Micropropagated walnut dependency on phosphate fertilization and arbuscular mycorrhiza for growth, nutrition and quality differ between rootstocks both after acclimatization and post-acclimatization

The English walnut (Juglans regia L.) is the main species cultivated for the production of edible nuts. In vitro micropropagation of walnut explants, necessary for mass propagation of high-quality walnut rootstocks, needs an ex vitro acclimatization phase followed by a post-acclimatization growth in greenhouse when plantlets become photoautotrophic. However, poor survival and slow growth rates are common difficulties encountered in nurseries when establishing micropropagated walnut saplings. As many other fruit and nut bearing trees, walnut exhibits a high dependency on symbiotic soil-borne arbuscular mycorrhizal fungi for better soil nutrient acquisition and development due to a coarse root architecture that notably limits soil inorganic phosphate (Pi) uptake. In the context of rootstock production, we investigated the establishment of seven walnut rootstocks of economic interest (RG2, RG6, R17, RX1, VLACH, VX211, and WIP3) inoculated or not with Rhizophagus irregularis under two contrasting Pi fertilization regimes. We demonstrated that inoculation with R. irregularis decreases micropropagated walnut rootstock dependency on Pi fertilization both at the acclimatization and post-acclimatization stages, together with improving quality, sapling biomass production and nutrition of walnut rootstocks. We also showed that these benefits are rootstock-dependent, indicating that walnut mycorrhizal dependency for Pi nutrition varies between cultivars.