Author: George Orel, Adam Marchant, Judyth McLeod, and Graeme Richards
E-mail: [email protected]

University of Western Sydney: Hawkesbury
PO Box 415 Richmond NSW 2753
Phone: +61 2-45701358
E-mail: [email protected]

Adem HH, Jerie PH, Aumann CD, Borchardt N 2000. High yields and early bearing for walnuts. Rural Industries Research Development Corporation Publication No 00/100. RIRDC project No Dav- 73A.

Aleta N, Girona J, Tasias J (1993) Forward. Acta Horticulturae 311: 3.

Bhattacharyya B, Johri BM 1998. Flowering plants: taxonomy and phylogeny. Berlin, Springer Verlag

Burbank L (1914) How plants are trained to work for man. New York: P. F. Collier & Sons.

Chiba MV (1997) Walnut research and walnut industry development in Australia. Proceedings of the Third International Walnut Congress, Alcobaca, Portugal, 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau CP editors. Acta Horticulturae 442: 429-433.

Cohen M, Comas M, Aleta N (1997) Water behaviour of three commercial cultivars and two preselected clones of Juglans regia trees. Proceedings of the Third International Walnut Congress, Alcobaca, Portugal, 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau CP editors. Acta Horticulturae 442: 143-152.

Darwin C (1877) The different forms of flowers on plants of the same specie. London: John Murray.

de Witt HCD 1963. Plants of the world. London: Thames and Hudson.

Forde HI (1975) Advances in fruit breeding. Indiana: Purdue University Press.

Harwood WS (1905) New creations in plant life. London: The Macmillan Company.

ITTO 1999. Summary of world timber situation. Draft Annual report. ITTO Quarterly (TFU).

Juglans CGC Report (1996)

Korac M, Cerovic S, Golosin V, Ognjanov V, Miletic R (1993) Prospective Yugoslav walnut selections with lateral fruit buds. Acta Horticulturae 311: 41-45.

Krussmann G 1985. Manual of cultivated broad leaved trees and shrubs. London BT Batsford Ltd.

Kuden A, Kaska N, Turemis N (1977) Walnut selection in Middle Taurus Mountains.Proceedings of the Third International Walnut Congress, Alcobaca, Portugal, 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau CP editors. Acta Horticulturae 442: 117-108.

Lacointe A, Ameglio T, Kajji A, Daudet FA, Cruiziat P, Archer P, Frossard JS (1997) Carbon and water fluxes in vegetating 3 year old walnuts. Proceedings of the Third International Walnut Congress, Alcobaca, Portugal, 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau CP editors. Acta Horticulturae 442: 153-158.

Loiko RE (1990) Apomixis of walnut. Acta Horticulturae 284: 86-94.

Lost crops of the Incas (1999) Little known plants of the Andes with promise for world wide cultivation. Report of an Ad Hoc Panel of the Advisory Committee on Technology Innovation 323-327.

Malvolti ME, Spada M, Beritognolo I, Cannata F 1997. Differentiation of walnut hybrids (Juglans nigra L. x Juglans regia L.) through RAPD markers. Proceedings of the Third International Walnut Congress, Alcobaca, Portugal 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau CP editors. Acta Horticulturae: 442: 43 Ð 52.

Mc Granahan G, Leslie C (1990) Walnuts (Juglans). Germplasm resources of temperate fruit and nut crops. The International Society for Horticultural Science, Wageningen Netherlands vol 2.

Mehmet S, Beyhan O (1993) A study on the selection of Darande walnuts. Acta Horticulturae 311: 5760.

Nicese FP, Hormanza JI, McGranahan GH 1997. Characteristics of walnut (Juglans regia L.) cultivars using RAPD. Proceedings of the Third International Walnut Congress, Alcobaca, Portugal 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau CP editors. Acta Horticulturae 442: 53-61.

Paunovic SA (1990) The walnut cultivars selected from indigenous populations of Juglans regia L. in Serbia, SFR Yugoslavia. Acta Horticulturae 284: 135-141.

Pham HH 1992. Flore du Cambodge du Laos et du Viet Nam. Paris: Museum National d’Historie Naturelle.

Piekko A, Czynczyk A (1990) Evaluation of selected types of walnuts in Poland. Acta Horticulturae 284: 143-144.

