This is a family of about 435 species of trees and shrubs in two genera, namely Populus L. and Salix L., distributed chiefly in northern temperate regions (Mabberley 1987). Members of both genera, but in particular members of the genus Salix are noted for their propensity to produce inter-specific hybrids. This makes accurate identification of plant material difficult. Thus, dermatological reports referring to Populus or Salix taxa that have not been properly identified by an authority on this plant family need to be interpreted with caution.
Members of the Salicaceae are perhaps best known as a source of salicin, the precursor of aspirin, which was at one time widely available as a purified chemical entity with the antipyretic, anti-inflammatory and analgesic properties we now associate with aspirin.
Although salicin does not appear to have any history of use as an externally-applied remedy, aqueous decoctions and alcoholic extracts of the buds, bark and twigs of both Populus and Salix species have been used to treat skin conditions. Salicin, which is a phenolic glucoside, co-occurs in the plant material with its aglycone saligenin, otherwise known as salicyl alcohol. Saligenin has been used as a local anaesthetic and is a known contact allergen. Certain Populus species produce a sticky resinous material on their buds and young twigs. This material is harvested by man for commercial purposes. It is also harvested by bees for use in their bee-hives. This bee-processed material is then too harvested by man for commercial purposes. The former product is termed balm of Gilead; the latter is termed propolis. Both balm of Gilead and propolis have been reported to induce contact sensitivity reactions in man. A variety of contact sensitisers of propolis and poplar bud extracts have been identified, these principally being caffeate but also isoferulate and cinnamate esters.
About 35 species are found in northern temperate regions, including some of the fastest-growing trees in these regions; many inter-specific hybrids are known (Mabberley 1987). In addition to the timber provided by many species, these trees are a botanical origin of propolis - bee glue - which is made by honey bees (Apis mellifera L.) from the sticky resinous material they collect from the buds and young twigs of certain Populus species, and which is in turn harvested by man from bee-hives. Propolis is a dark yellowish-brown substance with a fragrant smell suggestive of cinnamon (Cinnamomum). The resinous material may also be harvested by man directly from the buds and young twigs to produce a minor article of commerce known as balm of Gilead. Clearly, the precise botanical origin of any given sample of either propolis or balm of Gilead will be obscure unless positive attempts have been made by the supplier, user, or investigator to determine this information.
A balm of Gilead sample purchased in Oxford, UK and supposedly derived from Populus candicans exhibited chemical characteristics suggesting that it was actually derived from Populus nigra or a hybrid thereof (Whatley et al. 1989).
The composition of bee glue varies according to its plant source and includes wax, crude resins, essential oils, colouring matters and esters of cinnamic alcohol (Bunney 1968, Hjorth 1961). Trees given by common name as sources of bee glue are poplar (Populus), horse-chestnut (Aesculus), spruce (Picea), willow (Salix), lime (Tilia), sycamore (Platanus) and fruit trees (Ribbands 1953); and fir (Abies), alder (Alnus) and birch (Betula) (Rothenborg 1967). Whilst various Populus species or hybrids seem to be the principal source of propolis in Europe, propolis collected in different parts of the world may have an entirely different botanical origin and therefore chemical constitution. For example, propolis sourced from Brazil may be derived from one or other of the following species (Bankova et al. 1999, Park et al. 2002):
Propolis sourced from Venezuela, by contrast, appears to originate from Clusia minor L. and Clusia major L., fam. Guttiferae (see Tomás-Barberán et al. 1993).
Dermatitis from bee glue was reported by Umansky in 1934. In 14 cases of bee-keepers' dermatitis, positive patch test reactions were observed to samples of bee glue obtained from various parts of the United Kingdom and in all of 7 and 6 patients tested to resins extracted from the buds of balsam poplar (P. balsamifera) and lombardy poplar (query P. nigra) respectively. Additional sensitivity was observed to extracts of buds of spruce (Picea), willow (Salix), horse-chestnut (Aesculus), lime (Tilia) and apple (Malus × domestica (Suckow) Borkh., fam. Rosaceae) (Bunney 1968). She concluded that once a patient has become sensitised by the more potent or abundant allergen in bee glue derived from poplar, he may then react to the weaker or less abundant allergens in the resins of some other trees from which bee glue is derived. The allergenic fraction of bee glue is found in the volatile oil derived from it by distillation.
