THE ANATOMY OF THE DEVELOPING BUD UNION AND ITS RELATIONSHIP TO DWARFING IN APPLE

Light microscopy has been used to study the effect of dwarfing and sem i-dwarfing apple rootstocks on the early development of bud-unions wit h 'Gala', and the anatomy of 2-year-old bud-unions between 'Bramley' a nd the same rootstocks. The bridging of the cut edges of the cambia of bud and rootstock was achieved by differentiation of callus formed at an early stage in budding. New cambial cells were aligned at right an gles to the pre-existing cambia, with their long axes horizontal. Subs equently-formed xylem adopted this arrangement, so that fibres and Ves sels were arranged obliquely to the axis of the stem. At the interface between the bud and dwarfing rootstocks vessels with smaller than nor mal diameter were formed, indicating the presence of elevated levels o f auxin in this region. In addition, little xylem was produced in the adjacent rootstock tissue. In the case of semi-dwarfing rootstocks, th e rootstock produced normal xylem after a brief interruption. We sugge st that failure of auxin to cross the bud-union interface in the case of the dwarfing rootstocks leads to reduced rootstock xylem formation, and hence a poor supply of water and minerals to the scion, and this underlies the dwarfing effect.