the concentration of SMs in all parts of a tree is not uniform, and different amounts may be present in leaves, flowers, fruits, bark, heartwood, roots, branch bases and wound tissues. Variations in the content of SMs have also been found among species, between trees of a given species, and between different seasons.
Plant SMs are effective against pests and disease agents because they are either analogues of certain vital components of the cellular signalling system, or can interfere with vital enzymes and block metabolic pathways. In non-target species, however, many of the compounds exhibit certain useful biological activities. In fact, plant SMs have been used by humans for thousands of years as dyes (e.g. indigo, shikonin), flavours (e.g. vanillin, capsaicin), fragrances (e.g. essential oils of rose, lavender), stimulants (e.g. caffeine, nicotine), hallucinogens (e.g. morphine, tetrahydrocannabinol), poisons (e.g. strychnine, coniine) and medicines (e.g. quinine, atropine). Before the discovery of modern pesticides, plant extracts containing nicotine and pyrethrin were widely used in agriculture as insecticides. However, it was the potential use of plant SMs in health care and personal care products, and as lead compounds for the development of novel drugs, that led to a huge interest in their isolation and characterisation from major plant species over the past few decades. At present, the total number of identified SMs exceeds 100,000 (Winks 1999). These can be grouped into three main chemical classes:
  1. Phenolics
  2. Nitrogen containing compounds
  3. Terpenes (terpenoids)
Phenolics
These organic compounds are characterised by the presence of a hydroxyl (-OH) group, attached to a benzene ring or to other complex aromatic ring structures. Phenols with more than one hydroxyl group per aromatic ring are known as polyhydric phenols (e.g. catechol, resorcinol, hydroquinone, pyrogallol). Phenolic compounds range from simple phenol (MW 94, found in essential oil of Pinus sylvestris) to polyphenols such as anthocyanin pigments (MW 2,000) and tannins (MW up to 20,000). The term 'tannin' is derived from the historic use of these polyphenolics to produce leather from animal skins. Tannins are mainly found in bud and foliage tissues, seeds, bark, roots, sapwood and heartwood; but bark and heartwood often contain the highest levels. Tannins may be condensed or hydrolysable; condensed tannins are widespread in plants, whilst hydrolysable ones are limited to dicotyledonous plants. Both types can occur together in the same plant as in oak bark and leaf. Hydrolysable tannins are either gallotannins (glucose core surrounded by 5 or more galloyl ester groups) or ellagitannins (containing hexahydroxydiphenic acid). Condensed tannins (also called flavolans) are made of catechin units. Barks of many forest trees, such as silver birch, cherry, Douglas fir, larches, and Sitka spruce, provide a rich source of polyphenols and tannins; in particular alder bark and fruit have been reported to contain around 20% tannins (Usher 1974).
Another important type of polyphenolic compounds is flavonoids that have pharmacologically useful antioxidant properties. Of the 8000 known phenolic compounds, around 4000 are flavonoids (Harborne 1988). Flavonoids commonly occur in foliage, bark, sapwood and heartwood in trees. These polyphenolic compounds are essentially a class of water-soluble pigments from plants. The main properties reported for flavonoids include antioxidant, anti-inflammatory, antihistaminic, and antiviral. For example, quercetin (found in a number of forest trees, and in particular, the bark of Quercus spp.) has been reported to block the sorbitol pathway, which is linked to certain problems associated with diabetes. Quercetin may also protect beneficial low density liposomal cholesterol (LDL) in the body; similarly, rutin and many other flavonoids are known to protect blood vessels from oxidative damage.
Some phenolics also occur as glycosides; such as salicin, the bitter antipyretic compound found in relatively large amounts in many Populus and Salix species (Lee et al. 1993). Coniferyl and sinapyl alcohols may also be present as glucosides, presumably as immediate precursors to lignin synthesis. Stilbenes are also commonly found in the heartwood of Pinus species and may occur with phenolic hydroxyls, methylated or as glycosides.
Lignans are the products of oxidative coupling of propylphenols through β-β coupling of the side chain (neolignans are formed by coupling of the side chains other than β-β linkage). Examples of lignans include pinoresinol (in Pinus spp.), Lariciresinol (in Pinus, Larix and Picea spp.) and syringaresinol (in Quercus rubra and Salix sachalinensis) (Lee et al. 1993). Lignans display a wide range of biological activities including fungal growth inhibition, fish toxicity, insect antifeedant and juvenile hormone functions (Gottlieb & Yoshida 1989).
