The timing and pattern of chill and heat exposure affect the estimation of agroclimatic requirements and adaptability potential in Prunus cultivars

• Variability between years in CR estimates was more marked in the warm sites. • Timing and pattern of chill exposure emerges as crucial as the total amount of chill. • Early winter temperatures appear to be important for an adequate dormancy release. • Insufficient chill accumulation in warm sites resulted in low fruit set. A series of anomalies can occur in temperate fruit trees cultivated in mild winter regions where the fulfilment of chill requirements can be compromised in some seasons. Some symptoms include erratic bud break, bud abortion and ultimately, low fruit set. In this work, we experimentally determined the chill and heat requirements of 14 Prunus spp. cultivars, including apricot, almond and peach, across two locations with varying levels of chill and heat accumulation (cold/mild versus warm climatic conditions) during two successive growing seasons in Murcia (South-Eastern Spain). Chill accumulation varied considerably among the studied sites and seasons. In general, for the same cultivar, chill requirements (CR) were lower and heat requirements (HR) were higher in warm locations compared to cold/mild locations. Moreover, the results indicated that year-to-year fluctuations in both total amount of chill and chill exposure patterns had an impact on the quantification of CR and HR, particularly in warm areas. To understand the effect of the amount and dynamics of chill accumulation in the adaptability of the same cultivar in contrasting environments, we evaluated two climatic adaptation variables: the percentage of flower buds reaching flowers and the percentage of fruit set. Overall, the correlation between the initial number of flower buds and fruit set was notably stronger during the season with higher chill accumulation, especially in warm locations. The most obvious symptoms of poor adaptability caused by the lack of chill were observed during the season on which the accumulation of chill during November and December was particularly scarce. This information suggests that in chill-limited situations, the timing and pattern of chill accumulation rather than the total amount of chill during the season affects dormancy release, flowering and productivity. Finally, the results appear to show a distinct phenotypic plasticity in response to chill exposure among cultivars.