Beakbane and Thompson (1939, East Malling) had it right: scion vigour is physiologically linked to the xylem anatomy of the rootstock.

In 1939 A.B. Beakbane and E.C. Thompson published research that showed a correlation between the xylem anatomy of apple rootstocks and the vigour that those rootstocks imparted to scion cultivars grafted on them. However, Beakbane and his colleagues never made a clear functional link between rootstock xylem anatomical characteristics and the vigour that rootstocks impart to scions. Subsequently numerous researchers have reported relationships between rootstock xylem anatomy and scion vigour in several fruit tree species. Researchers have also reported that the leaf or stem water potential of trees growing on size-controlling rootstocks is often somewhat more negative than trees growing on vigorous rootstocks in the same environment. Recent research with dwarfing peach rootstocks has documented clear linkages between rootstock xylem anatomy, theoretical and measured rootstock axial hydraulic conductance, scion stem water potentials and scion shoot growth rates in composite peach trees. This paper will review the core evidence that links rootstock xylem anatomy to scion vigour and demonstrate that linkage using L-PEACH, a dynamic functional-structural tree model of tree physiology, architecture and growth.