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Juglans regia, the Persian walnut, English walnut, or especially in Great Britain, common walnut, is an Old World walnut tree species native to the region stretching from the Balkans eastward to the Himalayas and southwest China. The largest forests are in Kyrgyzstan, where trees occur in extensive, nearly pure walnut forests at 1,000–2,000 m altitude (Hemery 1998)—notably at Arslanbob in Jalal-Abad Province.

Description

Juglans regia is a large, deciduous tree attaining heights of 25–35 m, and a trunk up to 2 m diameter, commonly with a short trunk and broad crown, though taller and narrower in dense forest competition. It is a light-demanding species, requiring full sun to grow well.

The bark is smooth, olive-brown when young and silvery-grey on older branches, and features scattered broad fissures with a rougher texture. Like all walnuts, the pith of the twigs contains air spaces; this chambered pith is brownish in color. The leaves are alternately arranged, 25–40 cm long, odd-pinnate with 5–9 leaflets, paired alternately with one terminal leaflet. The largest leaflets are the three at the apex, 10–18 cm long and 6–8 cm broad; the basal pair of leaflets are much smaller, 5–8 cm long, with the margins of the leaflets entire. The male flowers are in drooping catkins 5–10 cm long, and the female flowers are terminal, in clusters of two to five, ripening in the autumn into a fruit with a green, semifleshy husk and a brown, corrugated nut. The whole fruit, including the husk, falls in autumn; the seed is large, with a relatively thin shell, and edible, with a rich flavor.

Taxonomy

The Latin name for the walnut was nux Gallica, “Gallic nut”;[1] the Gaulish region of Galatia in Anatolia lies in highlands at the western end of the tree’s presumed natural distribution.

For the etymology and meaning of the word in English and other Germanic languages, see our article “walnut”. In the Chinese language, the edible, cultivated walnut is called 胡桃 (hú táo in Mandarin), which means literally “Hu peach”, suggesting the ancient Chinese associated the introduction of the tree into East Asia with the Hu barbarians of the regions north and northwest of China. In Mexico, it is called nogal de Castilla,[2] suggesting the Mexicans associated the introduction of the tree into Mexico with Spaniards from Castile.

The Old English term wealhhnutu is a late book-name (Old English Vocabularies, Wright & Wulker), so the remark that the Anglo-Saxons inherited the walnut tree from the Romans does not follow from this name.

Cultivars

J. regia ‘Buccaneer’ produces an abundant crop of seeds. A self-fertile cultivar, it produces pollen over a long period and is thus a valuable pollinator for other cultivars. The tree is about the same size as an open-pollinated walnut, it comes into leaf very late and so usually avoids damage by late frosts.

Distribution and habitat

Original habitat

J. regia is native to the mountain ranges of Central Asia, extending from Xinjiang province of western China, parts of Kazakhstan, Uzbekistan and southern Kirghizia and from lower ranges of mountains in Nepal, Bhutan, Tibet, northern India and Pakistan, through Afghanistan, Turkmenistan and Iran to portions of Azerbaijan, Armenia, Georgia and eastern Turkey. In these countries, there is a great genetic variability, in particular ancestral forms with lateral fruiting. During its migration to western Europe, the common walnut lost this character by natural selection on account of competition with other vigorous forest species, such as oaks. They became large trees with terminal fruiting. A small remnant population of these J. regia trees have survived the last glacial period in Southern Europe, but the bulk of the wild germplasm found in the Balkan peninsula and much of Turkey was most likely introduced from eastern Turkey by commerce and settlement several thousand years ago.

Introduction around the world

In the fourth century BC, Alexander the Great introduced this “Persian nut” (Theophrastus’ καρυα ή Περσική[3]) in Macedonian and Greek ancestral forms with lateral fruiting from Iran and Central Asia. They hybridized with terminal-bearing forms to give lateral-bearing trees with larger fruit. These lateral-bearers were spread in southern Europe and northern Africa by Romans. Recent prospections in walnut populations of the Mediterrean Basin allowed to select interesting trees of this type. In the Middle Ages, the lateral-bearing character was introduced again in southern Turkey by merchants traveling along the Silk Road. J. regia germplasm in China is thought to have been introduced from Central Asia about 2000 years ago, and in some areas has become naturalized. Cultivated distribution now includes North and South America (Chile, Argentine), Australia, New Zealand, South Africa and Japan. So, the Persian walnut is grown from 30° to 50° of latitude in the Northern Hemisphere and from 30° to 40° in the Southern Hemisphere.

