|Authors: ||T. Tworkoski, G. Fazio|
|Keywords: ||Malus × domestica, abscisic acid, xylem conductance, xylem vessel elements|
The size-controlling effects of apple (Malus × domestica) 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 μmol CO2•m-2•s-1 and 4.2 and 3.1 mmol H2O•m-2•s-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 progressively pressurized up to 1.5 MPa in a pressure chamber, shoots were removed 100 cm above the graft union, and the hydraulic conductivity of roots was measured.
Xylem exudate was collected and analyzed for cytokinin, abscisic acid (ABA), and inole-3-acetic acid.
Lower hydraulic conductance was associated with the more dwarfing, RC-C, compared with the more invigorating RC-A rootstocks (0.58 and 1.41 ml•cmStem-1•
h-1•MPaLeaf-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•h-1, respectively). The concentrations of cytokinin and indole-3-acetic acid were variable but 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 using 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.
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