Micropropagation of mature Juglans hybrids

Juglans hybrids (Juglans nigra × J. regia) are valuable forest trees for high value timber production which is highly demanded in furniture industry. However, plant material is rare and therefore expensive since in Europe only a limited amount of hybrid seeds is produced. Vegetative propagation would allow a constant production of hybrid planting material. A micropropagation protocol was developed for two mature hybrid genotypes. The first one is a 60-year-old vigorous tree which produced stump sprouts after felling. The second hybrid is a micropropagated clone which demonstrated its good stem form and high growth rate in a clonal field trial. From both genotypes scions were grafted onto Juglans regia seedlings in order to induce rejuvenation of the plant material prior to micropropagation. Explant sources were young shoots taken from grafted plants in a greenhouse of both genotypes and from a 19-year-old tree in the clonal field trial of the second genotype. After surface sterilization, nodal segments with one axillary bud were placed on establishing media (Driver and Kuniyuki (DKW) and Rugini Olive Media (ROM)) containing 6-benzylaminopurine (BAP) and indole-3-acetic acid (IBA). Severe problems during the establishment phase were contamination and browning of explants. The proportion of surviving explants was very low. However, proliferating shoot cultures could be established from both genotypes with explants from grafted plants as well as from the 19-year-old tree of the second genotype. Plant growth regulator combinations and concentrations were tested with the aim of a continuous production of high quality explants and microcuttings, which are suitable for further propagation and rooting. For both genotypes, media combinations based on ROM and a combination of ½ ROM plus ½ DKW medium, respectively, were identified, which gave significantly higher propagation rates and shoot quality compared to standard DKW medium.