Early bearing genotypes of walnut: a suitable material for breeding and high density orchards

Walnut (Juglans regia) is a non-precocious bearing and vigorous tree. Standard seedlings of walnut have long juvenility and are hard to propagate by usual propagation methods like cutting, layering and somehow grafting. There is an excellent genetic diversity among Juglans regia populations from China to central and west Asia including Iran. Among the populations there are some genotypes of walnut that are precocious and have short juvenility phase. These genotypes usually have cluster bearing flowering habit, low vigor and good rooting ability that make them suitable for easy propagation by cutting and stool layering. Most of these genotypes have basitonic phenotype in nature. Such dwarf genotypes which have ability to induce precocity in scion can be used not only as a rootstock in walnut breeding programs to shorten the vegetative adult phase of the scion and reduce duration and costs of a breeding program, but also have potential for establishing high density orchards. Precocity trait also has a high heritability in walnut. Low vigor genotypes show the highest proliferation rate and smaller shoot size, lower callus formation and higher rooting percentage in vitro. Moreover, these genotypes showed in vitro flowering on microshoots. Therefore, these genotypes of walnut are an ideal material for molecular genetic investigations for breeders and geneticists to find the genes involved in precocity, growth, flowering habit, easy rooting and other valuable traits in walnut. In this article we review the researches done about these genotypes.