Adventitious rooting and clonal plant production of hybrid walnut (Juglans) rootstock selections

Micropropagated shoots of 15 Paradox (black walnut x Persian walnut) genotypes selected for disease or nematode resistance and 3 backcrosses to J. regia selected for virus tolerance were used to evaluate rooting methods. Substitution of fructose for sucrose strongly stimulated shoot growth of some genotypes but inhibited others. Rooting increased when I mM phloroglucinol was added to the multiplication medium. Rooting was most successful when shoots were induced on 5 to 40 ppm IBA for 3 to 5 days. Root development on a 1/4 strength basal medium:vermiculite mix was superior to either stationary liquid or conventional gelled medium. Genotypes differed but generally higher IBA concentrations increased rooting. Rooting of genotypes induced on 40 ppm IBA ranged from 100% to 19%. Rooting on 5 ppm IBA ranged from 80% to 0%. Microshoots that did not root in vitro were re-cut at the base, treated with IBA, and stuck in vermiculite in a fog chamber. Although percent rooting was generally not as high as with in vitro rooting methods, the roots produced were more fibrous with very little associated callus. Rooted plantlets were acclimated in a fog chamber for 2 weeks and then grown 3 months in a greenhouse to desired size for field planting. Survival by genotype ranged from 90% to 22% and averaged 65%. Plants were then chilled (10 degrees C) under short day-length and low light for 3 weeks to induce artificial dormancy. Dormant trees were stored successfully for up to 6 months at 5 degrees C. Over 5000 trees were produced last year for disease testing and field trials. Hardwood cuttings of the same genotypes were treated with IBA, stuck in Oasis Rootcubes and placed on bottom heat. Rooting percentages varied from 0 to nearly 100% and were influenced by IBA concentration.