Foliar Paclobutrazol Application Suppresses Olive Tree Growth While Promoting Fruit Set

In some agricultural practices, application of plant growth regulators is very useful as an efficient means to modulate olive tree vigour to optimize fruit production. However, the physiological mechanisms underlying these effects are still poorly understood. Here, we evaluated the effectiveness of paclobutrazol, chlormequat and prohexadione on the modulation of vegetative growth and fruit set, with a special emphasis on the underlying effects 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 effective 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 profiling revealed that not only all treatments were effective in reducing the endogenous contents of gibberellins, but also that the entire hormonal profiling was altered upon treatments. It is concluded that (i) various plant growth regulators can be effectively used to control olive tree growth, and (ii) paclobutrazol seems to be more efficient on vegetative growth retardation than the other tested treatments, being foliar application more effective than soil treatment.