Hormone and growth interactions of scions and size-controlling rootstocks of young apple trees

Abstract

Size-controlling rootstocks are critical to grow small, efficient trees that enable early and high yield in plantings of apple. Improved knowledge of rootstock-related size-control processes is important for breeding and cultural practices. The research objective was to determine hormone profiles within trees grafted on Malling rootstocks. Buds from ‘Gala’, ‘Fuji’, M.7, M.9, M.27, and MM.111 were grafted to rootstocks M.7, M.9, M.27, and MM.111 and then grown in a greenhouse and the field. After 2 years, heights of trees grafted to growth-controlling rootstocks decreased in the order M.9 > M.7 > MM.111 in the greenhouse, the reverse trend that was measured in the field. Although soil resources were not measured, ample availability of resources such as water and mineral nutrients in the greenhouse may have been responsible for the reversal of rootstock effects on tree height in greenhouse and field. Elevated abscisic acid (ABA) and ABA metabolites were associated with ‘Gala’, MM.111 and M.9 scion that were grafted on M.9 rootstocks. Abscisic acid and abscisic acid glucose ester were generally greater in root, rootstock stem below the graft union, scion above the graft, and xylem exudate of rootstock M.9 than MM.111. Reduced gibberellin (GA19) was found in roots and xylem exudate of ‘Gala’ grafted to M.9 than MM.111 rootstocks. These results support the hypothesis that hormone signals from rootstocks control tree growth but it is likely that stage of development, time after planting, and environmental resources will also interact to influence growth effects of size-controlling rootstocks. It is proposed that gene expression associated with hormone metabolism can be developed to further understand the underpinnings of size-controlling rootstocks and assist the selection of rootstocks for size control and, possibly, for other hormone-related characteristics.