Vol.:(0123456789) 1 3 Journal of Plant Growth Regulation https://doi.org/10.1007/s00344-020-10188-z Foliar Paclobutrazol Application Suppresses Olive Tree Growth While Promoting Fruit Set Amani Ajmi 1  · Ajmi Larbi 2  · Melanie Morales 3  · Erola Fenollosa 3  · Anissa Chaari 2  · Sergi Munné‑Bosch 3 Received: 13 January 2020 / Accepted: 1 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract In some agricultural practices, application of plant growth regulators is very useful as an efcient means to modulate olive tree vigour to optimize fruit production. However, the physiological mechanisms underlying these efects are still poorly understood. Here, we evaluated the efectiveness of paclobutrazol, chlormequat and prohexadione on the modulation of vegetative growth and fruit set, with a special emphasis on the underlying efects exerted by phytohormones, in one-year- old olive trees (Olea europea cv. Arbosana). Paclobutrazol and chlormequat were tested using both foliar spraying and soil irrigation, while prohexadione was applied by foliar spraying only, and all treatments were compared to a control. Among treatments, paclobutrazol and chlormequat were the most efective in promoting vegetative growth retardation, when applied through foliar spray. Growth retardation by foliar application of paclobutrazol was associated with proline accumulation and reduced gibberellin contents in leaves. Furthermore, hormonal profling revealed that not only all treatments were efective in reducing the endogenous contents of gibberellins, but also that the entire hormonal profling was altered upon treatments. It is concluded that (i) various plant growth regulators can be efectively used to control olive tree growth, and (ii) paclobutrazol seems to be more efcient on vegetative growth retardation than the other tested treatments, being foliar application more efective than soil treatment. Keywords Fruit production · Fruit set · Hormonal profling · Olea europaea L · Phytohormones · Vigour Abbreviations ABA Abscisic acid CC Chloride of chlormequat GA Gibberellins PBZ Paclobutrazol PGRs Plant growth regulators Pro-ca Prohexadione calcium Introduction The potential productivity of olive trees has increased in the last decade largely due to increases in cultivation surface and planting density. Densities of plantation in modern olive groves range nowadays from 200 to 2000 plants ha −1 (De la Rosa et al. 2007). The main advantages of the high-density planting system are high early yields, the totally mechanized harvesting and low harvest costs, to maximize short-term profts (De la Rosa et al. 2007; Proietti et al. 2012). How- ever, the major long-term problem presented by this planta- tion system lies in controlling tree size, to enable efcient harvesting by the machine and to ensure illumination of the canopy (De la Rosa et al. 2007). The main strategy used to control vigour is based on selection of low-vigour cultivars. Hence, the olive cultivars Arbequina, Koroneiki, Sikitita and Arbosana are recommended for planting in high-density orchards (Dag et al. 2006; De la Rosa et al. 2007; Panelli et al. 1994; Rallo et al. 2008; Tous et al. 2008; Larbi et al. 2011). Dwarfng rootstocks, developed for deciduous spe- cies such as apple and pear (Stern and Doron 2009; Webster 2001), have not yet been fully developed for olive (Pastor Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00344-020-10188-z) contains supplementary material, which is available to authorized users. * Sergi Munné-Bosch smunne@ub.edu 1 Institut Superior Agronomique Chott Mariem, BP 47, 4042 Chott Mériem, Sousse, Tunisia 2 Olive Tree Institute, BP 208, 1082 Tunis, Tunisia 3 Department of Evolutionary Biology, Ecology and Environmental Sciences, Faculty of Biology, University of Barcelona, Avinguda Diagonal 643, 08028 Barcelona, Spain