Freezing temperature treatment induces bud dormancy in 'Granny Smith' apple shoots

One-year-old, 'Granny Smith' apple (Malus × domestica Borkh.) shoots were selected randomly from commercial orchards in 1999 and 2003, cold stored at temperatures between 1 and 13 °C for varied periods, following a 12/12 h freezing temperature pre-treatment of −1/13 °C (supposedly non-chilling temperatures) for 1 or 2 weeks. The rate of budburst, determined after forcing at 25 °C, was used to follow the progression of bud dormancy. The freezing pre-treatment clearly enhanced (deepened) bud dormancy in all experiments. Clearly definable influences due to the presence of leaves during the pre-treatment were not observed on shoots cold stored at chilling temperatures. Trees were defoliated before leaf drop in late summer/autumn under field conditions by hand (2001) and using a spray containing 3% urea and 1.8% zinc sulphate in 2003. In 2001 the first two hand defoliation dates significantly enhanced dormancy, while in 2003 chemical defoliation had no effect. These data indicate that the leaves are not clearly involved in the perception of factors that are responsible for dormancy induction. As with dormancy release it is possible that the perception of induction factors, i.e. low temperatures, and as shown in these data, freezing temperatures, occurs within the buds themselves.