Submitted to: Journal of Horticultural Science and Biotechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/2/2007
Publication Date: 11/12/2007
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. Journal of Horticultural Science and Biotechnology. 82(6):960-966. Interpretive Summary: Orchard management and yield are strongly affected by the size, shape, and branch patterns of apple trees. Biological processes that are responsible for apple tree form are not well understood and improved knowledge can lead to increased orchard efficiency. Hormones were measured in trees with two genetically-distinct apple tree growth forms (upright, narrow and spreading, round) that were grown on three size-controlling rootstocks (dwarf, semi-dwarf, and seedling). Branching patterns were modified by supplementing hormones with applications of the synthetic plant growth regulator, benzyl adenine. Synthetic benzyl adenine stimulated bud break in both growth forms and bud break was increased more in upright, narrow than spreading, round growth forms. The results indicate that the auxin-to-cytokinin hormone ratio regulates bud break and branch development of different apple tree growth forms and that rootstock modified concentrations of these hormones. These results provide new information for pomologists studying apple tree training and management systems and can assist plant breeders in developing adaptable tree forms.
Technical Abstract: Scion from two siblings of a ‘Goldspur Delicious’ x ‘Redspur Delicious’ cross were budded to three size-controlling rootstocks [M.9, M.7, and Malus domestica ‘antanovka’ seedling] and planted in the field in 1997. The scion had two contrasting growth habits; one with narrow crotch angles, numerous short branches and an upright narrow canopy (UN) and the other with wide crotch angles, few short branches, and a spreading round canopy (SR). Shoot tips were collected at time of bud break in April 2004 and analyzed for auxin (AUX), cytokinins (CK), and abscisic acid (ABA) to determine relationships between scion growth habit, size-controlling rootstock, and shoot tip hormone concentrations. The UN growth habit had higher AUX, lower ABA, and equivalent CK as the SR growth habit. It is possible that the higher AUX-to-CK ratio (ACR) contributed to the UN growth habit which had more antigravitrophic shoot growth and appeared to have greater apical dominance than the SR growth habit. Either growth habit grown on seedling rootstock had nearly twice the ACR than on M.7 or M.9 rootstocks. The synthetic CK, 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 BA concentration stimulated bud break in both growth habits, compared with controls, and bud break was increased more in UN than SR growth habits. The results indicate that the ACR may be a factor regulating bud break and development of growth habit of apple scions and that rootstock modified the hormone concentrations in shoot tips.