Citation: Inês, C.; Gomez-Jimenez, M.C.; Cordeiro, A.M. Inflorescence Emergence and Flowering Response of Olive Cultivars Grown in Olive Reference Collection of Portugal (ORCP). Plants 2023, 12, 2086. https://doi.org/10.3390/ plants12112086 Academic Editors: Giora Ben-Ari and Georgios Koubouris Received: 7 May 2023 Revised: 19 May 2023 Accepted: 22 May 2023 Published: 24 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Inflorescence Emergence and Flowering Response of Olive Cultivars Grown in Olive Reference Collection of Portugal (ORCP) Carla Inês 1, * , Maria C. Gomez-Jimenez 2 and António M. Cordeiro 1 1 Instituto Nacional de Investigação Agrária e Veterinária,I.P. (INIAV), UEIS Biotecnologia e Recursos Genéticos, Estrada de Gil Vaz—Apartado 6, 7350-404 Elvas, Portugal; antonio.cordeiro@iniav.pt 2 Plant Physiology, Faculty of Science, University of Extremadura, 06006 Badajoz, Spain; mcgomez@unex.es * Correspondence: carla.ines@iniav.pt Abstract: In olive trees, fluctuations in the onset of phenological stages have been reported due to weather conditions. The present study analyses the reproductive phenology of 17 olive cultivars grown in Elvas (Portugal) in 3 consecutive years (2012–2014). Through 2017–2022, the phenological observations continued with four cultivars. The phenological observations followed the BBCH scale. Over the course of the observations, the bud burst (stage 51) occurred gradually later; a few cultivars did not follow this trend in 2013. The flower cluster totally expanded phase (stage 55) was achieved gradually earlier, and the period between stages 51–55 was shortened, especially in 2014. Date of bud burst showed a negative correlation with minimum temperature (Tmin) of November–December, and, in ‘Arbequina’ and ‘Cobrançosa’, the interval stage 51–55 showed a negative correlation with both the Tmin of February and the Tmax of April, whereas in ‘Galega Vulgar’ and ‘Picual’ there was instead a positive correlation with the Tmin of March. These two seemed to be more responsive to early warm weather, whereas ‘Arbequina’ and ‘Cobrançosa’ were less sensitive. This investigation revealed that olive cultivars behaved differently under the same environmental conditions and, in some genotypes, the ecodormancy release may be linked to endogenous factors in a stronger way. Keywords: phenology; olive cultivars; climate change; (endo)dormancy; ecodormancy; reproductive cycle; cold stress; Olea europaea L. 1. Introduction Olive orchards are a key component of agricultural systems of the Mediterranean basin, and since the end of the last century there has been considerable expansion of olive production at low latitudes [1,2]. Olea europaea L. is an evergreen tree species and its vegetative structures cease their growth in autumn and undergo a winter rest period lasting until favorable temperature conditions return in late winter–early spring [3,4]. Olive reproductive structures are axillary buds of the leaves of the previous year’s shoots [5]. Axillary buds are developed during the growing season (spring–autumn) and they last in a dormant state until reproductive bud burst in the next spring [3,4]. In the Northern Hemisphere, inflorescence (and flower) external development takes place in early spring and the release of floral buds from dormancy occurs whenever trees have been exposed to a long enough period of chilling temperatures [6,7]. During chilling accumulation, reversal of the cold already experienced might occur due to warm temperatures [8]. After bud break, the time to flowering decreases as temperatures increase, mainly maximum temperature [9,10]. When flowering begins, high temperatures shorten the process [11] and the tree quickly goes through the beginning of flowering to the first petals fallen stage and the end of flowering. Dormancy is a physiologic state that plants enter to protect their buds from winter har- diness. Environmentally induced dormancy onset and release coordinate growth cessation Plants 2023, 12, 2086. https://doi.org/10.3390/plants12112086 https://www.mdpi.com/journal/plants