ARS | Publication request: Physiological and morphological effects of size-controlling rootstocks on ‘Fuji’ scion

Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 8, 2008
Publication Date: January 4, 2012
Citation: Tworkoski, T., Fazio, G. 2011. Physiological and morphological effects of size-controlling rootstocks on ‘Fuji’ scion. Acta Horticulturae. 903:865-872.

Interpretive Summary: Small, efficient trees are necessary for early and high yielding apple trees in high density plantings. Apple tree size has been successfully controlled by grafting desirable stems (scions) to dwarfing rootstocks but new rootstocks are needed for tree size management under a variety of environmental conditions. To assist in developing new rootstocks, improved knowledge of the dwarfing process is needed. In this experiment ‘Fuji’ scion was used to assess root-produced chemical (hormonal) and water movement (hydraulic) signals and anatomical (size of water-conducting plant cells) changes from a range of size-controlling rootstocks. The objective was to compare hormonal, hydraulic, and anatomical effects of genetically different rootstocks on one type of apple scion. Results support previous work that hydraulic conductivity was reduced and anatomical measurements indicated that diameters of water-conducting cells were smaller in scion grown on dwarfing rootstocks. It is possible that higher levels of a growth-suppressing hormone (abscisic acid) produced in dwarfing rootstocks may play a role in altering scion anatomy which, in turn, may reduce water movement. Alternatively, reduced water movement caused by an undetermined factor may lead to water stress and induction of abscisic acid synthesis. The abscisic acid moving up the stems from size-controlling apple rootstocks may exert a growth inhibitory effect on apple shoots. Improved understanding of dwarfing processes will help develop and select rootstocks that provide trees with targeted sizes and that are adapted to specific environments.

Technical Abstract: The size-controlling effects of apple rootstocks were characterized in greenhouse experiments. One-year-old ‘Fuji’ scions on 25 size-controlling rootstocks, from the USDA apple rootstock breeding program in Geneva, NY, were grown for one season and shoot development was measured each month. Discriminant analysis associated the rootstocks into three clusters based on similarity of shoot length and diameter growth and tree height. Total shoot growth for one season was 367, 283, and 149 cm for rootstock cluster (RC) A, B, and C, respectively. RC-A had significantly greater photosynthesis and transpiration rates than RC-C (18.3 and 12.3 (micro)mol CO(2) / m(-2) leaf area / sec(-1) and 4.2 and 3.1 mmol H(2)O / m(-2) leaf area / sec(-1), respectively). At the end of the first growing season, trees were placed in controlled cold environments to provide chilling. At the beginning of the second growing season, trees were removed from chilling, the roots were pressurized, shoots were removed 100 cm above the graft union, and hydraulic conductivity was measured. Xylem exudate was collected and analyzed for cytokinin, abscisic acid (ABA), and inole-3-acetic acid. Less hydraulic conductance was associated with the more dwarfing, RC-C, compared with the more invigorating, RC-A, rootstocks (0.58 and 1.41 mL / cm(Stem)(-1) / hr(-1)/ MPa(Leaf)(-1), respectively). Abscisic acid flux was higher in xylem exudates from dwarfing RC-C than vigorous RC-A rootstocks (2.28 and 0.23 pmol / Ml(-1) / hr(-1), respectively). The concentrations of cytokinin and indole-3-acetic acid were variable and rootstock-related differences were not determined. Stem samples of scion and rootstock from above and below the graft were collected and anatomical differences of stem cross-sections were analyzed with scanning electron microscopy. Results indicated that vessel diameter was less in ‘Fuji’ scion on RC-C than RC-B and RC-A rootstocks. It is possible that elevated ABA altered xylem development and reduced hydraulic conductance in size-controlling processes of apple rootstocks.