Interactions between rootstock, inter-stem and scion xylem vessel characteristics of peach trees growing on rootstocks with contrasting size-controlling characteristics.

Background And Aims: The primary physiological mechanism influencing tree vigour in size-controlling rootstocks of peach has been related to the hydraulic conductance of the rootstock. Differences in rootstock hydraulic conductance are a function of rootstock xylem vessel characteristics. The present research examined whether the vigour and xylem vessel characteristics of the rootstock influence the xylem characteristics of the scion. We tested whether using a size-controlling rootstock genotype as an inter-stem influences the xylem vessel characteristics of either the rootstock below the inter-stem or the scion above it and vice versa.

Methodology: Anatomical measurements (diameter and frequency) of xylem vessels were determined above and below the graft unions of the trunks of peach trees with differing scion/rootstock combinations. The three peach rootstocks were 'Nemaguard' (vigorous), 'P30-135' (intermediate vigour) and 'K146-43' (dwarfing). The vigorous scion cultivar was 'O'Henry'. The inter-stem experiment involved trees with 'Nemaguard' (vigorous) as the rootstock, 'K146-43' (dwarfing) as the inter-stem and 'O'Henry' as the scion. Based on anatomical measurements, we calculated the theoretical axial xylem conductance of each stem piece and rootstock genotype with the Hagen-Poiseuille law.

Principal Results: Xylem vessel dimensions of rootstocks varied in conjunction with tree vigour. Scion xylem vessel dimensions of different scion/rootstock combinations were only marginally affected by rootstock genotype. The inter-stem sections from the dwarfing genotype ('K146-43') had narrower vessels and a lower calculated hydraulic conductance than the xylem from either the vigorous rootstock below ('Nemaguard') or the scion above ('O'Henry').

Conclusions: Rootstock genotype only marginally affected scion xylem vessel characteristics. Thus the xylem vessel characteristics of the dwarfing rootstock genotypes appear to influence tree growth directly rather than through an effect on the xylem characteristics of the scion. A dwarfing rootstock genotype used as an inter-stem appeared to work as a physical restriction to water movement, reducing potential xylem flow and conductance of the whole tree.