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Title:   Author  item
                                            Tworkoski, Thomasitem
                                            Fazio, Gennaro
  Submitted to: Plant Growth Regulation
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/6/2015
                                    Publication  Date: 5/26/2015
                                    Citation: Tworkoski, T., Fazio, G. 2015. Hormone and growth interactions of scions and size-controlling rootstocks of young apple trees. Plant Growth Regulation. DOI: 10.1007/s10725-015-0078-2.Interpretive Summary: Nearly all commercial cultivars, such as ‘Delicious’ or ‘Gala’, are grafted to rootstocks that control tree size and increase orchard yield, but new rootstocks are needed that can adapt to environmental stresses while still providing size control.  This research identified several chemical-hormone differences in plant parts of grafted trees that likely affect size-control and environmental adaptation and which also could be used as markers in rootstock breeding or which may serve as targets for cultural control.
                                Technical Abstract: Size-controlling rootstocks are critical to grow small, efficient trees that enable early and high yield in high-density plantings of apple.  Improved knowledge of scion and rootstock-related processes associated with size-control can be an important factor in rootstock 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 in 2008 and then grown in a greenhouse and the field.  In 2010, selected greenhouse-grown trees were evaluated for hormones (auxin, abscisic acid, cytokinins, and gibberellins) and associated compounds in roots, stems (below and above the graft union and distal stems), distal leaves, and xylem exudates in ‘Gala’, M.9, and MM.111 that were grafted on M.9 and MM.111 rootstocks.  After two years, heights of trees grafted to growth-controlling rootstocks decreased in the order M.9 greater than M.7 greater than MM.111 in the greenhouse, the reverse trend of rootstock impact on tree height was experienced after two years in the field.  Environmental resource availability likely was responsible for the reversal of rootstock effects on tree height in greenhouse and field.  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 scions 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.  These results indicate that the graft, per se, is not the dwarfing mechanism but the rootstock regulates scion growth differentially for each scion/rootstock combination.  Hormone results were somewhat variable but overall trends were observed.  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 ABAGE (abscisic acid glucose ester) were greater in root, rootstock stem below the graft, scion above the graft, and xylem exudate of M.9 than MM.111.  GA and auxin (IAA) were found in exudate of ‘Gala’ on MM.111 but not M.9.  These results support the hypothesis that hormone signals from rootstocks control tree growth but it is likely that stage of development and environmental resources may also interact to influence growth.  Marker genes associated with hormone metabolism can be developed to assist the selection of rootstocks for size control and, possibly, for tolerance of environmental stress.


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