Relationships between sap flow, hydraulic conductivity, and the anatomical characteristics of stems and roots in apple rootstocks of different vigour

To find a useful parameter that could be related to the degree of rootstock vigour, for rootstock breeding programmes, sap flow, hydraulic conductivity, and the anatomical characteristics of stems and roots were measured in 1-year-old trees of apple rootstocks of different vigour. Sap flow was less active in dwarfing rootstocks than in invigorating rootstocks, when taking leaf area into account, under a controlled temperature of 20 degrees C with the root zone at 15 degrees C. The hydraulic conductivities of stems and roots were not consistent with the degree of dwarfing. The hydraulic conductivity of the stem appeared to be related to the diameter and density of the large vessels in the xylem., On the other hand, the phloem-to-xylem ratio in roots with diameters of 1.5 - 2.0 mm, was closely-related to sap flow and the degree of dwarfing. The phloem-to-xylem ratio decreased as the degree of vigour increased. Moreover, the phloem-to-xylem ratio was considered to be related to the ability of roots to absorb and transport water, which was expressed as the root-specific sap flow. Therefore, the phloem-to-xylem ratio could be a useful index for evaluating the potential of apple trees as dwarfing rootstocks.