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Corylus L., the hazelnut, is one of the major world nut crops. World production of hazelnuts, which was 745,078 metric tons in 1998, is second only to almond. Turkey, which accounts for about 77% of this tonnage, is the leading producer and exporter and sets the world market. Italy is second; the United States, third; China, fourth (FAO, 1998). Spain, which had the third largest crop five to ten years ago, has reduced its acreage and is now fifth.
Corylus was one of the first genera to move northward following the retreat of the most recent glacial period about 17,000 to 18,000 B.C. (Thompson et al., 1996). The amount of hazelnut pollen deposited in strata associated with the pre-Boreal period, 8000 to 7500 B.C., rose sharply, and hazelnut became the dominant boreal vegetation until 5500 B.C. Since this period of glory, the number of plants have been greatly reduced due to human activity but the geographical range of the genus has not diminished significantly (Thompson et al., 1996).
The hazelnut has a long history intertwined with human civilization. Ancient Chinese and Greek manuscripts described the uses of this nut (Hummer, 1995). In China, the hazelnut was considered one of the five sacred nourishments, endowing temple priests with powers of clairvoyance. In the first century A.D., Dioscorides, the Greek physician noted the use of hazel in remedies to cure the common cold as well as baldness. Virgil wrote that the hazel was more honored than vine myrtle or bay. In Roman and Celtic cultures, hazelnut has been attributed with mystical powers, symbolizing fruitfulness in marriage, providing protection against lightning, and divining the hidden location of treasures, veins of metals and water (Hummer, 1995).
Corylus L., a member of the Betulaceae, or birch family, includes about 18 described species that are widely distributed throughout temperate regions of the northern hemisphere. Only eight or nine of these species are widely recognized by most taxonomists and include the shrubby species C. avellana L., C. americana Marshall, C. cornuta Marshall, C. heterophylla Fischer, and C. sieboldiana Blume; and the tree species C.colurna L. C. chinensis Franchet, and C.ferox Wallich (Thompson et al., 1996). The tree species C.jacquemontii Decaisne, native to the Indian Himalayan hills, is most recently considered to be a synonym of C. colurna L. var. lacera (Wall.) A. DC. (GRIN, 1999).
Hazelnuts are exceedingly polymorphic, as is evidenced by the numerous taxa described as species reported for geographic variants. Variability among these species enables breeders to select for disease resistance, cold hardiness, and rootstocks. Most Corylus species are diploid with 2n = 22. Aneuploids have been reported (Kasapligil, 1968; Thompson, 1996). Spontaneous tetraploids and triploids may appear in seedlings of C. avellana and C. heterophylla (Botta et al. 1986; Danielsson, 1946; Danielsson-Santesson, 1951). Danielsson (1946) and Danielson-Santesson (1951) induced tetraploids with colchicine in C. avellana. The species within Corylus have no crossing barriers and hybridize readily. Several hybrid species have been recognized (GRIN, 1999)
The main economically important species, the European hazelnut, C. avellana, naturally occurs in scattered locations from the Atlantic coast of Europe and North Africa to the coastal regions of Norway. The northern boundary includes the British Isles, Scandinavia and northern regions of the Russian Federation. The distribution is bounded in the east by the Ural Mountains in the Soviet Union. The southern boundary extends from Morocco, Algeria, and Spain in the west through Italy, Yugoslavia, Greece, and Turkey to North Iran and Transcaucasia (Thompson, 1996). While the distribution of this species is quite broad, the production of the cultivated hazelnut is limited to regions near large bodies of water with mild, humid winters and cool summers, e.g., the Black Sea coast of Turkey, the Mediterranean coasts of Italy and Spain, and the Willamette Valley of Oregon in the United States. This species was cultivated from early times in the Black Sea region of Turkey and the Caucasus and since classical times in the Mediterranean region. The important commercial cultivars throughout the world have either been selected directly or developed from crossed of this species. C. avellanahas superior quality, and larger size nuts than do other species in the genus.
Two species, C. americana and C. cornuta, are native to North America. The eastern species, C. americana ranges from Saskatchewan to Main, southward to Florida, to the mid-western states. This species is a small to medium-small shrub that freely suckers. The leaf-like husks are somewhat longer than the nuts and often split up one or both sides (Thompson et al., 1996). Nuts of this species are harvested from the wild for direct use and some cold-hardy selections have been made. This species confers cold-hardiness, productivity, good quality nuts, and resistance to eastern filbert blight, Anisogramma anomala (Peck) E. Muller. The western species, C. cornuta, overlaps the range of C. americana, and extends farther north and west. This species is shrubby, soloniferous and has very long husks surrounding the nuts. These long husks are tightly constricted and are covered with dense bristly hairs. This species has not been desirable for direct use, because of the low quality nuts. Selections from the northern extension of the range could contribute genes for cold hardiness. Some plants survive -50 o C. This species could also contribute genes for eastern filbert blight resistance and very early nut maturity.
