EFFECT OF PRUNING OF YOUNG APPLE TREES ON CONTENT OF NUTRIENTS AND GROWTH REGULATORS IN RELATION TO FRUIT BUD FORMATION

Young apple trees of ‘McIntosh’ and ‘Melba’ cultivars, grafted on A2 and M26 rootstocks, were treated with different methods of shoot heading or thinning or left untouched as a control. Dormant shoot heading had the most stimulative effect on shoot growth and reduced the number of spurs and flower buds and, consequently, yield. Shoot thinning and summer shoot heading did not influence the growth, fruit bud formation, or yield as much as shoot heading. At the beginning of the growing season, pruning decreased dry matter content in wood by about 15%, but later in the season the average content of soluble sugars, starch and total carbohydrates was almost equal for treated and control trees. These results indicate that photosynthetic ability tends to adjust to the demand of heavily pruned trees.

The inhibition of flower bud formation can be explained by some hormonal disorders caused by removal of growing points/buds. Our investigations showed that three hormones—cytokinins, auxins and gibberellins—were on a higher level in xylem and phloem tissue of the trunk and branches of heavily pruned trees. Free cytokinin-like substances were the most abundant in conductive tissue at the very beginning of growth. Pruning doubled the level of cytokinins in the spring. High levels of cytokinins in the trunk preceded the extension of new shoots. In mid-June, after a month of growth, auxin-like substances in the pruned trees rose to much higher levels than in controls. The high levels of auxins in shoots and trunk were followed by high levels of gibberellins from middle of June to the end of July. At that time the content of gibberellic acid in the shoots of pruned trees was almost twice as high as in the control.