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.