Effect of Fruit Thinning on Endogenous Plant Hormones and Bearing of the Olive (Olea europaea L.) ‘Nabali Muhasan’

Fifteen-year-old ‘Nabali Muhasan’ olive trees in their “On” year were thinned with 100 ppm Naphthalene Acetic Acid (NAA), 24 days after full bloom. Changes in Abscisic Acid (ABA), Indoleacetic Acid (IAA), Gibberellins (GAs) and cytokinins were determined from September until the following April in the lateral buds of the thinned and the un-thinned trees using High Performance Liquid Chromatography (HPLC). The results showed that free ABA concentration in the thinned trees was significantly higher than that in the un-thinned ones. A significant increase in IAA concentration in the thinned trees was recorded during October and February. However, IAA concentration in the thinned and the un-thinned trees throughout the sampling dates was almost the same. With the exception of December and February, free GAs concentration in the thinned trees was lower than that in the un-thinned ones. Cytokinins concentration in the thinned trees was significantly less than that in the un-thinned ones, however, the concentration increased sharply in the thinned trees during March. The thinned trees produced longer shoots, larger fruit size, higher yield and reduced number of retained fruits as compared to the un-thinned trees. In the following season, however, the thinned trees had a higher number of perfect flowers, fruits per shoots and return bloom, while it had a less number of staminate flowers per inflorescence as compared to the un-thinned trees. Thus, it seems that fruit thinning of the ‘Nabali Muhasan’ olive might regulate tree bearing and put control to some extent on alternate bearing by the alteration of endogenous plant hormones