S04 - Session P1 - New Tools I - Acclimatization of micropropagated walnut, a saucerful of secrets

Authors: Emma Mortier, Patrick Viellefosse, Edwige Grosbois, Jean Coulié, Laurent Jouve *

Walnut micropropagation is a technique that has been known for several decades. Nevertheless, the ex-vitro acclimatization process is still very delicate. We present in this study that this acclimatization can be carried out on a large scale, with a high survival rate of the plants above 85% in routine. This improvement of acclimation was possible due to two major points. Firstly, a pre-acclimatization and on the good practices of the laboratory allowing to propose to the nurserymen in charge of the breeding and the grafting of the seedlings of walnut trees a much more robust material than the material directly resulting from the in vitro. Secondly, the use of recent techniques of mycorrhization of walnut rootstocks by arbuscular fungi of the plants during pre-acclimatization. This technique resulted in a better growth and a better behavior in front of sanitary obstacles when leaving the in vitro conditions. Thus, a greater quality of the plants could be proposed to the nurserymen bringing in a second time a more valuable quality for the produced plants. We propose here a simple and reasonably cheap protocol that can be used by the large majority of nurserymen historically recalcitrant to in vitro plant acclimatization considering the difficult and costly mortality rates of this technique. This methodology for in vitro walnut acclimatization finds its place in the need of production of certified plants, rootstocks or varieties on its own roots, in connection with the directives of production of walnut and other fruit plants at the European level. Suggested Micropropagation of nuts has been known for decades. Nevertheless, the ex-vitro acclimatization process is still very delicate. our study allowed to obtain a routine survival rate of more than 85%. This was possible thanks to two important improvements. First, a pre-acclimatization treatment which yielded much more robust plants compared to material coming directly from in vitro. Second, mycorrhization with arbuscular fungi during pre-acclimatization. This technique resulted in improved growth and survival upon leaving the in vitro conditions. We thus propose here a simple and reasonably inexpensive protocol that allows for the production of plants that may convince the vast majority of growers to reconsider their preconceptions for in vitro plants. In this way, the need for certified plants, rootstocks or varieties on their own roots, which must comply with European directives, is met.