Effects of Gibberellic Acid and Indoleacetic Acid on Shoot Growth and Xylem Differentiation and Development in the Olive, Oha europaea L.1

Abstract Gibberellic acid (GA) applied as an aqueous spray to 4-year-old ‘Ascolano’ olive trees at concentrations of 250 or 500 ppm promoted shoot growth, mainly through internode elongation. Indoleacetic acid (IAA) at the same concentrations caused curling of young leaves and suppressed terminal bud growth for 10 to 14 days. Shoot growth promotion by GA was counteracted by IAA. GA at 100, 250 or 500 ppm significantly promoted xylem differentiation and development in newly developed regions of olive shoots. GA + IAA, both at 250 or 500 ppm, had a synergistic effect. IAA at all concentrations had no effect in this respect. Xylem lignification was normal in all treated shoots. It is suggested that the endogenous gibberellin level was the limiting factor for xylem development and that a certain balance between auxin and gibberellin is required to stimulate cambial activity with subsequent xylem development. Secondary phloem development was not influenced by IAA and/or GA. GA alone or in combination with IAA stimulated precocious periderm formation