Abstract
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.
Highlights
Most walnut plants produced worldwide arise from grafting under seedling rootstocks, with the associated genetic variability derived from their seed origin
Better results were attained using in vitro propagation protocols [5–11], which have allowed for the commercial-scale production of clonal rootstocks in some laboratories around the world
The data attained here allowed validation of the protocol as an alternative to produce self-rooted and grafted walnut plants, because even the plants where the graft failed but the rootstock rooted successfully can be sold as self-rooted clonal rootstocks
Summary
Most walnut plants produced worldwide arise from grafting under seedling rootstocks, with the associated genetic variability derived from their seed origin. Given the limited ability to develop adventitious roots, characteristic of the Juglans genotypes [1], clonal walnut propagation is a long-pursued goal with scarce results so far. Attempts to overcome this recalcitrant behavior were made by using stool layering [2] and hardwood or softwood cuttings [3,4]. While these are the most traditional and cheapest systems for walnut cloning, by no means are they used for large-scale propagation. Even with the availability of clonal rootstocks, the production of grafted walnut plants by traditional grafting methods is a long, seasonal, and time-consuming process [12–15]
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