Grafting “in vitro” shoots on acclimating plants: morphological and anatomical aspects of compatible and incompatible pear/quince combinations

Grafting is a necessary practice in fruit-tree management to assure high orchard yield efficiency and high fruit quality. While the use of scion-rootstock combinations has made it possible to solve many problems, it has also created others of a biological-functional nature. Key issues regarding rootstock-scion interaction include structural and molecular analyses of tissues grown together at the graft union; new vascular connections; xylem and phloem flow; xylem transport of the main nutrients and aminoacids; canopy gas-exchanges; water consumption and use efficiency; and the canopy/root ratio. The present study reports and discusses a new methodology for in vitro grafting of pear shoots on acclimating rootstocks to study graft-incompatibility. The pear cvs. Beurré Hardy and Beurré Bosc and the rootstocks quince MC and clonal seedling OHF40 were used. The cultivars show different levels of compatibility with quince: Beurré Hardy is compatible and Beurré Bosc incompatible; both are compatible with the clonal seedling. The rootstocks were obtained by micropropagation and acclimated in a 2-litre pots at 22°C in greenhouse. When the rootstocks were 15-20 cm tall, they were top grafted with “in vitro” shoots of the varieties. The upper part of the rootstock was removed and the stem was cut longitudinally 4-5 mm deep. The cultivar shoots were 2 cm long and wedge-shaped at the bottom. A rubber tube 3-4 mm in diameter was used to secure the graft union. This system prevents water loss from the cut surface and ensures good contact between the grafting parts. A plastic chamber covered the grafted plant to keep humidity high and prevent shoot dryness. There were 25 plants per graft combination. Morphological and anatomical analyses of both compatible and incompatible combinations were carried out after 2, 4, 8, 15 and 30 days to determine differences in newly forming vascular connections. Preliminary observations showed high callus proliferation of the compatible cv. Beurré Hardy with both rootstocks, while the incompatible cv. Beurré Bosc showed no activity until 4 days after grafting