Title: Endogenous Hormone Concentrations and Bud Break Response to Exogenous BA in Shoots of Apple Trees with Two Growth Habits Grown on Three Rootstocks Authors
Submitted to: Annual Cumberland Shenandoah Fruit Workers Conference
Publication Type: Proceedings
Publication Acceptance Date: November 21, 2006
Publication Date: March 1, 2007
Citation: Tworkoski, T., Miller, S.S. 2007. Endogenous Hormone Concentrations and Bud Break Response to Exogenous BA in Shoots of Apple Trees with Two Growth Habits Grown on Three Rootstocks. Annual Cumberland Shenandoah Fruit Workers Conference. P. 114. Technical Abstract: Apple tree size and shape are strongly affected by branch development. We evaluated potential hormone influence on branch development in two, genetically-distinct apple tree growth forms. Scion of two apple (Malus xdomestica) siblings were budded to three size-controlling rootstocks (M.9, M.7, and Malus antanovka) and planted in the field in1997. The scion had two contrasting growth habits; one with narrow crotch angles, few branches, and an upright narrow canopy (UN) and the other with wide crotch angles, numerous branches, and a spreading round canopy (SR). Shoot tips were collected at time of bud break in April 2004 and analyzed for indole-3-acetic acid (IAA), cytokinins (CK), and abscisic acid (ABA). The UN growth habit had higher IAA, lower ABA, and equivalent CK as the SR growth habit. The synthetic cytokinin, 6-benzyl adenine (BA), was applied to 30 cm shoot explants of both growth habits in a greenhouse in March 2006. An 8.7 mM concentration of BA stimulated bud break in both growth habits, compared with controls, and bud break was increased more in UN than SR growth habits. It is possible that the higher auxin-to-cytokinin ratio contributed to the upright, narrow growth form. Either growth form grown on invigorating rootstock had nearly twice the auxin-to-cytokinin ratio than on dwarf or semi-dwarf rootstocks. The difference in endogenous hormone concentrations may explain bud break response to exogenous BA treatments. When applications of exogenous hormones are used to modify bud break or canopy structure, endogenous hormone concentrations may affect efficacy and outcome of such treatments. Improved knowledge of how such messages interact can benefit efforts to grow fruit trees to specifications needed for efficient orchard management.