Raven PH, Evert RF, Eichhorn SE (1992) Biology of Plants. New York: Worth Publishers.

Rink G, Haines JK (1997) Variation in nut, kernel and shell characteristics of Juglans nigra L. Proceedings of the Third International Walnut Congress, Alcobaca, Portugal 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau editors. Acta Horticulturae 442: 197-204.

Sartorius R, Stosser R (1997) On the apomictic seed development in the walnut (Juglans regia L.). Proceedings of the Third International Walnut Congress, Alcobaca, Portugal 13-16 June 1995. Pereira JAG, Martins JMS, de Abreau editors. Acta Horticulturae 442: 225-229.

Smith JR (1950) Tree crops. Washington DC: Island Press.

Sykes JT (1975) Tree crops. Crop genetic resources for today and tomorrow. Cambridge: Cambridge University Press.

Stace CA 1996. Plant Taxonomy and Biosystematics. Melbourne: Press Syndicate of the University of Cambridge.

Thain M, Hickman M (1995) Dictionary of Biology. London: Penguin Books.

Tootill E (1984) Dictionary of Botany. London: Penguin Books.

Yong-Ling R, Wood BW, Payne JA (1992) Chinese Hickory an emerging nut tree crop. Fruit Varieties Journal 46: 116 Ð 22.

Three newly acquired species of walnuts from Central and South America, namely Juglans australis Griseb., J. neotropica Diels., J. olanchana Standl. et L.O. Williams, and two new accessions from Asia, J. sigillata Dode, and an unidentified J. sp., were assessed for the purpose of a future breeding programme. Data were generated by the analysis of morphological character assessment and DNA analysis. Chloroplast DNA indicated a very close relationship between the species tested. A combination of morphological and DNA fingerprint data indicated that the initial identifications of new accessions were correct and as such will permit the proposed breeding programme.

The fruit of Juglans regia is by far the most important specie for nut production. The fruit of Juglans nigra L. and Juglans hindsii (Jeps.) Rehder are grown also, but being less palatable than Juglans regia these are of lesser importance. Rink & Haines (1997) comment on the fact that recently, a number of genetic selections of Juglans nigra have been made but these were aimed at increased timber production. Large scale selection for increased nut production had been scarce.

Carya illinoensis (Wang) K. Koch, (Pecan) and Carya ovata (Ashe) Little. (Hickory) yield valuable nuts, (Bhattacharyya & Johri 1998), Carya illinoensis being the second most important nut crop after the walnut (Yong-Ling et al. 1992).

In regards to Juglans neotropica (and possibly other South American species) no cross pollination is required to produce fruit, but hybrids can be made if desired. Reliable identification (using morphology and molecular methods) and importation in quantity of 'new' species, should be encouraged. Evaluation of their characteristics potential for breeding should be the aim of future research.

To recapitulate some of the known facts: The family Juglandaceae belongs to the Angiospermae, class Dicotyledoneae, order Juglandales (Krussmann 1985). The order Juglandales comprises two families, namely Myricaceae with 2 genera and Juglandaceae with 6 genera and 60 species )Stace 1996, Bhattacharyya & Johri 1998). Manchester (1987) divided Juglandaceae into two subfamilies, namely the Platycaryoideae and the Juglandoideae.

There are two geographically distinct areas of distribution of Juglandaceae, including the genus Juglans (Krussmann 1985, Bhattacharyya & Johri 1998). Manchester (1987) is of the opinion that Juglandaceae originated in the Northern Hemisphere, with major diversification occurring during the Paleocene era.

The exact number of species belonging to the genus Juglans is a subject of contention. According to various authors the genus Juglans contains 15 species (de Witt 1963), 9 species (Pham 1992), 21 species (Malvolti et al 1997) or 20 species (Nicese at al 1997). Manchester (1987) divided the genus Juglans into three sections, namely Dioscaryon, Rhysocaryon and Cardiocaryon. Shape of fruit was used to determine to which section a particular specie may belong.

There are several reasons for the introduction of tropical and subtropical (low chill) genes from the Central and South America. Low chill varieties may be used in hybridisation to lower the chill requirements of the existing high chill varieties of Juglans species. Careful selection of F1 hybrids with low chill requirements is the desirable outcome.

