Statistical Approach to Assess Chill and Heat Requirements of Olive Tree Based on Flowering Date and Temperatures Data: Towards Selection of Adapted Cultivars to Global Warming

Delineating chilling and forcing periods is one of the challenging topics to understand how temperatures drive the timing of budburst and bloom in fruit tree species, especially when flowering data are limited to a short-term phenology data. Here, we investigated this question on olive trees, using flowering data collected over six years on 331 cultivars cultivated in the worldwide collection of Marrakech, Morocco. Using Partial Least Squares (PLS) approach on a long-term phenology (29 years) of Picholine Marocaine cultivar, we showed that the relevance of delineating chilling and forcing periods depends more on the variability of inter-annual temperatures than on the long-term datasets. In fact, chilling and forcing periods are similar between those delineated using datasets of 29 years and those of only six years (2014-2019). Furthermore, we demonstrated that the variability of inter-annual temperatures is the main factor explaining this pattern. We, then, used datasets of six years to assess chill and heat requirements, of 285 cultivars. We classified Mediterranean olive cultivars into four groups according to their chill requirements. Our results, using the kriging interpolation method, indicated that flowering dates of most of these cultivars (92%) were governed by both chilling and forcing temperatures.