MORPHOLOGICAL AND ANATOMICAL CHARACTERISTICS OF GRAFT UNIONS IN APPLE TREES ON DWARFING ROOTSTOCKS

R.K. Simons, M.C. Chu
Graft-unions of apples on dwarfing rootstocks exhibit growth patterns that are ultimately affected by scion and rootstock vigour. In order to develop a high-quality tree, evaluations are needed to determine graft-union response in the formative stages, which may reveal compatibility problems that might be better eliminated before the grower has expended several years' effort on a non-productive tree. For example, there is a question whether incompatibility exists between Granny Smith and M.26 (a tree recently in demand for apples popular in the current market). In isolated examples of this stock/scion combination where T budding was used, the wound response to rootstock removal has produced excessive growth on the opposite side of the tree trunk, creating an abnormal, poor-quality tree. This growth-habit affected development of the functioning phloem within the graft-union area which ranged from thick and well-developed (4 mm) to very thin (1 mm) on the distal side of the union. A cross-section of an M.26 trunk was 10–12 mm wide at the point where it was cut off, resulting in the contiguous functioning phloem being disrupted on both proximal and distal sides of the wound. Dessication of the rootstock wound persisted in the longitudinal axis of the trunk for as much as 1 inch (25.4 mm), producing a girdling effect. As adjacent tissues developed around the wounded area, radial growth of the vascular elements developed into a swirling effect found in fruit-bud initials. Vascular elements were at right angles to the normal longitudinal pattern of growth, causing constriction. In areas near the graft-union juncture, collapse of large, vascular elements was noted. These areas were characterized by radial type of growth which was offset from the longitudinal pattern and caused overgrowth within the union.

As uniting tissues were undergoing stress, whether mechanical or physiological, necrosis became apparent in the newly formed callus. As growth progressed, more necrosis became evident as a result of callus collapse in the union. New callus formed and adjusted around these necrotic areas with swirling vascular elements. These regenerative processes may be characterized as a successive growth response to initial formation of regenerative tissue. Carbohydrate content was variable in the graft-union area. However, the greatest accumulation occurred in lateral vascular branching

Simons, R.K. and Chu, M.C. (1981). MORPHOLOGICAL AND ANATOMICAL CHARACTERISTICS OF GRAFT UNIONS IN APPLE TREES ON DWARFING ROOTSTOCKS. Acta Hortic. 114, 198-199
DOI: 10.17660/ActaHortic.1981.114.27
https://doi.org/10.17660/ActaHortic.1981.114.27

Acta Horticulturae