Progression of apple (Malus x domestica Borkh.) bud dormancy in two mild winter climates

One year old, ca. 500 mm long shoots of 'Golden Delicious' and 'Granny Smit h' apple were selected randomly from commercial orchards in either Elgin (3 4 degrees S, 305 m, ca. 900 chill units (CU) in 1995) or the Bokkeveld (33 degrees S, 945 m, ca. 1500 CU in 1995) regions of the Western Cape, South A frica. Twenty shoots were harvested weekly from 29 March 1995 until 23 Augu st 1995 and forced at a constant 25 degrees C with continuous illumination until budburst was observed on four out of 20 shoots per bundle, i.e. days to 20% budburst. The progression of bud dormancy in Elgin differed from tha t observed in the Bokkeveld. Both cultivars reached maximum dormancy in the Bokkeveld before any considerable chilling accumulated (<100 CU), but in E lgin, ca. 600 CU accumulated before a maximum was attained. In Elgin temper atures that normally promote chill requirement satisfaction enhanced dorman cy. The chilling models used in this study were inadequate in considering c limatic conditions involved in the entrance to bud dormancy. In the Bokkeve ld, temperatures other than those used to calculate CU induced dormancy - p ossibly freezing temperatures or frost conditions.