Tuesday, 31 July 2018

Wild Service Tree (Sorbus torminalis)

WILD SERVICE-TREE or CHEQUERS (Sorbus torminalis (L.) Crantz)

Family: Rosaceae

(not to be confused with the "Serviceberry" shrub (Amelanchier alnifolia Nutt.)

Sorbus torminalis
(Michelham, October 2011)
The Wild Service Tree, also known as “Chequers” is probably the rarest of the approximately 40 Sorbus species native to the British Isles. Thinly distributed on clay and limestone soils in lowland areas across the Midlands and southeast England and Wales it is also found in the temperate zones of Mediterranean Europe (for distribution maps in England, see: Thomas 2017: 1808 or OnlineAtlas of the British and Irish Flora; for distribution maps over Europe, see EUFORGEN).

It is a deciduous woodland tree that is at its most conspicuous in the autumn, when the flame-shaped leaves change to fiery reds and oranges. In Britain today, it is most commonly found in groves, thickets and hedges, and generally reaches a height of 10-15m. Although considered shade-tolerant, it flourishes when grown in conditions of light and space, and free-standing trees are usually taller, sometimes topping at a massive 25 metres.  It is relatively drought-tolerant but vulnerable to frost, which is why in Britain it is limited to the lowlands of the southern and central latitudes (for a comprehensive review of the ecological characteristics of S. torminalis in the UK, see Thomas 2017).

The common name “Service” is from the Old English word “syrfe” (“serves” plural), a derivation of the Latin Sorbus; the species part of the Latin name “tormina”, refers specifically to the effectiveness of the fruit for treating colic (Grigson 1955).

Flowering and Fruiting
Around May the Wild Service Tree produces flowers on a corymb, the inflorescence resembling the creamy white trusses of its close relative Rowan (Sorbus aucuparia). The fruit, known as "Chequerberry" is a small oval or pear-shaped pome that develops in late summer, ripening in the autumn.  The pomes are highly variable in shape and size, even within the same corymb. More commonly obovate to round, and sometimes obovate or elliptical shaped, they measure an average of ~14mm long and ~11 mm wide but can measure anywhere between 8.20 - 19.60 mm long and 8.20 –15.50 mm wide (Bednorz 2007).
Sorbus torminalis – bletted fruits
(including 6 unripe, yellow fruits).
(Michelham, December 2005)
Food Uses: a cautionary tale
In the early stages of development, the fruit is a yellowish colour and too firm to be edible, as well as exceedingly astringent, sour and somewhat bitter (Plants For A Future [PFAF] database). However, with maturation the pome turns a yellow- or purplish-brown colour and becomes so soft that it is almost impossible to pick without squashing. Once bletted (the natural process of browning and softening) the fruits are highly palatable and can be eaten fresh, straight from the tree (with caution: see below).  The naturalist Richard Mabey (1989: 38) describes the taste of the bletted fruit as “…unlike anything else which grows wild in this country, with hints of damson, prune, apricot, sultana and tamarind.” 

Chequers pomes can produce up to six pips (seeds), although one or two is more typical. They are soft and easily chewed and swallowed compared to, e.g. the hard seeds of apple, but unless processed they are potentially toxic. Like the seeds of many other fruit in the Rose family (including apple, cherry, peaches, apricot, pear and plum), S. torminalis seeds contain cyanogenic glycosides, which, if consumed over the long-term, even in small amounts, can cause serious health problems (Bolarinwa et al. 2015; Gleadow and Møller 2014). 

Sorbus torminalis – seeds from
bletted fruits. (Michelham, December 2005)

Thomas (2017: 1824) summarised the historical food uses of Chequerberry in Britain and Europe, noting that although it was often made into jams and jellies, the fruit was primarily consumed in drink form, more commonly an alcoholic cider but also wine and brandy. Fermentation is a way to remove cyanogenic glycosides, which may explain its consumption as an alcoholic drink.

