Chilling and heat requirements of apricot cultivars for flowering

Chilling requirements for breaking of dormancy and heat requirements for flowering were studied for 3 successive years in 10 apricot cultivars which spanned the range of flowering times in this species. Different methods for estimating chilling requirements were evaluated and compared, and correlations between chilling requirements, heat requirements and flowering date were established. The cultivars examined showed a range of chilling requirements (chill units, CU), between 596 CU (Currot) and 1266 CU (Orange Red), though most of them showed chilling requirements between 800 and 1200 CU. The results obtained in different years by the Utah and Dynamic models were more homogeneous with respect to the hours below 7 °C model. The heat requirements for flowering ranged between 4078 and 5879 growing degree hours (GDH). The apricot cultivars showed important differences concerning flowering date, and the results indicate a high positive correlation between chilling requirements and flowering date, as well as a negative correlation between chilling requirements for breaking of dormancy and heat requirements for flowering.