Estimation of chilling and heat requirements of six sweet cherry (Prunus avium L.) cultivars

Warmer winters without sufficient chilling due to climate change in Mediterranean regions represent a threat for deciduous fruit tree species such as sweet cherry (Prunus avium L.) with high chilling requirement, since it will affect fruit tree phenology leading to dramatic yield reduction and fruit industry income loss. Quantifying chilling requirements to overcome winter dormancy is crucial for identifying suitable cultivars for a given site, for predicting the necessity and timing applications of rest-breaking chemicals and the possible consequences of climate change. The aim of the following study was to determine the chilling and heat requirements of six sweet cherry cultivars grown in a germplasm collection field located in central Italy. The cultivars analyzed in this study were: ‘Kronio’, ‘Kordia’, ‘Galuciu Precoce’, ‘Ferrovia’, ‘Sandra’ and ‘Ciliegio D’Ottobre’. Historical temperature records and blooming dates were recorded in order to estimate the chilling and heat needs of each cultivar. Temperature-based models, developed by Richardson (1974) and Richardson et al. (1982) for peach, were used to calculate the chilling and heat requirement of the cultivars under investigation; the obtained results were compared with experimental data achieved from a parallel study conducted in controlled environment. The study in controlled environment was started using 2 years old cuttings collected in the abovementioned germplasm collection field. Several cuttings (50 for each cultivar) were artificially chilled in a refrigerated chamber at 6°C. Every 100 chilling hour intervals a sample of cuttings were transferred into a growing chamber at 26°C and flower development was rated weekly. ‘Galuciu Precoce’, ‘Kronio’, ‘Kordia’ and ‘Ciliegio D’Ottobre’ resulted to be the cultivars with low chilling requirements while ‘Ferrovia’ and ‘Sandra’ showed medium-high chilling requirements. Growth chamber experiment showed that, in all cultivars, the need of Growing degree hours (GDH) to achieve bloom was inversely related with chilling units (CU) accumulation. Only the cultivar ‘Ciliegio D’Ottobre’ showed an atypical pattern which required further investigation. From our field calculations, the specific chill accumulation requirements varied among cultivars, showing a minimum chill requirement value for ‘Kronio’ (700 CU) to a high chilling requirement of ‘Ferrovia’ (1200 CU). Field estimated chilling requirement values of each cultivar confirmed the growth chamber experiment results.