Suboptimal winter chilling impedes development of acrotony in apple shoots
In 1997, 1-year-old, unbranched, ≃1-m-long shoots of the apple (Malus x domestica Borkh.) rootstock 'Merton-Immune 793' (MI.793) were selected at random from two commercial stoolbeds in the Western Cape, South Africa, during the dormant period. One site has mild winters [307 Utah Chill Units (CU) in 1997, 34°S, 300 m] while the other is moderately cold (1497 CU in 1997, 33°S, 950 m). In 1998, 'Granny Smith' shoots were collected from a mature orchard in another warm area (574 CU in 1998, 34°S, 116 m). Shoots were prepared and forced at 25°C with continuous illumination. During dormancy the developmental rate was determined 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 terminal bud increased shortly before spring budburst such that a weak acrotonic tendency was established. The shorter dormant period, as experienced with the mild winters common to the apple-growing regions of the Western Cape, impeded the full development of acrotony and subsequent apical control. With less chilling (mild areas) a basitonic tendency remained. Budburst was slower and more erratic, and inhibition by the distal shoot parts was accentuated. Delayed foliation may be due more to correlative inhibition than to endodormancy. When lateral buds are released from paradormancy they exhibit a growth potential similar to, or, with less chilling, even greater than, that of the terminal bud. This permits a greater expression of autonomy between shoots.