Experiments with the aim to develop hybrids, which would tolerate low temperature conditions, were conducted on the turn of the 20th century by Luther Burbank. Hybrids of peaches, nectarines and plums were bred so they could tolerate lower temperatures (Harwood 1905). Thus the aim of this experiment was in opposition to that of Burbank.


The source of materials used in this study was quite varied. J. sigillata Dode and an undescribed species ("J. sp.") were acquired in Southern China. J. olanchana Standl. et L.O. Williams was collected in temperate North America, J. neotropica Diels. and J. australis Griseb. came, respectively, from tropical and temperate Central and South America.

DNA analysis

The information gained from the sequencing of a region of chloroplast DNA indicated a very close relationship between all species of Juglandaceae tested. No certain sequence differences were seen in three tRNA coding regions, or in the trnL (leucine) to trnF (phenylalanine) intergenic spacer. The intron in trnL had three informative nucleotide substitutions, which matched a division between the species of European/Asian origin and the Juglansspecies of the New World. The data also indicated that the new American accessions were much more closely related to each other than their European/Asian counterparts.

Morphological analysis

76 morphological characters were obtained from in situ observations of new accessions and the reliably identified species grown under field conditions. The plants investigated were seed grown and had been evaluated for a period of 6 years. The six new accessions were still at a juvenile stage and lacking in adult characters, e.g. size, branch ramification, bark texture and flowers.

The results of morphological studies clearly demonstrated the division between the 'New World' species and the species of European/Asian origin. Relationships between the four American species are not definitively resolvable.

The placing of all new accessions within taxonomical trees was in accordance with their geographical origin.

The existence of problems regarding reliable identification of new horticultural materials is well known. Results of this work have confirmed the previously supposed identity of species examined. The results also indicate definite correctness at the generic and sectional levels. This work clarifies our position, re future breeding programmes, which will utilise clonal materials currently present in this country.

Production and use in Australia

Juglans regia and its many varieties and cultivars have spread to every continent due to world wide popularity and adaptability of trees to diverse environmental conditions. The total nut crop in the USA in the year 1909 was 63 328 000 Lb, of which Juglans regia accounted for 22 026 524 Lb (Burbank 1914). The production of J. regiain the year 1994 reached 800 000 tonnes (Aleta et al. 1993). 1996 annual world production reached 560, 000 tonnes, the main producers being the USA, China and India (Juglans CGC Report 1996).

In Australia during 1992-1993, total production of walnuts ( Juglans regia) (with shell) amounted to only about 72t. In the same period of time, 407 t was imported. The projected figure for production during year 2000 was 470 t (Chiba 1997), yet the figures for the year 1999 show that only 110 t was actually produced, while the imports accounted for some 2 500 t worth almost $10 million (Adem et al. 2000).

The fruit of Chinese Hickory Sarg. is a highly prized nut crop also, especially in the South East Asia. Carya cathayensis is the most important specie of the six Carya species indigenous to this region. (Yong-Ling et al 1992). Cultivation of Chinese hickory (Carya cathayensis) is localised in Zhejiang Province, with 19 000 hectares under cultivation and yields of 600kg to 2 400kg per hectare (Yong-Ling et al 1992).


There is no need to reiterate the well known facts, however walnut crop’s dependence on water supply should be mentioned. Cohen et al (1997) stated that to produce walnut crop, large amounts of water are needed. The availability of water may be a limiting factor in regions with scarce water, eg. the Mediterranean, South Australia. To redress the water deficit water irrigation is needed. In an experiment conducted by Lacointe et al (1997) it was established, that circa 7 litres of water were transported through the transpiration stream of 3 year old Juglans regia L. cv. Lara seedlings every 24 hours.

Wood characteristics and usage

The wood of Juglandaceae is often ring-porous, with only few medium to large vessels. The intervascular pitting is alternate. Parenchyma is predominantly apotracheal, or as broken bands intermediate between apotracheal and paratracheal. The rays are 2 to 7 cells wide and heterogenous. Genus Platycarya possesses spirally thickened vascular tracheids (Bhattacharyya & Johri 1998).

The related genera of Juglans, Carya and Engelhardtia supply timber of high quality (Bhattacharyya & Johri 1998).