Patients contact sensitive to Populus and to bee glue derived from Populus have been observed to show positive patch test reactions to balsams of Peru and Tolu (see Myroxylon balsamum Harms, fam. Leguminosae) (Rothenborg 1967). Benzyl salicylate and cinnamic acid derivatives may be mutually responsible. According to Rothenborg (1967) allergic contact dermatitis from unpurified beeswax contaminated by bee glue can occur in patients who have no contact with bees but who become sensitised by beeswax in cosmetics and medications or who are cross-sensitised by balsamic resins derived from other plants.
Beeswax was suspected as a cause of dermatitis in a man who worked inside a barrel which contained resin and beeswax (Queries and Minor Notes 1931). Early reports of dermatitis from beeswax (Greenberg and Lester 1954) probably refer to raw or unpurified beeswax. Patch test reactions to purified beeswax were negative in 2,634 patients who had dermatitis (Rothenborg 1967). It seems, therefore, that purified beeswax is unlikely to be a significant source of cosmetic dermatitis. Beeswax has been reported to produce untoward reactions when used as an injection vehicle (Fernstrom 1957); injection of honey produced a state of shock (Mazzi 1964). Beeswax used for art work caused dermatitis in a woman who was employed in making mouldings (Camarasa 1975).
A bibliography is provided by Rothenborg (1967), Bunney (1968). Bee-keepers' dermatitis has been reported from Hungary (Racz 1960).
Perioral dermatitis from propolis (bee glue) in creams was reported by Wanscher (1976). In these patients, balsam of Peru (from Myroxylon balsamum Harms, fam. Leguminosae) produced negative patch test reactions. She states that propolis is collected by bees from resinous exudations of horse chestnut (Arbor castanea [sic]) (Aesculus hippocastanum), spruce (Abies nobilis), willow (Salix), larch (Larix), fir (Pinus) and especially poplar (Populus).
Schulmann & Détouillon (1932) noted that "le peuplier" — poplar — is commonly implicated in woodcutter's eczema. Dermatitis from poplar sawdust in a carpenter was referred to by Senear (1933). Lovell et al. (1955) observed a strong (4+) patch test reaction to "poplar pollen oil" in a patient with allergies to various tree pollen oils who presented with an airborne contact dermatitis.
Dermatitis of the hands and forearms followed by exfoliative dermatitis and lymphadenopathy resembling lymphoma occurred in a wood-worker from the sawdust. Positive patch test reactions to dry and moistened wood dust were observed (Weber 1953). This author noted that closed patch test with moistened wood-dust of various woods often produced irritant reactions and he questioned some of his own findings as to the allergenicity of some native woods. A cabinetmaker developed dermatitis from poplar saw-dust which was derived from a factory situated above his workplace (Caro MR, in discussion of Weber 1953). Cross-sensitivity has been observed between poplar (Populus), aspen (Populus) and willow (Salix) (Tan and Mitchell 1968). Two woodmen who had dermatitis had been in contact with resin of white poplar buds showed positive patch test reactions to purified balsamic resins and essential oils derived from the tree and to bee glue (propolis) produced from the tree by honey-bees (Winkler 1956). There was no history of contact with bee-hives in these patients.
According to Stuart (1979), possibly from Anon (1960), balm of Gilead of commerce is derived from P. candicans, the air-dried closed winter leaf buds from which provide a minor article of commerce known as balm of Gilead buds (Populi Gemma). Anon (1960) also notes that Populus tacamahacca similarly provides balsam poplar buds. The balsam separates from the buds when they are immersed in boiling water (Felter & Lloyd 1898). It (the balsam) is also known as tacamahaca.
