Juglans regia, known by various common names including the common walnut, English walnut, or Persian walnut amongst other names, is a species of walnut. It is native to Eurasia in at least southwest and central Asia and southeast Europe, but its exact natural area is obscure due to its long history of cultivation. The species has numerous cultivars which produce the edible walnut consumed around the world and produced predominantly in China. It is widely cultivated across temperate regions throughout the world including those of Eurasia, Australia, and the Americas.
Etymology The genus name, Juglans, is derived from two Latin words, jovis, which means Jupiter, the chief god of the ancient Roman religion; and glans meaning an acorn or nut. The specific epithet, regia, is defined as regal or royal in references the quality of the fruit and edible nuts.
Description Juglans regia is a large deciduous tree, attaining heights of 10–25 metres (35–80 feet), and a trunk up to 2 m (6+1⁄2 ft) in diameter, commonly with a short trunk and broad crown. The tallest accurately measured specimen is in the Lagodekhi Protected Areas of the country of Georgia, 29 m (95 ft) tall, and the stoutest in Spain, 9 m (30 ft) in girth. Heights of 34 m in the wild in Kyrgyzstan and 32 m in cultivation in Britain have been reported, but not verified. 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 creamy-white at first, becoming brown in older twigs. The leaves are alternately arranged, 20–45 cm (8–17+1⁄2 in) long, odd-pinnate with (3–)5–9 (most often 7) leaflets, arranged in opposite pairs with one terminal leaflet. The largest leaflets are the three at the apex, 10–20 cm (4–8 in) long and 6–10 cm (2+1⁄2–4 in) broad; the basal pair of leaflets are much smaller, 5–8 cm (2–3 in) long. The margins of the leaflets are entire, but with toothed margins on seedlings, and in the cultivar 'Laciniata'. The leaves open fairly late in the spring (typically mid May in Britain), and are red-brown at first, becoming their mature dark yellow-toned green by mid June. The male flowers are in drooping catkins 5–10 cm (2–4 in) long, and the female flowers are terminal, in clusters of two to five, ripening in the autumn into a fruit with a green, semi-fleshy husk and a brown, corrugated nut. The whole fruit, including the husk, falls in autumn with the husk breaking open; the seed is a large and edible nut with a rich flavour; the shell is thick in wild populations, while most cultivated plants have been selected for a thin shell.
Distribution and habitat
Original habitat One of the centres of origin and diversity of J. regia is Iran, and another further northeast in Central Asia. In Central Asia, genetic diversity is very patchy, with the highest diversity found around Sariosiyo in Uzbekistan, and conspicuously lower in the Arslanbob forests in Kyrgyzstan (these now thought to be of cultivated origin). Another native glacial refugium population with high genetic diversity is found on the southern fringes of the Alps in northeast Italy. However, as with other old and widespread cultivated plants, it is not easy to reconstruct the original distribution and determine the borders of the past natural ranges. There are many reports concerning the earliest fossil pollen and nuts of J. regia, and the conclusions that various authors draw are somewhat contradictory. Taken together these finds suggest that J. regia possibly survived the last glaciations in several refugia, as the compilation of the data shows most likely southern Europe, the Near East, China, and the Himalaya. The largest forests are in Kyrgyzstan, where trees occur in extensive forests at 1,000–2,000 m (3,300–6,600 ft) altitude, notably at Arslanbob in Jalal-Abad Province.
Global introduction The earliest evidence of walnuts in the Levant comes from wood remains dated to the Middle Bronze Age, discovered in northern Israel. The walnut is also referenced a single time in the Hebrew Bible. In the fourth century BC, Alexander the Great introduced this "Persian nut" (Theophrastus' καρυα ή Περσική) in Macedonian, Ancient 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 the Romans. Recent prospections in walnut populations of the Mediterranean 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 travelling along the Silk Road. J. regia germplasm in China is thought to have been introduced from Central Asia about 2,000 years ago, and in some areas has become naturalised. Cultivated J. regia was introduced into western and northern Europe very early, probably in Roman times, and to the Americas in the 17th century, by European colonists. Important nut-growing regions include California in the United States; France, Serbia, Greece, Bulgaria, Romania and Hungary in Europe; China in Asia; Baja California and Coahuila in Mexico; and Chile in Latin America. Lately, cultivation has spread to other regions, such as New Zealand and the southeast of Australia. It is cultivated extensively from 30° to 50° of latitude in the Northern Hemisphere and from 30° to 40° in the Southern Hemisphere. Its high-quality fruits are eaten both fresh or pressed for their richly flavoured oil; numerous cultivars have been selected for larger nuts with thinner shells.
Genetic diversity J. regia is highly diverse genetically, and has been extensively cultivated for possibly as long as 2,000 years in parts of west Asia and southern Europe. Its closest relative is Juglans sigillata from western China and the far northeast of India; it differs chiefly in leaves with 9–11 leaflets, and nuts with a much thicker, harder shell.
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