Endogenous hormone concentrations and bud-break response to exogenous benzyl adenine in shoots of apple trees with two growth habits grown on three rootstocks

Scions from two siblings of a 'Goldspur Delicious' X 'Redspur Delicious' cross were budded to three rootstocks with different levels of vigour [M.9, M.7, and Malus domesticaAntanovka' seedling] and planted in the field in 1997. The scions had two contrasting growth habits: one with narrow crotch angles, numerous short branches and an upright narrow (UN) canopy; and the other with wide crotch angles, few short branches, and a spreading round (SR) canopy. Shoot tips were collected at the time of bud-break in April 2004 and analysed for auxin (AUX), cytokinins (CK), and abscisic acid (ABA) to determine relationships between scion growth habit, size-controlling rootstock, and shoot tip hormone concentrations. Although not statistically different, the UN growth habit had numerically higher AUX, lower ABA, and equivalent CK levels as the SR growth habit. These differences resulted in statistically higher AUX:CK ratios (ACR). It is possible that the higher ACR contributed to the UN growth habit, which had more anti-gravitrophic 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 the UN than the SR growth habit. The results indicate that the ACR may be a factor regulating bud-break and the development of growth habit in apple scions, and that rootstock modified the hormone concentrations in shoot tips.