THE MECHANISM OF FRUITING INHIBITION CAUSED BY PRUNING IN YOUNG APPLE TREES

Four cultivars of young apple trees grafted on Antonovka seedlings and on M7 rootstocks were either severely pruned (shoot heading and thinning) or control (very light shoot thinning). The effect of pruning on growth of shoots, trunk, foliage area, area of branch spread, fruit bud formation, fruit setting and yield was studied. Heavy pruning increased the total and mean length of shoots as well as their number, but reduced trunk thickening, area of branch spread and number of spurs. Foliage area of pruned trees was smaller at the beginning of the season but larger at its end, when compared to the control. Pruning reduced the number of fruit buds, fruit set and yield. The following explanation of fruiting inhibition caused by shoot heading is suggested: Dormant shoot heading disrupts the natural growth correlations between the buds on each shoot treated. As shoot heading reduces the number of actively growing meristems, each meristem is better supplied with growth hormones transported from the roots. The buds that remain on the headed shoots are also better supplied with nutrients because the distance between the site of buds and the tree base is shorter. In this condition, almost all the buds on the headed shoot produced long laterals that were not able to form flower buds. Flower buds already formed also were removed with pruned shoots. Yield reduction was mainly due to fewer flower buds, but also to reduced tree size.