Effect of Exogenous Gibberellic Acid Application On King Dominance and Endogenous Gibberellic Acid Level of ‘Granny Smith' Apple

Generally, gibberellic acid (GA) is not considered a primary hormone in the control of fruitlet abscission driven by king dominance. However, the increase in the amount of indol-acetic acid (IAA) exported from fruit after the application of GA, suggests that GA could function in the king dominance as a modulator for synthesis and polar IAA transport. The aim of this study was to find out how exogenously applied GA₃+GA₄ affects fruitlet abscission and king dominance related to the endogenous GA level of the seeds. Treatments were done at different times (5, 15, 25 and 35 days after full bloom - dAFB) and at their combinations. Exogenous GA₃+GA₄ application significantly reduced the rate of fruitlet abscission especially for lateral fruits by multiple treatments. King fruit abscission did not differ between GA treatments applied to the whole cluster or only to the king fruit. However, the rate of abscission of lateral fruitlets was significantly reduced by the spraying of the lateral fruits only. Even though 5 and/or 15 dAFB treatments were effective in lateral fruitlet retention this effectiveness lasted until the end of June only. These fruits were subject of June drop. Spraying at four times was the most effective way in preventing of fruit abscission. Seed numbers of king and lateral fruits were not significantly different from each other, and were not influenced by GAs treatments either. Increasing differences between king and lateral fruits in fruit quality parameters (fruit weight and diameter) were observed with multiple application times compared to single treatments. Differences, however, were not found for L:D ratio, skin color, soluble solids content and firmness. GA₃ content of seeds was affected by treatments; the greatest amount was measured 10 days after treatments, then GA content slightly decreased possibly due to the breakdown or transformation into another form of the exogenously applied GAs.