Adventitious Root Formation in Walnut Hybrid Rootstocks (Juglans hindsii X J. regia)

Abstract

The walnut hybrid “Paradox” is used extensively as a rootstock for English walnut cultivars grown in California orchards. “Paradox” rootstock produces more vigorous trees and increases yield compared to both J. hindsii (California black) or J. nigra (black). A major obstacle in the development of a clonal “Paradox” rootstock with superior disease resistance is that it has not been able to be rooted with much success. In addition, rooted cuttings usually died when transferred to the field. Recently we have been successful in rooting several genotypes of “Paradox” as well as other individual trees selected as potential rootstocks in the Walnut Improvement Program. Rooting percentages varied considerably ranging from 2 to 92%. Genotype was a major factor accounting for the variable results. Reliable, high (over 70%) rooting percentages, however, were obtained in several genotypes. The method of application was also an important factor in obtaining high rooting percentages. Application of IBA using a novel toothpick method was superior in most cases to that of a quick dip. The highest rooting percentages were first obtained with semi- hardwood cuttings. Careful attention to the amount of water applied to the basal sections of hardwood cuttings was essential for rooting of hardwood cuttings. Other factors that were critical for obtaining high rooting percentages included hedging of the stock plant, water status of the stock plant, and application of several compounds including abscisic acid, ferulic acid or spermine to hardwood cuttings. The addition of these organic compounds increased the rooting percentages significantly. Successful transfer in the field required the use of fiber plugs that kept the roots from being damaged when transferred.