Anatomical structure associated with vegetative growth variation in macadamia

Abstract

Low-vigour scion cultivars and dwarfing rootstocks are a significant contributing factor to the success of modern temperate orchard systems. Planting density and canopy efficiency are currently limited in macadamia by a lack of low-vigour cultivars and rootstocks. The relationships between xylem vessel diameter, hydraulic conductivity and vigour are implicated in dwarf cultivars and low-vigour rootstocks of peach and apple. This study aims to determine relationships between vessel anatomy and tree-scale vigour in subtropical macadamia, to stimulate the development of low vigour cultivars and dwarfing rootstocks. Stem sections from the most recent mature flush were collected from five replicates of three macadamia cultivars: ‘D4’ (high vigour), ‘B25’ (intermediate vigour) and ‘B63’ (low vigour). Sections were imaged under a fluorescence microscope in order to measure the number and size of all xylem vessels in the stem. The three cultivars showed distinct differences in height over 5 years of measurement (P\u2009<\u20090.001). Change in tree height related to the xylem vessel characteristics of mean vessel area of the ten largest vessels per section (R2\u2009=\u20090.57, P\u2009<\u20090.001), pith size (R2\u2009=\u20090.70, P\u2009<\u20090.001) and total additive vessel area (R2\u2009=\u20090.41, P\u2009<\u20090.01). This study indicates that relationships between anatomical structure and vigour in macadamia are likely to be similar to those of well-studied temperate tree crops and that anatomical traits may be useful in the selection of low-vigour cultivars. Future work should investigate hydraulic relationships with vigour and rootstock-scion interactions in macadamia.