A tasty fruit snack can be prepared by pounding the fruit into a mush with a pestle and mortar, shaping the mush into cakes or dampers (unleavened loaves) and baking them, e.g. on hot stones, in ashes or in an oven (Hillman, unpublished field notes). Pounding or otherwise crushing whole fruit and then processing it with heat (roasting, baking, boiling) is another effective method of detoxifying the seeds because it mixes the cyanogenic glycosides with endogenous enzymes, which in turn transforms the cyanide contents into a chemical form that, when baked, are volatized by the heat (Bolarinwa et al. 2015: 22). Indeed, most historically reported solid foods made with Chequers fruit in Britain and Europe involve these processes, e.g. jellies and conserves, and a German source reported their use in a sweet and sour porridge (Düll 1959, cited in Thomas 2017: 1824).
         Processing Sorbus domesticus fruits – bletted and pounded.                   
Processing Sorbus domesticus fruits – bletted, pounded and ready for eating.
Processing Sorbus domesticus fruits – bletted, pounded and baked.
Recent scientific interest in the bioactive and antioxidant potential of the pomes of S. torminalis and several other Sorbus species suggests that there are significant health benefits from eating the fruit, both raw and processed. Like most edible fruit, Chequerberry is higher in Vitamin C and other antioxidants when fresh than as fermented or processed products; nevertheless, Chequerberry jams and jellies are high in Vitamin C compared to similar processed food products from other fruit (Mrkonjic et al. 2017).  Chemical analyses show that fruit of Chequerberry and some of its relatives meet the criteria for being named a “functional food” because they have high levels and types of endogenous phenolics and other compounds that are known to promote the prevention and treatment of infections and diseases (MikulicPetkovsek et al. 2017; Mrkonjic et al. 2017; Olszewska and Roj 2013; Raudonė et al. 2014).

Nutraceutical, Nutritional and Medicinal Benefits
As noted above, the species nomenclature “torminalis” is a reference to the medicinal use of Chequerberry fruit in treating colic (“tormina” in Latin) (Grigson 1955). In fact, Chequerberry fruit as well as the leaves have numerous reported traditional medicinal applications across Europe and the Near East, including the treatment of coughs, diarrhoea, fever, bronchitis, colic and kidney stones, and they are also effective as a diuretic (Olszewska and Roj 2013).

The effectiveness of S. torminalis fruit for preventing as well as treating a range of medical conditions has been confirmed by recent studies of the chemical components of fruit, flowers and leaves. Among these, Mrkonjic et al. (2017) identified compounds in the fruit that are known to decrease the risk of type 2 diabetes and promote antimicrobial activity against infection by certain strains of staphylococci and E-coli.  In another study, Hasbal et al. (2015) identified S. torminalis (and several other species in this genus) as a potent natural source of Acetylcholinesterase inhibitor compounds (anti-AChEs), chemical substances that are widely used in present-day medicines for the treatment of neurodegenerative disorders, e.g. Alzheimer’s and Parkinson’s diseases.

The value of the leaves for traditional medicine is exemplified by their uses in the Kırklareli Province of northern Turkey, where they are boiled into a decoction to treat diabetes and stomach ache (Kültür 2007: 358). Scientific analyses of extracts from the leaves confirm that they are an excellent source of antioxidants as well as having other useful medicinal components (Olszwska et al. 2012), as are the leaves of various other Sorbus species (Raudonis, et al. 2014).

Archaeological Occurrence
Chequerberry seeds are seldom reported archaeologically, and in most cases are recovered from waterlogged contexts, rather than charred. A remarkable exception, which also represents the earliest known archaeological occurrence, is the recovery of 18 charred Chequerberry seeds from the mid/late Neolithic hilltop settlement a Heilbronn-Klingenberg, in southwest German. The seeds were found in pit features, which the excavators interpreted as cellar storage structures used primarily for cereals (although cereals dominated the seed assemblage from this site, a wide range of wild edible plants were also found) (Stika 1996).

Another remarkable find is a waterlogged Chequerberry fruit skin and seeds that were recovered from a 16th century cesspit in Göttingen, southern Lower Saxony, Germany, (skilfully illustrated by Hellwig 1997: 11). They were recovered in association with a substantial number of other woodland fruits, which suggests that the collecting and consumption (and probably processing) of wild edible plants were important for the subsistence, diet and possibly social aspects of this Medieval urban community (Hellwig 1997).

Although Sorbus sp. seeds are not uncommon in British archaeological sites, S. torminalis is rarely reported. One (waterlogged) seed was recovered from a latrine at Dudley Castle, in the Midlands, dating from the British Civil war in the mid17th century (Moffet 1992).

The rarity of these seeds in the archaeological record is probably due to identification problems given the similarity in size and shape seeds in the Sorbus genus. Moreover, there is great within-species variability and S. torminalis seed size and shape can vary significantly even within fruit from a single corymb (Bednorz 2007; Bednorz et al. 2006; Maciejewska-Rutkowska and Bednorz 2004).  Bednorz et al. (2006: 4) reported that the seeds range in shape from mostly obovate (±77%), to elliptical (±12%), to oblanceolate (±6%), to wide obovate (±4%) and sometimes occur as oblong or roundish (±1%); their average size is ~ 6.20 mm long and ~3.25 mm wide, but they range from 3.20–8.10 mm in length and 1.70–5.70 mm in width.