Corylus heterophylla, the Siberian hazelnut ranges over vast areas of China, the Soviet Far East, Korea, and Japan (Thompson et al., 1996). This species has many similarities to C. americana, with good quality nuts with relatively open husks. Nuts from this species are collected directly from wild plants and sold on domestic markets by local people. Currently active breeding programs in Korea and China emphasize this development of genotypes derived from this species.
Another Asian species, C. sieboldiana, is similar to the North American, C.cornuta. This plant also has tubular husks constricting the nut. This species is less desirable for direct harvest or breeding because the nuts have poor quality. However, this species has extreme cold hardiness.
Corylus fargesii, occurs in Guizhou, Sichuan, and Hubei at high elevations. This shrubby species has been observed very little outside China and is virtually unknown in the west (Thompson et al., 1996).
Corylus colurna, the Turkish tree hazel, naturally occurs in mixed temperate forests from Romania, through the Balkans, northern Turkey, Transcaucasia, and Northern Iran (Thompson et al., 1996). Some taxonomic authorities consider C. jacquemontii, one of the tree hazels native to the Indian Himalayan hills, to be a variety of this species (GRIN, 1999). C. colurna is a large, attractive, tree reaching 14 m in height, with a pyramidal-shaped crown. The bark is gray, corky, and furrowed. The nuts are sometimes gathered and eaten locally but the lack of precocity, small nut size, adherance of the husk, and thick shell make this species undesirable for direct commercial production. Forms of this species, which have a non-suckering habit and are graft compatible with C. avellana, have been selected as rootstock. The species is more drought tolerant and cold hardy than C. avellana.
Corylus chinensis, the Chinese tree hazel, occurs sparsely at higher elevations in Sichuan, Yunnan, and Hubei. In these provinces the climate is relatively warm and the soils are rich. The trees grow very large, reaching 40 m. The wood is valuable for construction and furniture (Thompson et al., 1996).
Corylus ferox, another Himalayan tree hazel, occurs in high latitudes in India, Nepal, Bhutan, and northwestern Yunnan Province in China (Thompson et al., 1996). This species is readily distinguished by spiny husks which resemble a chestnut bur. The very small nuts are gathered directly and sold in local markets. This species may have potential for breeding or as a rootstock but only a few genotypes have been available to researchers in western countries.
Only one Corylus species is listed by the World Conservation Monitoring Center in the 1997 IUCN Red List of Threatened Plants. C. chinensis, the Chinese tree hazel is a vulnerable species in the provinces of Henan, Hubei, Hunan, Shaanxi, Sichuan, and Yunnan in the People's Republic of China.
Over 20 countries conserve C. avellana cultivars or wild species in genetic resource collections ( Bettencourt and Konopka,1989). The most extensive cultivated collection resides in Giresun, Turkey, which preserved 739 selections with over 700 genotypes of C. avellana (Hummer, 1995; Thompson et al. 1996). At the U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository in Corvallis, Oregon, the United States preserves more than 485 cultivars including representatives of the major eight species. Spain preserves more than 124 selections of C. avellanaand two tree species at Villaviciosa. Other hazelnut collections are located in Torino, Italy; Bordeaux, France; Sukhumi, Sochi, Maykop, Krasnodar, and Yalta in the Russian Federation; Seoul, South Korea, and Dalian, China. These primary collections are preserved as plants in an orchard. The most efficient spacing for seedling trees in 0.9 m in the row and 3 m between rows. Closer spacing results in crowding as trees mature. The 3 m between rows allows sufficient light exposure and permits the movement of farm equipment (Thompson, 1996).
Hazelnut seed cannot be stored for more than one year without substantial loss in viability (Mehlenbacher, 1991). Thus, open stored or refrigerated seed cannot be used for medium- or long-term germplasm preservation. Hazelnut seeds are considered desiccation-tolerant but LN2 sensitive (Stanwood, 1985).
Backup or secondary germplasm preservation of specific genotypes is accomplished through cold storage of in vitro cultured plants. This medium-term storage can be successful from 8 months to 2.7 years (Reed and Chang, 1997).
The floral biology of hazelnut is distinct from other orchard crops. The plants are monoecious, wind-pollinated and bloom in midwinter, from December to March in the Northern hemisphere. Staminate flowers are borne in catkins at nodes on one-year wood. Pistilate clusters are borne on one-year wood, on catkin peduncles or on very short spurs from older wood. At anthesis several stigmatic styles emerge and can remain receptive for up to three months. If the exposed parts of the styles are damaged by frost, the lower parts subsequently emerge as functional tissues. Pollen tubes grow to the base of the styles and rest for five months. The ovary becomes mature in the spring. Fertilization occurs when the nut is about one-half of the mature size (Thompson, 1996). Compatible pollination is essential for the development of the ovary or nut shell.
Most cultivars are dichogamous, but some overlapping of staminate and pistillage anthesis occurs. Self-incompatibility encourages the need for cross-pollination in orchards. Cross-incompatibility of clones sharing the same S allele is important for orchard production or planning parental combinations for breeding programs (Mehlenbacher and Thompson, 1988).