Care should be taken not to confuse the balm of Gilead poplar with the balm of Gilead fir (Abies balsamea Mill., fam. Pinaceae). This fir is the source of Canada balsam, which is also known as balm of Gilead. Similarly, care should be taken not to confuse balm of Gilead derived from the balsam poplar with balm of Gilead, otherwise known as balm of Mecca, derived from Commiphora opobalsamum (L.) Engl., fam. Burseraceae.
The fragrant resinous leaf-buds from this tree preserved in rum or alcohol were a popular household remedy for external use in New England (White 1887). The application of this preparation to bruises, cuts, and sprains produced a smarting sensation but could also produce a very severe inflammation. White noted that dermatitis from balm of Gilead remained localised to the site of application whereas dermatitis from Tincture of Arnica (see Arnica montana L., fam. Compositae) became disseminated. He observed one or two cases of bullous dermatitis each year from this source. Felter & Lloyd (1898) referred to a similar application prepared from poplar buds using oil or lard rather than alcohol, this presumably not containing the salicin and other phenolic glycosides that one would expect otherwise to be extracted into an alcoholic solvent.
Four of 14 patients contact sensitive to balsam of Peru (from Myroxylon balsamum Harms, fam. Leguminosae) showed positive patch test reactions to Balm of Gilead USP (Hjorth 1961).
The flowers are used as a styptic (Smith 1969).
North American Indians rub the sticky sweet-smelling buds on the face to prevent sunburn. The hairy fruit pods can irritate the hands (Turner and Bell 1973).
Rothenborg (1967) described dermatitis in a beekeeper resulting from contact with bee glue derived from "Populus canadensis". Positive patch test reactions were observed to the leaf and bud of the tree and to raw beeswax from his hives.
Many cultivated forms of this tree are to be found, it now being regarded as a hybrid arising from P. deltoides Bartram ex Marshall × P. nigra L. (Polunin 1969). Accordingly, it is difficult if not impossible to identify retrospectively the particular cultivar to which Rothenborg (1967) refers. The best known cultivar is probably Populus × canadensis cv. Serotina (syn. Populus serotina Hartig), which is known by the common names black Italian poplar and black hybrid poplar. Populus × canadensis cv. Carolinensis (syns Populus carolinensis Foug., Populus × euramericana Guinier) is known by the common names Carolina poplar and Canadian poplar.
According to Le Coulant et al. (1966), the allergens of liverworts belonging to the genus Frullania growing on the bark of "carolins d'Italie" (possibly this poplar) penetrate into the wood and can cause dermatitis in match-makers.
The balm of Gilead poplar is now regarded as a hybrid arising from Populus balsamifera L. × Populus deltoides Bartram ex Marshall (Mabberley 1987). It is cited in the literature as a source of balm of Gilead (see Populus balsamifera L. subsp. balsamifera above).
About 400 species of trees, shrubs, and shrublets are found mostly in northern temperate regions; many inter-specific hybrids are known (Mabberley 1987).
Flück & Jaspersen-Schib (1976) recorded that a decoction of the bark of Salix alba L., Salix fragilis L. and other species has used in traditional medicine as an application to wounds.
A feature of Salix species is the presence in the bark, twigs, leaves, leaf buds, and flower buds of phenolic glycosides, in particular salicin and salicortin (Binns et al. 1968, Julkunen-Tiitto 1986, Julkunen-Tiitto 1989). Salicin was at one time widely used for its antipyretic and analgesic activity as an alternative to aspirin. However, in part because of its propensity to produce skin rashes following oral administration (Todd 1967), it has fallen out of use.