J. regia was introduced into western and northern Europe very early, by Roman times or earlier, and to the Americas by the 17th century, by English colonists. Important nut-growing regions include France, Serbia, Greece, Bulgaria, and Romania in Europe, China in Asia, California in North America, and Chile in South America. Lately, cultivation has spread to other regions, such as New Zealand and the southeast of Australia.[4] It is cultivated extensively for its high-quality nuts, eaten both fresh and pressed for their richly flavored oil; numerous cultivars have been selected for larger nuts with thinner shells.

Nutritional value

A study of ten cultivars of J. regia in Turkey showed significant variations in fatty acid content of the nuts:[5]

  • 62% – 71% fat
    • saturated fat(as a percentage of total fatty acids):
      • 5.2% – 7.3% palmitate
      • 2.6% – 3.7% stearate
    • unsaturated fat(as a percentage of total fatty acids):
      • 21.2% – 40.2% oleate (monounsaturated)
      • 43.9% – 60.1% linoleate (diunsaturated)
      • 6.9% – 11.5% linolenate (triunsaturated)

[edit] Potential biological effects

Walnuts and other tree nuts are important food-allergen sources that have the potential to be associated with life-threatening, IgE-mediated systemic reactions in some individuals.[6][7]

Certain extracts of walnuts have in vitro antioxidant and antiproliferative activity due to a high phenolic content.[8]

In vitro tests of walnut extract have shown a high antiatherogenic potential and osteoblastic activity, suggesting a potential beneficial effect of a walnut-enriched diet on cardioprotection and bone loss.[9]

The extract from walnut leaves is an antioxidant, decreases the blood sugar level and has a positive impact on lipid metabolism. The extract suppresses functional insufficiency of liver, links synthethising enzymes, increases the antitoxic action of hepatocytes and improves the functional insufficiency of kidneys.[10] The ethanolic extract from leaves of J. regia has an antidiabetic effect on diabetes-induced rats.[11]

Bark and leaf crude extracts of J. regia ,and J. mollis , showed in vitro activity against Mycobacterium tuberculosis.[12]

Culture

In Skopelos, a Greek island in the Aegean Sea, local legend suggests whoever plants a walnut tree will die as soon as the tree can “see” the sea. This has not been proven as fact; however, it might take some time to find a local arborist willing to take on the job of planting a walnut tree. Most planting is done by field rats (subfamily Murinae). In Flanders, a folk saying states: “By the time the tree is big, the planter surely will be dead.” (Dutch: Boompje groot, plantertje dood). This saying refers to the relatively slow growth rate of the tree.

Cultivation

Walnut trees grow best in rich, deep soil with full sun and long summers, such as the California central valley. In the U.S., J. regia is often grafted onto a rootstock of a native black walnut, Juglans hindsii to provide disease resistance. Other plants often will not grow under walnut trees because the fallen leaves and husks contain juglone, a chemical which acts as a natural herbicide. Horses that eat walnut leaves may develop laminitis, a hoof ailment. Mature trees may reach 50 feet in height and width, and live more than 200 years, developing massive trunks more than eight feet thick.

Other uses

Walnut heartwood is a heavy, hard, open-grained hardwood. Freshly cut live wood may be Dijon-mustard color, darkening to brown over a few days. The dried lumber is a rich chocolate-brown to black, with cream to tan sapwood, and may feature unusual figures, such as “curly”, “bee’s wing”, “bird’s eye”, and “rat tail”, among others. It is prized by fine woodworkers for its durability, luster and chatoyance, and is used for high-end flooring, guitars, furniture, veneers, knobs and handles.

Methyl palmitate, which has been extracted from green husks of J. regia has insecticidal properties: at a concentration of 10 mg/ml, it killed 98% of Tetranychus cinnabarinus (carmine spider mites) in one study.[13]

Written on February 23rd, 2012 , Botany, Forestry Tags:

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