Production of grafted plants of mature Quercus robur L. 'Fastigiata' through serial grafting on juvenile rootstocks and effect on micropropagation

Scions from a hundred-year-old Quercus robur 'Fastigiata' tree (clone QRFA) were serially grafted onto 3-4 week-old seedlings, using a miniaturized version of cleft-grafting. Two kinds of scions were used for the initial grafts: a) axillary shoots (2-3 cm long; 1-2 mm diameter) from forced branch segments and b) woody shoots (similar in size to the former) from the previous season. The grafts were done in the laboratory and subsequently placed in a growth chamber. About forty days later, new scions (2 cm long; 1 mm diameter) were obtained from successful grafts, which permitted to carry out new grafting cycles. Every 40 days, the new scions were regrafted. Twelve serial grafts were done during this experiment. Grafting success of initial grafts using axillary shoots from forced branch segments was 63%. In subsequent graft cycles grafting success ranged from 69% to 95%. Serial grafting allowed for the exponential production of grafted plants in a very short term. Alternatively, parts of the grafted plants, obtained after graft cycles 1, 3, 5, 8, and 12, were used to obtain shoots for micropropagation. Nodal explants from elongated shoots obtained after graft cycles 8 and 12 showed higher multiplication rates than the explants from ungrafted and single-grafted plants along at least 18 subcultures, suggesting a reinvigoration effect of grafting on this parameter. Regarding in vitro rooting ability, no significant differences were found between axillary shoots from (serial) grafted and ungrafted plants, regardless the number of subcultures previous to rooting experiments.