The INRA's Walnut Germplasm Collection, a Reservoir of Diversity Essential for the Future of French Walnut Industry

Since ancient times, walnut has been the subject of many legends. Greek mythology tells us that Dionysus fell in love with King Dion's youngest daughter, Carya, whose two sisters were turned into rocks because of their jealousy. Later, Carya dies of sadness and Dionysus immortalizes her in a fertile walnut tree. She later gave her name to Artemis, who is the godess of walnut and fruit bearing trees, under the name of Artemis Karyatis. A temple dedicated to her shows walnut columns, carved in the shapes of the goddess, erected to support the capital. These are the famous caryatids, which are also found in stone at the entrance to the Erechtheion of the Acropolis of Athens and in many classical buildings such as the Louvre.

The cultivated walnut, Juglans regia L., is believed to be native to Central Asia, from Kashmir, in the foothills of the Himalayas, to present-day Kyrgyzstan. It first spread to Uzbekistan, northern Iran, the Caucasus region and eastern Turkey, and then to northern India, Nepal and western China. Its introduction in Europe dates from the last ice age, about 50,000 years ago. Archaeological evidence found in Périgord in France suggests that Cro-Magnon man could have consumed ancestral forms of the fruit of Juglans regia 17,000 years ago. In Ancient Greece, the walnut became more abundant thanks to Alexander the Great and its dispersion then increased in Spain, Portugal and North Africa. In the 16th century, the conquest of the New World allowed its arrival in South America, notably Chile, before its introduction in California in the 19th century.

World walnut production has grown steadily over the last decades and exceeded 866,000 metric tons in 2017/18 (kernel basis), according to INC data. France production was estimated at 17,500 metric tons in 2017/18, placing it sixth in the world and second in Europe, behind Ukraine. Although the United States, Chile and Eastern Europe dominate the world exports, the French walnut nevertheless resists global competition thanks to its recognized organoleptic qualities.

There are two appellations of controlled origin corresponding to the two main production regions, the AOC ‘Noix du Périgord’ and ‘Noix de Grenoble’. France exports 80% of its production, mainly to Europe and mainly destined for the table nut market. The old variety ‘Franquette’ provides about 70% of the production and the areas allocated to walnut increased by 19% between 2000 and 2010, reaching more than 20,000 hectares. This development makes it the second largest fruit crop after apples, with more than 1,800,000 trees.
 
Éric Germain, a researcher from Institut National de la Recherche Agronomique (INRA)- Bordeaux, led the first walnut improvement program in France. From 1977 to 1995, he made crosses, mainly between old French varieties such as 'Franquette', 'Grandjean' and 'Marbot', appreciated for their late budbreak date and quality, and Californian varieties such as 'Pedro' and 'Chandler', known for their ability to produce fruit all along the shoot. There are indeed two main types of fruiting in walnut: in terminal position, at the end of the current season’s shoot, as is the case in old French varieties, and in lateral position, all along the one-year shoot. This is the case of the American variety 'Payne', present in the pedigree of most American varieties. Two cultivars with lateral fruiting were registered in 1995: 'Fernette' and 'Fernor', resulting from a cross between 'Franquette' and 'Lara®', the latter having 'Payne' in its pedigree. A third cultivar with lateral fruiting and rather small kernels, 'Ferjean', resulting from the crossing between 'Grosvert' and 'Lara®', was registered in 1999. Finally, the last four cultivars were released in 2010: 'Feradam', 'Ferbel', 'Ferouette' and 'Fertignac'. Between 1996 and 2007, Éric Germain used a germplasm collection resulting from important prospecting work that he did in many countries between 1988 and 2000. The most interesting varieties have been crossed with plant material from different regions of the world, such as the Mediterranean, the Middle East, Central Asia and Japan. This diversification aimed to reduce the inbreeding problems previously observed in crossbreeding between French and American materials. Following INRA’s strategic decisions, the improvement program was stopped in 2007 although the experimental stations had identified some promising hybrids.

The INRA’s walnut collection is however still maintained by INRA – Bordeaux, more precisely at the Prunus–Juglans Genetic Resources Center. Aware of the importance of safeguarding biodiversity, Éric Germain knew that creating a reservoir of genetic diversity is an essential basis for initiating an improvement program. Thus, this rich collection represents a large part of the species of the genus Juglans, twelve different species and more than 400 accessions. For the only cultivated walnut, Juglans regia, the collection has nearly two hundred accessions coming from all over the world.

Visually, this collection presents a wide diversity of observable characteristics, or ‘phenotypes’. Within the species Juglans regia, we notice trees with purple foliage and others with laciniated foliage. Some produce nuts with red kernels. The largest nuts can reach 10 centimetres in diameter (e.g., nuts of the 'Bijou' variety), and others do not exceed the size of a hazelnut. The earliest trees emerge in the first week of March while the later trees wait until the last week of April. This great phenotypic variability is important for the breeder, who has a wide range of possible genitors to create a new variety as needed. The INRA’s collection is undeniably one of the most important in the world.

Today, the French walnut industry is facing increased global competition. It also has to cope with the already visible effects of climate change, with an increase in average temperatures and the appearance of late spring frosts, which can cause production losses. In 2017, production in the Périgord peaked at 12,000 tonnes instead of the usual 20,000 tonnes because of late spring frosts. In addition, future and existing pathogens must be taken into account in a possible breeding program. Examples include damage caused by bacterial infections caused by Xanthomonas arboricola pv. juglandis, and damage caused by two forms of anthracnose, the fungi Gnomonia leptostyla and Colletotrichum acutatum. It is therefore in the interest of the sector to have varieties with a short cycle, capable of avoiding spring frosts while offering the fruit on the market stalls relatively early in the season to resist American competition. If these criteria are essential, a French variety should also have high productivity, great vigour of the tree and an organoleptic quality always present. Nowadays, focusing on the knowledge of genetic diversity seems to be a promising strategy to help the breeder to achieve these varieties, even if orchard management improvements have to be made in parallel. INRA and the Ctifl (Technical Center for Fruits and Vegetables) are now exploring the genetic diversity of the INRA’s germplasm collection to conduct association genetics studies and to choose new genitors suitable to face new challenges of walnut industry. The new breeding program will be led by the Ctifl.