APPLE PRODUCTION UNDER CONDITIONS OF SUB-OPTIMAL WINTER CHILLING IN SOUTH AFRICA

Abstract

Apples are widely accepted to need ca. 1000-1200 Utah model chill units to break dormancy, yet in South Africa apples are often produced successfully with 500 chill units. Numerous chilling models are used locally to quantify winter chilling. These models are discussed in light of their shortcomings under low chill conditions. Branching in spring on year-old shoots is acrotonic (from the most distal buds). Dominance of these distal buds develops slowly during winter and is clearly defined before budburst in spring. It is the lack of an extended dormant period associated with the mild local winters that impedes the development of acrotony. This is due to an inherent low growth rate of the buds (increased endodormancy), and less synchronization among buds at the time of spring budburst, resulting in both a delayed and more erratic budburst. Furthermore, a considerably increased paradormant inhibition by the distal shoot parts is observed. Paradormancy accounts largely for the inhibition of budburst of the upper lateral buds under conditions of sub-optimal chilling. The causes of delayed foliation may reside more in the shoot piece than in the buds themselves. Cytokinins are known to release buds from dormancy, especially when applied in late winter. Also, under normal winter conditions, a flush of endogenous cytokinins is observed shortly before budburst. This cytokinin flush is triggered by the application of rest-breaking agents. The predominant system currently used is a tree with a single central leader and fruiting branches directly situated on the leader. The tree is similar to the French Solaxe, but is unique in the sense that its evolution is the result of an improved understanding of the reaction of the apple tree to a lack of winter chilling, hot growing conditions and poor soils. Finally, other practical manipulations of branching on both young and mature trees are discussed.