Seed coat structure appears to be the most diagnostic characteristic, which may explain why identifications of seeds from waterlogged contexts are more common, i.e. in cases of charring the seed coat may be damaged or burned off (see Maciejewska-Rutkowska and Bednorz’s 2004 study of five Polish Sorbus species, which provides a detailed key for seed identification accompanied by useful SEM micrograph images).

References cited
Bednorz, L., Walkowiak, R., Maciejewska-Rutkowska, I. and Moliński, K. 2006. Seed variability of the Polish species of the genus Sorbus (Rosaceae). Dendrobiology 55: 3–9.

Bednorz, L. 2007. Morphological variability of fruits and seeds of Sorbus torminalis in Poland. Dendrobiology 57: 3-14.

Bolarinwa, I.F., Orfila, C. and Morgan, M.R.A. 2015. Determination of amygdalin in apple seeds, fresh apples and processed apple juices. Food Chemistry 170: 437 - 442.

Düll, R. 1959. Unsere Ebereschen und ihre Bastarde. A. Ziemsen Verlag, Wittenberg, Germany.

EUFORGEN: European Forest Genetics Resources Programme. http://www.euforgen.org/ [accessed: 10.07.18]

Gleadow, R.M. and Møller, B.L., 2014. Cyanogenic Glycosides: Synthesis, Physiology, and Phenotypic Plasticity. Annual Review of Plant Biology 65: 155-185.

Grigson, G. 1955. The Englishman’s Flora. Phoenix House, London, UK.

Hasbal, G., Yilmaz-Ozden, T. and Can, A. 2015. Antioxidant and antiacetylcholinester activities of Sorbus torminalis (L.) Crantz (wild service tree) fruits. Journal of Food and Drug Analysis 23: 57 – 62.

Hellwig, M. 1997. Plant remains from two cesspits (15th and 16th century) and a pond (13th century) from Göttingen, southern Lower Saxony Germany. Vegetation History and Archaeobotany 6: 105-116.

Kültü, S. 2007. Medicinal plants used in Kırklareli Province (Turkey). Journal of Ethnopharmacology 111: 341-364.

Maciejewska-Rutkowska, I. and Bednorz, L. 2004. SEM and stereoscope microscope observations on the seeds of the Polish species of the genus Sorbus L. (Rosacea). Acta Societatis Botanicorum Poloniae 73: 293-300.

Mabey, R. 1989. Food for Free. London. HarperCollins.

MikulicPetkovsek, M., Krska, B., Kiprovski, B. and  Veberic, R. 2017. Bioactive components and antioxidant capacity of fruits from nine Sorbus genotypes. Journal of Food Science 82: 647-658.

Moffat, L. 1992. Fruits, vegetables, herbs and other plants from the latrine at Dudley Castle in central England, used by the Royalist garrison during the Civil War. Review of Palaeobotany and Palynology 72: 271-286.

Mrkonjic, Z.O., Nađpal, J.D., Beara, I.N., Sabo, V.S.A., Četojević-Simin, D.D., Mimica-Dukić, N.M. and Lesjak, M.M.  2017. Phenolic profiling and bioactivities of fresh fruits and jam of Sorbus species. Journal of the Serbian Chemistry Society 82: 651-664.

Olszewska, M. A., and Roj, J. M. 2011. Phenolic constituents of the inflorescences of Sorbus torminalis (L.) Crantz. Phytochemistry Letters 4: 151–157.

Online Atlas of the British and Irish Flora. Botanical Society of Britain & Ireland. https://www.brc.ac.uk/plantatlas/ [accessed: 10.07.18]

Plants For A Future (PFAF). http://www.pfaf.org/ [accessed 16.07.18]

Raudonis, R., Raudonė, L., Gaivelytė, K., Viškelis, P. and Janulis, V. 2014. Phenolic and antioxidant profiles of rowan (Sorbus L.) fruits. Natural Product Research 28: 1231-1240.

Raudonė, L., Raudonis, R., Gaivelytė, K., Pukalskas, A., Viškelis, P., Venskutonis  P.R. and  Janulis, V. 2015 Phytochemical and antioxidant profiles of leaves from different Sorbus L. species. Natural Product Research 29: 281-285.

Stika, H-P. 1996. Vorgeschichtliche pflanzenreste aus Heilbronn-Klingenberg: archäobotanische untersuchungen zum Michelsberger erdwerk auf dem Schlossberg (Bandkeramik, Michelsberger Kultur, Späthallstatt/Frühlatène). Materialhefte zur Archäologie in Baden-Württemberg 34.

Thomas, P.A. 2017. Biological Flora of the British Isles: Sorbus torminalis. Journal of Ecology 106: 1806-1831.

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