Responses of shoot growth, return flowering, and fruit yield to post-pruning practices and growth regulator application in olive trees

Abstract

Winter mechanical hedge pruning of olive trees can result in a strong post-pruning shoot growth response and a low return flowering intensity the following year. However, post-pruning practices including tip heading, watersprout thinning, and the application of the growth inhibitor mepiquat chloride (MC) may positively reduce subsequent vegetative growth and promote flowering. Two experiments were carried out with the aim of addressing the following questions: (1) Do these post-pruning practices applied on trees that received simulated mechanical pruning during the winter reduce growth and increase flowering in the spring of the following year?; and (2) What is the optimal MC dose in unpruned trees needed to increase flowering the following spring? In the first experiment, five-year-old olive trees (cv. Arbequina) growing in a fairly low density orchard (208 trees ha−1) were hedge pruned at the end of winter using manual clippers on the east and west sides to simulate mechanical disk pruning, and the post-pruning treatments were applied at the pit hardening stage (early summer). In the second experiment, foliar or soil application of MC was performed at pit hardening in unpruned trees. The results show that tip heading of new shoots led to a large number of lateral shoots that flowered little, or not at all, the following spring. In contrast, the watersprout thinning treatment had sufficient flowering to significantly increase fruit number and yield the following year compared to tip heading. Foliar application of MC (1500 ppm) after winter pruning did not inhibit new shoot growth, return flowering was low, and yield was less than the watersprout thinning treatment. The lack of a post-pruning response to foliar MC was likely related to the dose used (1500 ppm). In the second experiment, a foliar application of MC at 3000 ppm was associated with greater return flowering and yield than an untreated control. In conclusion, thinning of vigorous watersprouts at the pit hardening stage three months after simulated winter mechanical hedge pruning can improve fruit number and yield the next year, especially on a per canopy volume basis. Furthermore, the responses to foliar MC application in unpruned trees suggest that more detailed post-pruning studies with MC application doses in the 2000–3000 ppm range and at additional phenological stages should be performed over several consecutive years to assess whether this practice could be a suitable management tool.