Walnut wood is valued for its hardness and is also easy to cut (Harwood 1905). The timber is compact, homogenous and finely grained, varying from dark to light colour (Malvolti et al 1997). The wood from Juglans nigra, which is similar in quality, is described as relatively easy to manipulate (Malvolti et al 1997). It is used for cabinet making, furniture, carving, gun stocks etc. (Bhattacharyya & Johri 1998).

Juglans neotropica may be used as a plantation timber crop also. Trials conducted in New Zealand showed, that seed germinates readily and the seedlings grow rapidly. A 1.5m growth in the first year was registered with plants grown in the Auckland region. After 10 years the trees were 10m tall and bore their third harvest of fruit. Their speed of growth was comparable to that of Pinus radiata D. Don. (Lost crops of the Incas 1999). The wood of Juglans neotropica is also highly prized for carving and cabinet making throughout the Andean region of South America (Lost crops of Incas 1999). The wood of Carya ovata (Hickory) is a valued timber for axe handles (Bhattacharyya & Johri 1998).

As the production of tropical sawnwood,veneer and plywood is dropping (ITTO 1999) the importance and demand for traditional timbers e.g., the walnut timber, may increase.

Hybrid walnuts

The occurrence of natural hybrids of many species of plants is quite common. Darwin (1877) mentions the extraordinary number of hybrid forms of Primula species in Europe. According to his sources (Kerner) in the 19th century some 24 hybrid forms of wild Primula could be found in the European Alps alone.

Compared to 'pure species', hybrid plants usually display rapid growth and altered appearance. Second generation hybrids exhibit great variability (Burbank 1914, Harwood 1905, Tootill 1984, Thain & Hickman 1995).

Several cultivated hybrids of the following cold-climate species Juglans regia, Juglans nigra, Juglans ailantifolia Carr., Juglans cinerea L and Juglans mandshurica Maxim. are named: Juglans x intermedia is a cross between Juglans nigra and Juglans regia; Juglans x notha is a hybrid of Juglans ailantifolia and Juglans regia and Juglans x sinensis is a cross between Juglans mandshurica and Juglans regia (Krussmann 1985).

Although some hybrids may grow four times faster than their parents, the quality of wood is comparable to that of their parents (Burbank 1914). The wood of the hybrid walnuts is akin to the timber of some tropical woods, namely the mahogany (Harwood 1905).

Crosses between Juglans neotropica and Juglans regia have been reported. Also crosses between Juglans neotropica and Juglans nigra are possible. It was suggested that these hybrids may possess superior grade of timber and fruit ("Lost Crops of the Incas" 1999).

Heterosis, the theory behind the process.

The following definitions were included, to establish the meaning of the general terms used in reference to the subject of hybridisation and heterosis.

The term hybridisation can be defined as a natural or artificial process that, leads to the formation of an individual, which is produced from genetically different parents. This individual is termed a hybrid (Tootill 1984, Raven et al 1992).

Hybridisation is commonly used where the parents are from different taxa. The term may also apply to inversion hybrids, where offspring are heterozygous for a chromosome inversion (Thain & Hickman 1995). A hybrid is an offspring of two parents, which differ in one or more heritable characteristics (two different varieties or two different species) (Raven et al 1992). A hybrid has been defined as a zygote produced by the union of dissimilar gametes (Stace 1996).

Different species of forest trees may be interbred and modified and used as material for building a new specie that would more suited to the conditions of reafforestation than any existing species (Burbank 1914). The notion that interspecific hybrids are rare is not accurate. Stace (1996) recorded 780 interspecific hybrids among vascular plants of Great Britain, which number some 2500 native and alien species.

Heterosis (in Greek alteration) denotes the superiority of the hybrid over either parent in any measurable character.

Hybrid plants exhibit vigorous growth, greater yield and increased disease resistance. This effect is thought to be due to an accumulation of dominant alleles, each having additive effects. For example, if a characteristic is effected by two genes X and Y and the parents are XXyy and xxYY, the hybrid would be XxYy. Selfing will produce extremely variable F2 generations. To maintain optimum heterosis, two different and pure parental lines are used, producing F1 hybrids. Heterosis may result from heterozygous advantage, or from fixation of different deleterious recessives in the inbreds. The process of heterosis is not fully understood (Burbank 1914, Raven et al 1992, Thain & Hickman 1995, Tootill 1984).

