Comparison between two methods of estimation of chilling and heat requirements for flowering in almond

A precise determination of thermic requirements (chilling and heat) for breaking dormancy is quite difficult under field conditions. However, quantification under laboratory conditions should be considered with caution and it is costly and time consuming when analyzing a large number of genotypes. For this reason, statistical models based on the analysis of historical blooming dates have been developed to fit the responses of tree species to local weather conditions. Chilling and heat requirements for breaking dormancy and flowering were studied in five local almond cultivars with a widespread time of full bloom (‘Pizzuta d’Avola’, the earliest, ‘Tribuzio’, ‘Tuono’, ‘Cristomorto’ and ‘Rana Gentile’, the latest) in Apulia region (southern Italy). The chilling portions (CP), determined by dynamic model, and the growing degree hours by Richardson model (GDH) were used to determine chilling and heat accumulations, respectively. Using both full bloom dates and temperatures of previous seasons for nine years, two methods for estimating thermic requirements were compared: the Ashcroft method (AM), and a new version (Ashcroft method modified, AMM) that takes into account in a higher extent the GDH accumulation in comparison with AM, and allows selecting the optimal requirements when alternative choices are possible. The two methods generally underestimated the blooming date, but with a difference from the real one within 5 days for most of the cases studied. AMM resulted more effective than the original method in Apulian semi-arid conditions. The estimation of thermic requirements using temperature data and full bloom dates seemed to be useful, easy and not too expensive to be implemented for a wide number of cultivars.