The hydraulic resistance of young olive trees grafted on rootstocks with contrasting size-controlling potential was measured 30, 90, 360 and 480 days after grafting. Olive (Olea europaea L.) clones inducing plant vigorous growth (Leccino 'Minerva', LM) or dwarfing (Leccino 'Dwarf', LD) were studied in different scion/rootstock combinations (LD/LD, LD/LM, LM/LD, LM/LM). Plants growing on LD root systems developed lower leaf surface areas (about 50% less) than plants grafted on LM rootstocks. Graft union represented the largest fraction (up to 85%) of whole-plant hydraulic resistance 30 days after grafting, and still represented an important fraction of it 2 months later (40-55%). Four-hundred and eighty days after grafting, the hydraulic resistance of the graft union became negligible (< 3%) with no difference among the different scion/rootstock combinations. Our data reinforce the idea of graft hydraulics not playing a role in depressing the vigour of the scion as observable in adult plants grafted onto dwarfing rootstocks. The long time required for the vascularisation to be complete, on the contrary, represents the main risk for successful olive breeding.
