Chilling and heat requirements for local and foreign almond (Prunus dulcis Mill.) cultivars in a warm Mediterranean location based on 30 years of phenology records

Most temperate fruit and nut trees require fulfillment of chilling and heat requirements during their dormant phase in order to flower regularly and produce economically satisfying yields. Recent and expected temperature increases are cause for concern for many orchard managers, especially in warm growing regions, because they may compromise the trees' ability to fulfill their climatic needs. To explore temperature responses across different cultivars, we applied Partial Least Squares (PLS) regression to correlate bloom dates of 12 local and 25 foreign almond (Prunus dulcis Mill.) cultivars in Sfax, Tunisia with daily chill and heat accumulation based on more than 30 years of phenology records from 1981 to 2014 and long-term daily minimum and maximum temperatures between 1973 and 2016. We used three chilling models (the Chilling Hours, Utah and Dynamic Models) and one forcing model (Growing Degree Hours; GDH) to quantify climatic needs. Chilling and forcing phases derived from the PLS outputs appeared discontinuous for all almond cultivars and were shorter for the local almond cultivars than for the foreign cultivars. The Dynamic Model provided the most precise estimates of chilling requirements but still appeared to have some shortcomings. According to the Chilling Hours Model, chilling needs were very low, but still higher than for the Utah Model, where the negative chill contributions by high temperatures implied negative chilling requirements. The Chilling Hours and Utah Models therefore do not seem suitable for the climate of the Sfax region. For local almond cultivars, chilling requirements were estimated at between 3.4 and 15.5 Chill Portions (CP) and heat needs between 3962 and 8873 GDH. For foreign cultivars, chilling requirements varied from 6.7 to 22.6 CP and heat needs from 2894 to 10,504 GDH. High temperatures during the chilling phase showed a significant bloom-delaying effect on most of the local and the foreign almond cultivars.