Chilling requirements

Winter dormancy allows olive trees to withstand low temperatures in winter and later to bud when chilling requirements are met and when warmer temperatures favourable to growth and development occur. The chilling requirement is the amount of accumulated chill required by a variety to release dormant buds from dormancy. Chilling accumulation is therefore closely related to temperature. A wide range of temperatures, from 7ºC to 15ºC, contributes to chilling accumulation, with the optimum temperature being 12.5ºC. Temperatures exceeding 16ºC do not allow the accumulation of chilling, and higher temperatures can negate accumulated chilling. Empirical or statistical methods can be used to estimate the dormancy period. Recent studies on the chilling requirements of cultivars have proposed a new methodological approach using a leafy explant cutting method under forced growth conditions to better understand the release from dormancy. This method has shown that olive cultivars have different ranges of chilling requirements (Table 1). To meet the challenge of climate change, cultivar adaptation needs to be assessed in experimental trials in different regions. Recent studies have shown that high winter temperatures may prevent the accumulated chilling requirements from being sufficiently met, resulting in asynchronous and prolonged reproductive budburst, flowering and fruit ripening, thus hampering olive cultivation (Figure 3).

Figure 3: Asynchronous flowering due to insufficient chilling causes the presence of different phenological stages within the same tree (from Medina-Alonso et al., 2020).

Table 1: Classification of chilling requirements, flowering  and date of budburst in the World Olive Germplasm Bank of Córdoba (Spain) and Marrakech (Morocco).