III.4 Micropropagation of Juglans cinerea L. (Butternut)

The genus Juglans L. (family Juglandaceae) comprises about 20 species of deciduous trees. These monoecious trees are native to North and South America, and from southeastern Europe to eastern Asia (Bailey and Bailey 1976). They are grown as ornamentals, for the edible nuts, and some species for the fine-grained wood highly valued for furniture, veneer, gunstocks, and cabinet work. The Persian walnut (J. regia L.) is the most horticulturally developed and widely cultivated species for nut production (McGranahan and Leslie 1990). Worldwide production of walnuts in 1992 was listed at 918 180 metric tons, with the United States accounting for approximately 20% of world production at 181 400 metric tons (FAO 1992). Black walnut (J. nigra L.) is one of the most valuable hardwoods produced in the United States (Williams 1990). Eastern black walnut is also grown for the edible nuts, but it is the species valued economically for its high quality wood, prized for fine furniture, gunstocks, cabinets, and veneer. Butternut (J. cinerea L.) (Fig. 1A), a species native to North America from New Brunswick to Georgia, and west to Minnesota and Arkansas (Rink 1990), is valued economically and ecologically for its wood and edible nuts. The nut is an important wildlife mast and the wood is marketable for many uses including furniture, cabinets, fine woodworking, and paneling. In areas where quality butternut wood is available, it ranks eighth out of the top 28 species for prime veneer and sawlogs (Peterson 1990). Butternut, also known as white walnut, is relatively slow-growing with an average height of 12 — 18 m, but can attain heights of 30 m (Rink 1990). Moist, rich soils of hillsides and stream banks are the preferred growing sites, although butternut can grow quite well on dry, rocky soils. Male and female flowers (Fig. 1B) are borne separately on the same tree, but mature at different times (McDaniel 1956). Male flowers (catkins) are preformed on the previous year’s wood, appearing as small, scaly, cone-like buds (Dirr 1983). Female flowers occur in two- to eight-flowered spikes borne on the current year’s shoots. Butternut can be distinguished from black walnut by stems having a chambered, chocolate-brown pith and a large, conspicuous 1