Chilling period influences the progression of bud dormancy more than does chilling temperature in apple and pear shoots

One year old, ca. 50 cm long shoots of 'Doyenne du Comice' pear (Pyrus communis L.) and 'Granny Smith' apple (Malus X domestica Borkh.) were selected randomly in autumn and winter 1999 from commercial orchards in either Elgin (34degreesS, 320 m) or in Somerset West (34degreesS, 80 m), South Africa. Shoots were cold-stored at 1, 4, 7 or 10degreesC for periods of 0, 1, 2 or 3 months after a daily 12/12 h freezing temperature pre-treatment of -1/13degreesC (supposedly non-chilling temperatures) for periods of 0, 1, 2 or 3 weeks. In 2000, 'Granny Smith' apple and 'Packham's Triumph' pear shoots were harvested in autumn from orchards in Elgin and cold-stored, without a freeze treatment, at 1, 4, 7, 10 or 13degreesC for periods of 0, 1, 2, 3 or 4 months. After the different treatments the shoots were forced to budburst at 25degreesC with continuous illumination. To determine the progression of bud dormancy, the rate of budburst, final percentage budburst, and the synchronization of budburst between shoots were used in 1999, but only the rate of budburst in 2000. In all the trials the storage period was the most important factor influencing the progression of dormancy. While in some cases the effects of both storage temperature and the freeze treatment were significant, the contribution to differences in the progression of dormancy was negligible. When our data were fitted to the chilling models currently used in South Africa the difference in temperatures between -1 to 13degreesC was over-emphasized relative to the period of exposure to these chilling temperatures.