Rootstock Effects on Shoot Morphology and Spur Quality of 'Delicious' Apple and Relationships with Precocity and Productivity

Additional index words. Malus domestics, flowering, spur density, trunk cross-sectional area Abstract. Two-year-old branch sections of 'Starkspur Supreme Delicious' apple (Malus domestics Borkh.) trees growing on 17 rootstock were studied over 6 years to determine the effects of rootstock on shoot morphology and spur quality and describe how these factors may be related to precocity and productivity. Shoot length was affected by rootstock and was positively related to trunk cross-sectional area within each year, but the slope of the regression line decreased as trees matured. The number of spurs on a shoot was largely a product of shoot length. Spur density was inversely related to shoot length, where rootstock with longer shoots had lower spur densities. Flower density was not related to spur density, and shoot length only accounted for a minor part of the variation in flower density. The proportion of spurs that produced flowers was closely related to flower density, indicating that rootstock influence flower density by affecting the development of individual buds rather than by the production of more buds. More vigorous rootstock generally had spurs with larger individual leaves and higher total leaf area per spur, but fewer spur leaves with lower specific leaf weights. More precocious rootstock were also more productive over a 10-year period when yields were standardized for tree size. Tree size was the best indicator of precocity and productivity, which could be predicted with a high degree of certainty as early as the 4th year. Apple rootstock are used primarily for reducing tree size and increasing precocity and yield efficiency. Compared with widely spaced large trees, intensive plantings of small trees resulting from the use of dwarfing rootstock intercept more incident light, which is closely related to dry matter production (Jackson, 1980). Smaller trees generally have less internal shading. Shading in full-sized trees results in areas of the tree canopy characterized by low productivity and poor fruit quality (Ferree, 1989; Heinicke, 1964; Warrington et al., 1995). In addition to improved light interception and distribution, increased partitioning of photosynthates to fruit rather than wood also may contribute to the high yield efficiencies commonly observed with dwarfed trees (Forshey and McKee, 1970; Preston, 1958, 1968). For example, on 27-year-old trees, the ratio of crop to the weight of trunk and branches was 13.7 for trees growing on the dwarfing rootstock, M.9, while for those growing on the vigorous M. 16 the ratio was 4.7. Spur quality and shoot morphological attributes may be one way in which rootstock influence productivity. In the past, spur quality has been measured in terms of spur leaf number, leaf area, mean leaf size, leaf dry weight, specific leaf weight (SLW) and terminal bud diameter (Barritt et al., 1987; Ferree, 1989). Spur density (spurs/m) and the proportion of flowering spurs (flowering spurs/spur) have been used as descriptors of shoot morphology (Ketchie 1984; Warrington et al., 1990), but measurements of shoot length, spur number, flower number, flower density, and lateral shoots may enable a more complete characterization.