Suboptimal winter chilling impedes development of acrotony in apple shoots

In 1997, 1-year-old, unbranched, approximate to 1-m-long shoots of the appl e (Malus x domestica Borkh.) rootstock 'Merton-Immune 793' (MI.793) were se lected at random from two commercial stoolbeds in the Western Cape, South A frica, during the dormant period. One site has mild winters [307 Utah Chill Units (CU) in 1997, 34 degrees S, 300 m] white the other is moderately col d (1497 CU in 1997, 33 degrees S, 950 m). In 1998, 'Granny Smith' shoots we re collected from a mature orchard in another warm area (574 CU in 1998, 34 degrees S, 116 m). Shoots were prepared and forced at 25 degrees C with co ntinuous illumination. During dormancy the developmental rate was determine d of the terminal bud, and of both distally and proximally situated lateral buds, with or without the inhibitory influence of a distal disbudded shoot piece (10 cm long). In the moderately cold area, the growth rate of the te rminal bud increased shortly before spring budburst such that a weak acroto nic tendency was established. The shorter dormant period, as experienced wi th the mild winters common to the apple-growing regions of the Western Cape , impeded the full development of acrotony and subsequent apical control. W ith less chilling (mild areas) a basitonic tendency remained. Budhurst was slower and more erratic, and inhibition by the distal shoot parts was accen tuated. Delayed foliation may be due more to correlative inhibition than to endodormancy. When lateral buds are released from paradormancy they exhibi t a growth potential similar to, or, with less chilling, even greater than, that of the terminal bud. This permits a greater expression of autonomy be tween shoots.