Walnut

Abstract

All species of walnuts (Juglans) produce nuts, but the Persian or English walnut (J. regia L.) is the only species widely cultivated for nut production. Persian walnuts are grown in North and South America, Europe, and Asia. The considerable variation within J. regia, particularly in nut size and shape, led taxonomists to describe six additional species that have not been widely accepted but that illustrate some of the diversity. Persian walnuts are native to the mountain valleys of Central Asia. They were introduced into Europe by the Greeks and introduced into North America by the colonists. Breeding of walnuts is relatively recent, although it is probable that in the past, walnuts from the best genotypes were selected both for food and planting. The first breeding programs started at the University of California (The USA) and the Fruit and Vine Research Station in Bordeaux (France). Because Persian walnuts are native to the mountains of Central Asia, considerable effort in the USA has been directed toward collecting material from that area. The major breeding objectives are to increase yield, quality, and range of harvest dates while decreasing the amount of chemical input required to control pests and diseases. Isozymes and molecular markers have been used for identification of cultivars and genetic diversity analysis. Molecular markers have been developed for Walnut blackline disease that causes necrosis at the graft union. Gene transfer in walnut using Agrobacterium tumefaciens gene insertion into cells regenerated into plants via somatic embryo cultures has been successful. Traits of interest that have been tested in walnut include expression of a Bt gene from Bacillus thuringiensis for insect resistance and use of RNAi gene silencing.