Table 1. Phenolic secondary metabolites (from Harborne 1991)
Class Biological Activity Example
Coumarins Allergenic, antifungal
Hepatotoxic
Blood anticoagulants
Xanthotoxin
Aflatoxins
Dicoumarol
Flavanones Bitter tasting Naringin
Flavones and flavonols Antioxidants
Antimicrobial
Enzyme inhibitors
Genotoxic
Quercetin
Kaempferol
Hydroxyquinones Allergenic
Photoactive
Allelopathic
Primin
Hypericin
Juglone
Lignans Antitumor
Antiviral
 
Phenols Allergenic
Antimicrobial
Urushiol
Phenol
Stilbenoids Antifeedant
Antifungal
Pinosylvin
Tannins Feeding deterrents
Astringent taste
Bind to proteins
Molluscicides
 
Xanthones Cytotoxic
Anti-inflammatory
 
Nitrogenous compounds
The most important nitrogen containing SMs in plants are alkaloids; with over 10,000 known structures, but they are only found in 20% of the angiosperms. Alkaloids are generally present in higher concentrations in bark, seeds, roots and leaves than in wood. All alkaloids contain nitrogen heterocycles, and are mainly present in plants as salts of carboxylic acids (such as citric, lactic, oxalic, acetic, malic and tartaric, fumaric, benzoic acids). Alkaloids have a wide variety of chemical structures e.g. monocyclic, dicyclic, tricyclic, tetracyclic, and more complex cage structures, and are classified according to the type of ring (pyrrolidine, piperidine etc) and their biosynthetic origin. Alkaloids and amines often affect neuroreceptors as agonists or antagonists, or modulate other steps in the signal transduction e.g. ion channels and enzymes (Winks & Schimmer 1999). This is because alkaloids are derived from the same amino acid precursors as neurotransmitters, and their structures often mimic those of neurotransmitters. For example, alkaloids have been shown to bind to acetylcholine receptors (both nicotinic and muscarinic) mimicking the structure of the natural ligand acetylcholine; to adrenergic receptors, mimicking the natural ligands noradrenaline and adrenaline; to dopamine receptors mimicking the natural ligand dopamine; to serotonin receptors mimicking the natural ligand serotonin; and to GABA receptors mimicking the natural ligand gamma-aminobutyric acid (Winks, 1999). Furthermore alkaloids may affect the function of ion channels by inhibiting neurotransmitter-degrading enzymes (such as acetylcholinesterase, monoamine oxidase) or by modulating enzymes involved in signal transduction (such as adenylyl cyclase, phosphodiesterase, protein kinase, phospholipase) (Winks 1999). Alkaloids are well known for potent pharmacological activities, such as analgesics, antimalarial, antispasmotics, and treatment of hypertension, mental disorders and tumours.
Other nitrogen-containing compounds such as non-protein amino acids, cyanogenic glycosides (containing cyanide bound to sugars, which is released when tissue is damaged; such as (R)-prunasin from leaves and bark of Prunus spp.), and glucosinolates are less common in plants. The non-protein amino acids, which are analogues of other amino acids used in protein synthesis, may block their uptake and transport and disturb biosynthesis. Non- protein amino acids also become incorporated into polypeptides to produce faulty proteins. Cyanogenic glycosides are stored in vacuoles. They are broken down by β-glycosidase enzymes to release HCN, which is a potent poison for the attacking pests.
Table 2. Nitrogenous secondary metabolites (from (Harborne 1991)
Class Biological Activity Example
Quinolizidine alkaloids
Piperidine alkaloids
Steroidal alkaloids
Tetratogens Anagyrine
Coniine
Jervine
Nitro-compounds
Azoxy-compounds
Neurotoxic Miserotoxin
Cycasin
Protease inhibitors
Phytohaemagglutinins
Antinutritional Trypsin inhibitors
Bean lectin
Terpenes (terpenoids)
Terpenes are widely used in the food, pharmaceutical and perfume sectors, as well as in a wide range of pharmacological applications. Terpenes are the largest group of natural products from plants with over 20,000 known structures, comprising essential oils, flavours, fragrances, and lipid-soluble plant pigments. These hydrophobic compounds are usually stored in plants in resin ducts, oil cells or glandular trichomes (Winks & Schimmer 1999). Terpenes are derived from 5-carbon isoprene units [CH2=C(CH3)-CH=CH2], such as C5 hemiterpenes, C10 monoterpenes, C15 sesquiterpenes, C20 diterpenes, C25 sesterpenes, C30 triterpenes, C40 tetraterpenes, and C50 Band over polyterpenes. Whilst lower terpenoids are found in volatile emissions and essential oils, higher terpenes are mainly present in plant's lipid soluble pigments.
Examples of monoterpenes include limonene in turpentines from some Pinus spp., α-terpinene from Pinus sylvestris, terpinolene in some Pinus turpentines, β-phellandrene in Pinus contorta turpentine, and α- and β-pinene, camphene and 3-carene that are abundant in turpentines from coniferous woods (Dev, 1989). The composition of turpentines is highly variable amongst plant species (Fengel & Wegener 1984). For example, the monoterpene fraction of Pinus balfourniana contains up to 81% α-pinene and 1.9% β-pinene, whilst that of Douglas fir Pseudotsuga menziesii contains 31% α-pinene and 36% β-pinene.
