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All walnut species of the genus Juglans are trees or large shrubs having shoots with chambered piths, large aromatic compound leaves, staminate catkins on one year old wood and female flowers on the top of the current year's twigs. The husked fruit is a false drupe containing a large woody-shelled nut. All Juglans produce edible nuts, although size and extrability differ considerably. Most species are highly regarded for their timber.

The genus Juglans consists of approximately 20 species grouped taxonomically into four sections: Rhysocaryon, Cardiocaryon, and Trachycaryon, Dioscaryon. All these species are diploid with 2n = 2x = 32 chromosomes. They can hybridize. The section of Rhysocaryon (black walnuts) is composed of 16 North and South American Juglans species. The most important are Juglans nigra L., Juglans hindsii Jeps and Juglans major Heller used as rootstocks of Persian walnut. The section of Cardiocaryon consists of species originating from Japan (Juglans sieboldiana Maxim), China (Juglans catchayensis Dode) and Mandchuria or the Korean peninsula (Juglans mandshurica Maxim) used as wood producers and sometimes as rootstocks. Juglans cinerea L. or butternut, the only species of the Trachycarion section is present in the forests of north-western Canada and USA. The section Dioscaryon consists solely of the commercially valuable Persian walnut, Juglans regia L.

Juglans 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 mountains of Nepal, 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 fruitfulness. During its migration to western Europe, English walnut lost this character by natural selection on account of competition with other vigourous forest species such as oaks. They became big trees with terminal fruitfulness. A small remnant population of these J. regia trees have survived the last glacial period in Southern Europe but the bulk of the wild J. regia germplasm in the Balkan peninsula and much of Turkey was most likely introduced from eastern Turkey by commerce and settlement several thousand years ago. Four centuries BC Alexander The Great introduced in Macedonia ancestral forms with lateral fruitfulness from Iran and Central Asia. They hybridized with terminal bearing forms to give lateral bearing trees. 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 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 naturalized. Cultivated distribution now includes North and South America (Chile, Argentine), Australia, New Zealand, South Africa and Japan. So Persian walnut is grown from 30 to 50 degrees of latitude in the Northern hemisphere and from 30 to 40 degrees in the Southern hemisphere.

Over 1 000 000 metric tons are producted annually (FAO, 1997). According to FAO, China leads production with 350 000 tonnes, followed by the United States of America (California) with 210 000 tonnes, Turkey (114 000 tonnes) and Iran (82 000 tonnes). Altogether Europe produces over 250 000 tonnes, the majority from Ukraine (77 000 tonnes), Romania (28 000 tonnes), France (26 000 tonnes), Yugoslavia (22 000 tonnes), Greece (20 000 tonnes), Russia (15 000 tonnes), Italy (14 000 tonnes), Germany (10 000 tonnes), Spain (9 000 tonnes), Moldavia (8 000 tonnes). Several of the major producers, notably Turkey, Iran and the Balkan countries, consume the bulk of their walnut production domestically. Leading exporters of walnut include the USA (105 000 tonnes), China (40 000 tonnes), France (14 000 tonnes), India (12 000 tonnes) and Chile (8 000 tonnes). Among significant importers of walnut are Germany, Spain, the United Kingdom, Canada and Japan.

Though Juglans regia L. is self compatible, its dichogamy encourages allogamy. Thus, the genetic structure of the Persian walnut is fairly heterozygous. Cultivated varieties generally well adapted to climatic conditions of the different production areas, often lack some important agronomic characteristics. It is therefore useful to select in natural populations or create through hybridization new cultivars combining characters of improved climate adaptation (late budbreak, low chilling requirement or winter hardiness), early fruiting and high productivity (lateral fruitfulness), disease tolerance (blight and anthracnose), hypersusceptibility to Cherry leaf roll virus, high fruit and kernel quality. This is possible given the very large and so far unexploited variability within the Juglans regia L. species. Hybridization programs to create new walnut varieties are relatively less developed than those of the other fruit species. The main programs are carried out at UC Davis (USA) and at the Fruit and Vine Research Station in Bordeaux (France). On the other hand, during the last decade, many prospections were carried out in natural populations in the main producer countries in particular in Greece, Iran, Italy, Morocco, Portugal, Romania, Spain, Turkey, and ex-Yugoslavia (Yugoslavia was left as country of origin in all tables in cases where no further geographical indication was available to identify the current country). New interesting genitors or cultivars are beginning to be released from these breeding and selection works.

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