Title: Interaction of scions and size-controlling rootstocks of young apple trees Authors
Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: August 30, 2013
Publication Date: N/A
Technical Abstract: Size-controlling rootstocks are critical to grow small, efficient apple trees that enable early and robust yield in high-density plantings. Improved knowledge of scion and rootstock-related processes associated with size-control can assist rootstock breeding and tree management. Growth regulation of scions by rootstocks can encompass chemical signals such as hormones, water conductance and allocation of carbon among shoots, roots and fruit. The objective of this work was to determine tree growth and hormone response of two cultivars and four rootstocks (i.e. used as scions) grafted to four size-controlling 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 for three years. Tree heights of scions of M.9 and M.27 were much less than scions of M.7 and MM.111, irrespective of rootstock. Heights of scion grafted to its own rootstock (e.g. M.9 on M.9) did not differ from an intact tree of the same genotype that was not grafted. Greenhouse-grown trees were evaluated for hormones (auxin (IAA), abscisic acid (ABA), cytokinins, and gibberellins (GA), and associated metabolites in roots, stems, leaves, and xylem exudates in scions of ‘Gala’, M.9, and MM.111 that were grafted on M.9 and MM.111 rootstocks. From the greenhouse trees, it was determined that elevated abscisic acid (ABA) and reduced gibberellin (GA) were associated with the more dwarfing rootstocks. The combination of ABA and its glucose ester were greater in root, rootstock stem below the graft and scion above the graft of rootstock M.9 than rootstock MM.111. GA19, GA53 and IAA were found in exudate of ‘Gala’ on MM.111 but not M.9. GA4 was the only biologically-active GA found, albeit in small quantities; but closely associated metabolites were found including GA34, a 2-hydroxylated metabolite from active GA4. GA34 was 5-fold higher in exudate of M.9 and MM.111 scions on M.9 than MM.111 rootstock, suggesting that M.9 rootstocks may have higher capacity for GA degradation than MM.111 rootstock. It is also possible that GA20 oxidase may be less active in dwarfing M.9 than MM.111. Elevated levels of GA53 were measured in xylem exudate of MM.111 but not M.9 rootstocks. The enzyme GA20 oxidase catalyzes conversion of GA53 to GA9 and GA20 that are precursors to active GA4. Hormone signals, however, were not clearly the sole size-controlling factors in that height and diameter growth and internode length were not reduced in young trees that were grown on dwarfing rootstock in the greenhouse. Tree stage of development and environment may interact to affect the influence of size-controlling hormone signals. Further work will focus on identification of marker genes associated with hormone metabolism that can be developed to assist the selection of rootstocks for size control and, possibly, for tolerance of environmental stress.