The diterpene acids; abietic, pimaric, communic and lambertianic acids are also known as resin acids, found in rosins from gymnosperm woods, particularly the pines. Resin acids are obtained as by-products from Kraft pulping of wood, and are used as sizing agents to control absorption of water in paper products. Betulin is an example of a pentacyclic triterpene found in bark of the birch Betula alba. Sterols are found in woods of a number of gymnosperms and angiosperms including Larix, Picea, Pinus, Fagus, and Quercus spp. Phytosterols are different from animal sterols in that they have an extra methyl or ethyl substituent in the side chain. The major sterol component in a number of conifers is β-Sitosterol, along with campesterol, dihydrobrassicasterol, 24-methyistanol and 5-α-sitostanol (Nes 1989). Sitosterol and other derivatives are essential components of plant cell wall, and also play an important role in cell growth. The concentration of sterols in heartwood is generally low (often >0.1%), but relatively large amounts may be isolated as by-products from the tall oil from Kraft pulping process. Tall oil is used to produce a variety of resins and these products are often referred to as 'Naval Products', which derives from their traditional use in shipbuilding.
Table 3. Terpene Secondary Metabolites (from (Harborne 1991)
Class Biological Activity Example
Monoterpenes Feeding deterrents Camphor
Sesquiterpenes
Sesquiterpene lactones
Toxic to livestock
Vertebrate poisons
Allergenic
Feeding deterrents
Ngaione
Hymenoxin
Parthenolide
Glaucolide-A
Diterpenes Poisonous
Irritant and co-carcinogens
Antitumor
Anticancer
Atractyloside, Grayanotoxins
Phorbol esters
Diterpenes (Larix spp.)
Taxol (Taxus brevifolia)
Triterpenes Poisonous
Antifeedants
Anti-inflammatory
Hemolytic
Lantadenes A&B
Papyriferic acid
Friedelin (Quercus spp.)
Dammarane triterpenes
(Betula pendula)
Tetraterpenoids
(Carotenoids)
Antioxidant β-Carotene
Phytosterols Estrogenic Mirosterol
Most terpenes disturb fluidity of membranes and efflux of ions, whilst some may cause cell death (cytotoxic, antimicrobial). Ruminants are known to avoid high terpene diets because they kill microbial populations in their gut that are needed to digest cellulosic materials. Some glycosides of triterpenes or steroids (called saponins) are haemolytic, whilst others can inhibit the vital enzyme Na+/K+ ATPase in the pests to deter herbivory.
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References
Adams,D.C., Pfister, J. A., Short, R. E., Cates, R. G., Knapp, B. W., & Wiedmeier, R. D. (1992) Pine needle effects on in vivo and in vitro digestibility of crested wheatgrass. Journal of Range Management 45, pp. 249-253
Fengel,D. & Wegener, G. (1984) Wood: Chemistry, ultrastructure and reactions. Walter de Gruyter, Berlin
Gottlieb,O.O. & Yoshida, M. (1989) Lignans. Natural products of woody plants. (ed Rowe,J.W.), pp. 349-511. Springer-Verlag, Berlin
Gottlieb,O.R. (1990) Phytochemicals: differentiation and function. Phytochemistry 29, pp. 1715-1724
Harborne,J.B. (1988) The flavanoids: Advances in research since 1980. Chapman & Hall, London
Harborne,J.B. (1991) The chemical basis of plant defense. Plant defenses against mammalian herbivory. (eds Palo,R.T. & Robbins,C.T.), pp. 45-59. CRC Press, New York
Lee,H., Watanabe, N., Sasaya, T., & Ozawa, S. (1993) Extractives of short-rotation hardwood species, I. Phenolics of Salix sachalinensis. Fr.Schm.Mokuzai Gakkaishi 39, pp. 1409-1414
Nes,W.R. (1989) Steroids. Natural products in woody plants. (ed Rowe,J.W.), pp. 808-842. Springer-Verlag, Berlin
Schales,C., Gerlach, H., & Koster, J. (1993) Investigation on the antibacterial effect of conifer needle oils on bacteria isolated from feces of captive capercaillies (Tetrao urogallus). Journal of Veterinary Medicine 40, pp. 381-390
Usher,G. (1974) Dictionary of plants used by man. Constable, London
Winks,M. (1999) Function of Plant SMs and their exploitation in biotechnology. Sheffield Academic Press, Sheffield
Winks,M. & Schimmer, O. (1999) Modes of action of defensive secondary metabolites. Function of Plant SMs and their exploitation in biotechnology. Annual Plant Reviews. pp. 17-133. Sheffield Academic Press, Sheffield