Apple mosaic virus (ApMV), is a virus of concern in cultivated Corylus. This virus is present in high frequency in cultivars from Turkey, southern Italy, and eastern Spain (Postman and Cameron, 1987). Infected trees are frequently symptomless or may exhibit symptoms on only one branch. Thermotherapy and micrografting are now routine for the generation of pathogen-negative plants (Thompson, 1996).
Corylus orchards begin to bear from four to six years after planting. Regular sucker removal shortens the juvenile period (Mehlenbacher and Smith, 1992).
Hazelnut seeds have a hard seed coat, internal dormancy and irregular germination with increasing length of storage (Williams et al., 1973). To overcome this dormancy, the following procedure is followed for C. avellana in Oregon (Thompson et al., 1996). Half-brown nuts are harvested in August and are refrigerated. In late November, the nuts are soaked for two to four days in water then stratified in moist vermiculite at 4oC for three to five months. After three months the seeds are warmed for 5 days and those with visible root tips are planted in flats in the greenhouse. Ungerminated seeds receive further stratification. Seedlings are transplanted after they reach 25 cm.
An alternative method for that is difficult to germinate hybrid Corylus seed involves a gibberellic acid soak. Shells are removed and kernels are soaked in 25 or 50 ppm Progibb (GA 4+7) or GA 3for 16 to 24 h. Seeds can then be germinated by planting directly into flats in the greenhouse or by placing on moist filter paper in germinating boxes. The latter method is preferred so that the seeds that fail to germinate initially can be stimulated by a second GA treatment (Thompson et al., 1996). Some seeds, such as C. avellana cv. Gem, or forma heterophylla and pendula, tend to decay in stratification and must be treated with GA. Freshly harvested seeds treated with GA have the highest germination rate, but germination of stored, non-stratified seed is enhanced.
Trees that are forced all winter and planted in the spring will cease growth. However, seedlings that have been hardened and subjected to chilling temperatures for several months before transplanting will resume growth (Thompson et al, 1996).
Simple layering is the commercial propagation technique of choice (Hartmann and Kester, 1975). The suckers arising from the base of vigorous young trees that are four to eight years old, are layered in early spring. The basal portion of suckers are scarred or a hog-nose ring is applied encircling the stem of the young shoots. A 0.5 m tall tarpaper collar is placed at the base of the suckering tree. Sawdust is added to the brim of the tarpaper. Roots form on the suckers during the summer. The tarpaper and sawdust are removed in late August and the rooted layers can be removed from the tree. Sometimes less-well rooted suckers are placed in a nursery bed for about a year for further root development (Hartmann and Kester, 1975).
Grafting can be difficult with Corylus. At the NCGR-Corvallis, whip and tongue grafting is performed in mid-winter. Care is taken to insure that the rootstock and scion are completely dormant prior to grafting. If the rootstock has sap moving upwards in the phloem, a successful callus bridge will not form between the stock and scion. In some cases a sharp knife is used to cut a score about 1 cm below the graft union to reduce sap flow. The surface areas of the stock and scion must be cut cleanly. A dull blade could mash cells and may prevent the joining of the callus from the stock and scion. A budding rubber is wrapped securely but not tightly at the union. The graft is placed in a warm greenhouse for several weeks while the graft union becomes secure.
Corylus when grown in field conditions is subject to many internal contaminants such as Xanthamonas and can be difficult to culture for in vitro or cryogenic purposes. Forcing of buds or grafting greenhouse grown plants can provide cleaner material for excision to establish laboratory cultures.
Bettencourt and Konopka. 1989. Directory of Germplasm Collections. 6. 11. Temperate Fruits and Tree Nuts, Rome: International Board for Plant Genetic Resources.
GRIN Genetic Resources Information Network. 1999. www.ars-grin.gov/npgs.
Hartman, H. and D. Kester. 1975. Methods and Rootstocks for the Important Fruit and Nut Species: Filbert. p. 554. In: Plant Propagation. Third Edition. Prentice- Hall, Inc. Englewood Cliffs, New Jersey.
Hummer, K. 1995. The mystical powers and culinary delights of the hazelnut: a globally important Mediterranean Crop. Diversity 11(1&2):130.
Kasapligil, B. 1968. Chromosome studies in the genus Corylus. Sci. Rep. Faculty Sci. Ege Univ. 59.
Mehlenbacher, S. A. and M. M. Thompson. 1988. Dominance relationships among S alleles in Corylus avellanaL. Theor. Appl. Genet. 76:669-672.
Mehlenbacher, S. A. and D. C. Smith. 1992. Heritability of ease of hazelnut pellicle removal. HortScience 23: 1053-1054.
Postman, J. D. and H. R. Cameron. 1987. Apple mosaic virus in U. S. filbert germplasm. Plant Dis. 71:944-945.
Thompson, M. M., H. B Lagerstedt, and S. A. Mehlenbacher. 1996. Hazelnuts. pp. 125-184, Chapter 3. In: Fruit Breeding, Vol. III: Nuts, J. Janick and J. Moore (eds.). John Wiley & Sons, Inc.