Temperature requirements for blooming in 12 sweet cherry cultivars


Sweet cherry flowering occurs once a year in spring. The flowering date depends on temperatures during the previous months: accumulation of winter chill enhances the dormancy-breaking process in flower buds, and subsequent heat promotes the resumption of growth. The effect of temperature on the phenology of temperate fruit trees has received particular attention in recent years, driven by the unclear but potentially consequential impacts of global warming. To a considerable extent, our ability to cope with these impacts depends on accurate determination of cultivarspecific temperature requirements. We used statistical procedures to establish the temperature requirements of 12 sweet cherry cultivars (‘Brooks’, ‘Cristobalina’, ‘Fercer’, ‘Ferrovia’, ‘Hartland’, ‘Margit’, ‘Primi Giant’, ‘Primulat’, ‘Royalton’, ‘Shato Nishiki’, ‘Starking Hardy Giant’, and ‘Sonata’), for most of which (10 out of 12) no such information was previously available. We used partial least squares (PLS) regression to correlate a 12-year record of flowering dates with daily temperature accumulation during the previous months at the mid-Ebro valley (Zaragoza, Spain). We used a PLS analysis with the Dynamic model to establish the chilling period, and another PLS analysis with the forcing model Growing degree hours, to determine the forcing period. Chilling requirements were quantified during the established chilling periods with the three most common models: the Chilling hours model, the Utah model, and the Dynamic model. The chilling model results showed characteristic patterns of PLS coefficients, which indicated a discontinuous chilling period. For the forcing model, the analysis indicated a consistent period during which heat strongly determined flowering dates. Estimates ranged from 567±106 to 893±99 chilling hours, from 694±100 to 1229±126 chilling units, and from 42.1±3.9 to 59.9±4.8 Chill Portions, while heat requirements ranged between 3653±684 and 6993±1281 growing degree hours.