Chilling and heat requirements for flowering in apricot cultivars

Abstract

Chilling requirements for breaking dormancy and heat requirements for flowering were studied in 10 apricot cultivars in the region of Belgrade during a 2-year period. Chilling requirements were determined using the Utah chill unit (CU) model. To determine the date of dormancy breaking, 1-year-old shoots were collected weekly from December to February and kept under controlled conditions. Heat requirements were calculated as growing degree hours (GDH) accumulated from the breaking of dormancy to the beginning of flowering using a base temperature of 4 degrees C. The cultivars studied showed a range of chilling requirements, from 834 CU in 'Ninfa' and 'Goldrich' to 1442 CU in 'Re Umberto'. The heat requirements for flowering ranged between 2624 GDH in 'Magyar kajszi' and 3469 GDH in 'Tomcot'. The latest flowering cultivar ('Re Umberto') finished dormancy 42 days after the earliest flowering cultivar ('Ninfa'). The results indicate that flowering time of apricot in the studied region is influenced more by chilling requirements than by heat requirements. Identification and selection of cultivars with higher chilling requirements and later flowering is important for apricot production, especially in regions with the frequent occurrence of late spring frosts.