Quantification of dormancy progression in terminal apple buds. Changes in growth rate and water stratus.

Abstract

During May 2000, unbranched year-old apple [Malus sylvestris subsp. mitis (Wallr.) Mansf.] shoots of the cultivars Royal Gala, Braeburn, Cripps' Pink and Granny Smith were harvested from bearing orchards in the Koue Bokkeveld, Ceres and Elgin regions of the Western Cape, South Africa, and cold stored at 4-7°C. Every two weeks two replicate bundles were removed and forced at 25°C until budburst had occurred on at least 25% of the shoots per bundle. The change In the rate of budburst over time was calculated for each orchard, and to this response either a linear or a quadratic function was fitted. Non-correlated arbitrarily chosen variables were selected from the functions. Using these variables, the orchards were separated into cluster groupings representing a dormancy pattern. First low and high chill requirement cultivars separated, followed by separation within these groups for region of origin. During dormancy in 2001, terminal buds from the same cultivars were harvested from the same orchards every two weeks and the water potential and water content was measured. A definite chilling influence was observed; water potential and content of buds from the colder Koue Bokkeveld was initially lower and increased with chilling whereas buds from the warmer Elgin showed less change in water potential or content.