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

Abstract

SummaryOne year old, ca. 500 mm long shoots of ‘Golden Delicious’ and ‘Granny Smith’ apple were selected randomly from commercial orchards in either Elgin (34°S, 305 m, ca. 900 chill units (CU) in 1995) or the Bokkeveld (33°S, 945 m, ca. 1500 CU in 1995) regions of the Western Cape, South Africa. Twenty shoots were harvested weekly from 29 March 1995 until 23 August 1995 and forced at a constant 25°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 that observed in the Bokkeveld. Both cultivars reached maximum dormancy in the Bokkeveld before any considerable chilling accumulated (<100 CU), but in Elgin, ca. 600 CU accumulated before a maximum was attained. In Elgin temperatures that normally promote chill requirement satisfaction enhanced dormancy. The chilling models used in this study were inadequate in considering climatic conditions involved in the entrance to bud dormancy. In the Bokkeveld, temperatures other than those used to calculate CU induced dormancy – possibly freezing temperatures or frost conditions.