Ex Vitro Rooting and Simultaneous Micrografting of the Walnut Hybrid Rootstock ‘Paradox’ (Juglans hindsi × Juglans regia) cl. ‘Vlach’

In vitro micropropagation is already a current multiplication tool for walnut self-rooted cultivars and rootstocks, but walnut grafting is still performed in the field or in greenhouses, mainly using seedlings as rootstocks. The present work describes a new approach to obtain clonal walnut-grafted plants, involving in vitro shoot production of ‘Paradox’ (Juglans hindsi × Juglans regia) cl. ’Vlach’, to be used as rootstock, and J. regia cv. ‘Chandler’, to be used as scion. After completing the in vitro multiplication phase and a seven-day root induction treatment, ‘Vlach’ explants are transferred to ex vitro conditions for root expression while being simultaneously grafted using the in vitro produced ‘Chandler’ scions. The importance of the presence of leaves on both the scion and the rootstock for the success rate of the technique was evaluated. Under optimal conditions, average success rates of 82% for rootstock rooting, 72% for micrografting survival, and 84% for grafted plant acclimatization were achieved. This rooting/grafting combination technique seems able to compete with the traditional techniques of nursery grafting, allowing obtaining high-quality walnut-grafted plants independently of the external weather conditions in a significantly shorter time.