Surveys of Salix species have demonstrated that levels of salicin and other phenolic glycosides vary widely between species and also show within-species and geographical variation as well as seasonal variation. This variability was known to Pereira (1842) who advised that those barks possessing the greatest bitterness should be selected for use (salicin being the sought-after bitter principle). The same advice was later repeated by Felter & Lloyd (1898). The following species were listed in these early publications as having the greatest repute:
Salicortin spontaneously releases salicin, 6-hydroxy-2-cyclohexenone (6-HCH), and pyrocatechol when the leaf or bark is damaged (Ruuhola et al. 2003). Salicin in turn (but less readily) produces saligenin (= salicyl alcohol) through the action of a glucosidase. 6-HCH is recognised by many insects as an antifeedant cue. Phytophagous insects that do not recognise this cue can reportedly utilise the salicortin to produce their own defensive secretions by converting salicin to salicylaldehyde (see, for example, Brückmann et al. 2002). Whilst salicortin, saligenin, and pyrocatechol can occur in quite large quantities in intact plant material (see, for example, Pearl & Darling 1970), free salicylaldehyde seems to occur only in very low concentrations (see Zenk 1967). However, salicylaldehyde may be formed from saligenin by the action of an oxidase once plant material is damaged. Both 6-HCH and catechol are potential contact allergens but do not appear to have yet been investigated for such activity in the context of sensitisation to Salix (or Populus) species. Saligenin is also known as 2-methylol phenol, a substance with recognised contact allergenic properties in the context of sensitisation to phenol-formaldehyde resins. Patients sensitised to phenol-formaldehyde resins and reacting to 2-methylol phenol also reacted to salicylaldehyde (Bruze & Zimerson 1997).
Saligenin was at one used as a local anaesthetic for minor surgery (Martindale & Westcott 1924).
Felter & Lloyd (1898) note that a decoction prepared from the bark of this willow has proved efficient as a local application to foul and indolent ulcers.
In traditional Chinese medicine, the leaf and fibrous root of this species are known as liu ye. Preparations of the material are used externally to treat urticaria, and on wounds to protect against infection (Huang 1993). Perry & Metzger (1980) note also that in China, a decoction of the young shoots or leaves is used for washing abscesses, ulcers, skin diseases, and varnish poisoning; and also that a decoction of the bark of the twigs is added to a bath to treat parasitic skin eruptions.
Confusion may arise from the use of the common name weeping willow because it usually refers to Salix alba L. cv. Tristis (Hunt 1968/70). More recent literature (Mabberley 1987) refers to the common weeping willow as Salix × sepulcralis Simonkai, a hybrid derived from Salix alba L. × Salix babylonica L.
In Indian traditional medicine, a decoction of the bark is used for cataplasms against obstinate dermatopathies and ulcers (Nadkarni 1976).
Felter & Lloyd (1898), referring to "Salix nigra L." by the common names black willow and pussy willow, recorded that "the bark of black willow is recommended as a poultice in gangrene, and as an external application to foul and indolent ulcers and rhus poisoning, in which it stands unrivaled. It is made by simmering the powdered bark in cream." Wren (1975) appears to have described the same preparation in a monograph on Salix discolor Muhlenb., the common name of which also being pussy willow, identifying "Salix nigra Marshall" as a synonym. This seems to be an example of the kind of problem that reliance on a common name rather than the scientific name for identification of a plant can generate.
Perry & Metzger (1980) note that in China, the bark serves as a styptic and anodyne on wounds.
Biswas & Mukherjee (2003) include this species, which is known locally as jalabetas, in a list of plants of Indian origin used for their wound healing activity in Ayurvedic medicine. The stem bark and flowers are the parts used.
This species is widely cultivated in Europe as a source of raw material for basket-making (Polunin 1969). Three cases of occupational dermatitis of the hands in women working in the basket-making industry were described by Gonçalo et al. (1986). These patients were patch tested with salicylic acid. They were also patch tested with various states of the raw material they used. All tests failed to elicit any reaction.
Watt & Breyer-Brandwijk (1962) recorded that in Basutoland, the bark of this species is used in making a preparation for the treatment of burns.