Chilling and heat requirement in apricot and peach varieties.

The aim of this research was to characterise the phenological behaviour of five peach and five apricot cultivars and to quantify their chilling and heat requirements by evaluating and comparing the applicability of different temperature models in Northwest Italy. Meteorological stations installed in the experimental orchards recorded climatic data during three seasons. Hourly temperature data were measured from the end of summer till full-bloom. Phenological data were determined weekly in the orchard, following Baggiolini stages. The Utah model was used to determine the starting date of chilling units (CU) accumulation. To calculate chilling unit requirements (WCR), several methods were compared (< 7.2°C, 0-7.2°C, Utah, North Carolina). To determine when the chilling requirement was satisfied, ten twigs of each cultivar were collected weekly from November to February and put under controlled conditions. Dormancy was considered broken when 70% of the twigs and 50% of the flower buds on each twig began to flower (Stage F). To assess heat requirements, growing degree hour accumulations (GDH), and growing degree day accumulations (GDD) were calculated using base temperature of 4.4°C. Results show different amounts of CU, GDH and GDD for apricot and peach cultivars. The date of dormancy breaking presented wide variations among varieties; from the end of December to the beginning of February in apricot and from the end of December to middle January in peach. The comparison of the four methods to calculate WCR showed that the < 7.2°C method for apricot, and the Utah and the 0-7.2°C methods for peach were the most reliable. GDH and GDD both seemed to be reliable methods for calculating heat requirements.