Chill and heat requirements of Japanese plum-type cultivars growing at two areas in Spain

Japanese plum, like other temperate fruit trees, needs to accumulate chill during endodormancy and heat during ecodormancy for proper flowering and subsequent fruit set. These requirements are cultivar-specific and provide information about the possible adaptation of each cultivar to particular areas for appropriate agronomic behavior. However, climatic requirements have been poorly studied in Japanese plum. The objective of this work was to determine the chill and heat requirements for endodormancy breaking and flowering in eight Japanese plum-type cultivars in two plum-growing areas: Badajoz (southwest Spain) and Zaragoza (northeast Spain) over two years. Endodormancy release was determined by monitoring branches collected weekly during the winter and transferred to growth chambers with warmer temperatures. The date of chill fulfillment was established when the weight of flower buds increased by at least 30% after 8 days in the growth chamber. The accumulation of chill until this date was calculated according to three models: chill hours (CH), chill units (CU), and chill portions (CP). Heat requirements were calculated as growing degree hours (GDH) accumulated from the date of chilling fulfillment to the date when 50% of flowers were opened (F50). The fulfilment of chill requirements for the endodormancy release was quicker in Zaragoza than in Badajoz. The chill requirements ranged from 359 to 557 CH, 601 to 792 CU, and 33.9 to 43.1 CP, with some differences between locations. The heat requirements ranged from 6,130 to 7,635 GDH. Knowledge of the temperature requirements of cultivars in particular areas is of increasing interest to orchard design due to the winter chilling decrease caused by global warming in many areas.