Diverse patterns in dormancy progression of apple buds under variable winter conditions

Abstract

Apple trees that do not receive adequate winter chill show poor bud break, uneven and delayed blooming that impact negatively on tree architecture and fruit production. Previous research indicates that the endodormancy progression of such trees differs from trees grown under adequate winter chill condition. This study aims to produce dormancy progression models for the diverse South African apple growing regions, including areas with inadequate winter chill. One-year-old ‘Granny Smith’ and ‘Royal Gala’ shoots were harvested from 24 farms across South Africa for five consecutive seasons and subjected to standard forcing conditions while monitoring the time to budbreak. A two linear joint line model was fitted producing 11 variables for each farm. Principle component and cluster analysis were used to further interpret the data. The results indicate a very diverse dormancy progression pattern across the South African apple growing regions. Three different dormancy progression clusters were identified. The clusters separate well according to maximum depth of dormancy and rate of dormancy release and could be partially explained by altitude. High laying areas showed a dormancy progression similar to that expected from areas with adequate winter chill and lower laying areas portraying significantly lower levels of endodormancy with protracted release periods. The results also indicated that the Utah chill model is not a reliable indicator of chill accumulation under these climatic conditions. Characterising the South African apple growing regions in terms of dormancy progression is not just a helpful tool for local orchard planning but also contributes to a better understanding of the impact of warmer winter conditions on apple production in general.