Root pruning of apple trees grown at ultra-high density affects carbohydrate reserves distribution in vegetative and reproductive growth

Five-year-old ‘Braeburn’, ‘Royal Gala’, ‘Oregon Red Delicious’, ‘Splendour’, ‘Granny Smith’, and ‘Fuji'/MM. 106 apple (Malus domestica Borkh.) trees growing under an ultra-high density planting system (20 000 trees/ha) were root pruned in September 1994. Trees were pruned to a depth of 30 cm with a sharp spade along both sides of the row and between trees, 20 and 30 cm from the trunk. Root pruning resulted in decreased tree height, shoot length, branch number, trunk and shoot diameter compared to the control. Root pruning increased floriferousness e.g., the number of flowering spurs in subsequent season. Total carbohydrates (TCHO) and TCHO concentration in tissues of the trees were altered by root pruning treatments. Elevated shoot TCHO concentration was associated with higher flower initiation (flowering spurs per tree in the following season) whereas higher root TCHO was associated with higher total yield. The mean value for root reserves in August 1995 for the control trees was 111 g/plant and 84 g/plant for the 20 cm root-pruned trees. Fruit yields were 4.1 and 2.6 t/ha respectively. Total yield and average fruit size were reduced by root pruning. The data suggest that root pruning created a lack of balance between reproductive characters and potential yield. This may be explained by reduced shoot leaf area (fewer bourse and shoot leaves) limiting carbohydrate supply and the effects of root pruning on TCHO, yield, and flowering are consistent with the predictions of an earlier model. © 1998 Taylor & Francis Group, LLC.