Selection of a breeding system depends on the level of additive and non additive genetic effects. Genus Juglans possesses a high proportion of additive genetic traits. This means, that a single character may not be determined by a single gene, but by a number of genes. The selection of the parents from each generation should be based on the basis of the individual's performance as a whole, rather than being based on a single specific phenotypic trait (Forde 1975).

Apomixis, or production of seed without fertilisation, occurs spontaneously in the walnuts (Loiko 1990). This may occur either by the formation of a diploid embryo or embryo sac by a somatic cell, or by suppression or modification of the meiotic process to give unreduced megaspores (Tootill 1984, Thain & Hickman 1995). Sartorius & Stosser (1997) observed apomixis in some isolated trees of Juglans regia in Germany.

'The foundation principles of plant breeding are simple and may be stated in a few words, the practical application of these principles demands the highest and most refined efforts of which the mind of man is capable... The plant breeder is an explorer into the infinite'(Harwood 1905).

Selection and breeding of the genus Juglans

The bulk of research done with the members of genus Juglans concerns J. regia, due to the fact, that its fruit and wood is superior to any other species in this genus. Some work was also done on J. nigra, J. hindsii and J. ailantifolia. In the case of J. regia selection and breeding work spans many centuries over a wide geographical area (Smith 1950).

The aims of breeding experiments is to produce higher fruit yield, to produce wood in quantity and of high quality, to heighten disease resistance, to achieve late leafing in colder climates and to produce desirable growth habit in order to lessen pruning requirements (Burbank 1914, Forde 1975, Sykes 1975 ).

Here one must distinguish between the 'passive' selection of 'naturally' occurring hybrids and the 'active' creation of new hybrids, followed by the selection of the most desirable individuals thus created.

Initially selections were made from populations of wild walnut trees for those with the most desirable nuts and growth habit. Seed for cultivation was collected on the basis of plants' phenotype. This was a type of mass selection, in which the trees most desirable were selected out. No progeny testing was undertaken and no new genotypes were created (Richards 1998 personal communication).

Mc Granahan and Leslie (1990) stated, that J. regia now growing in the Balkans and Turkey, originated in Iran, when seedlings were taken there by ancient Greeks. In the same way, Romans were responsible for the introduction of walnut into Europe.

Aleta et al (1993) comment, that the quality of crop (in this case the fruit) has gradually improved, due to the selection of seed from superior trees and the gradual destruction of inferior producers (wood harvesting).

Selection of superior clones from wild populations is continuing. Modern techniques eg. field surveying, aerial surveying, detailed statistical analysis, analysis of native tree crop populations and random sampling of accessions, are frequently employed. Because of the high cost of genetic studies and procedures, plants are selected on basis of their phenotype (Sykes 1975).

Some 200 cultivars of J. regia were selected in the years 1990-1991 from the Darende region, in Turkey. The aim was to produce fruit of superior quality (Mehmet & Beyhan 1993).

Kuden et al (1997) write of a similar experiment where germplasm from Taurus Mountains was used. The aim of this experiment was to determine the walnut population of this area and select and evaluate pomological characteristics of several clones. Lateral bud production, late bud break and fruit qualities were looked at.

Selection of germplasm, selecting for vigour, disease resistance, flowering time, frost resistance and fruit quality in Poland, was documented by Piekko & Czynczyk (1990).Similar work in the USA using Juglans hindsii was recorded by Forde (1975). Paunovic (1990) writes of an extensive research program concerning the selections of Juglans regia in the former Republic of Yugoslavia. Amongst other goals, this research targeted the regularity and size of yield, disease resistance and the resistance to frost. Selection for plants prone to produce lateral buds in more than average quantity and consequently with increased fruit yields, were also made (Korac et al 1993).

Experiments with the aim to develop hybrids, which would tolerate low temperature conditions, were conducted on the turn of the 20th century by Luther Burbank. Hybrids of peaches, nectarines and plums were bred so they could tolerate lower temperatures (Harwood 1905). Our aim is the eventual breeding of low chill hybrids of Juglans. The newly developed low chill varieties may be used in hybridisation, to lower the chill requirements of the existing high chill varieties of Juglans species. Careful selection of F1 hybrids with low chill requirements is the desirable outcome.