Heat requirements for flowering of European and Japanese plum cultivars in the Belgrade region

This study aims to estimate heat requirements for flowering in 35 European and 15 Japanese plum cultivars grown in the Belgrade region. Eight years of phenological and temperature observations (2011-2018) were used to perform the study. The heat requirements were quantified using the growing degree days (GDD). The base temperature (Tb) and starting date of heat accumulation, necessary for GDD calculation, were determined by the CV method (the least coefficient of variation in GDD). For both European and Japanese plum, the least CV was obtained for the Tb of 0°C when taking January 1 and February 1 as starting dates. For European plum, a slightly smaller CV value was obtained for February 1 than for January 1, while the opposite was true for Japanese plum. Cultivars of Japanese plum flowered earlier than cultivars of European plum (9 days on average) and required smaller temperature sums for flowering. The heat requirements for flowering calculated from January 1 for Tb of 0°C ranged from 462 to 548 GDD for cultivars of European plum (501 GDD on average), and from 381 to 428 GDD for cultivars of Japanese plum (401 GDD on average). The average temperature sum for Tb of 0°C accumulated from February 1 was 411 GDD for European plum, ranging from 372 to 458 GDD among cultivars, while for Japanese plum, it ranged between 291 and 338 GDD, with an average value of 311 GDD.