Chilling, thermal time and metabolic changes in five apricot cultivars

A field study on chilling unit requirement (WCR), growing degree hours accumulation (GDH), and growing degree days accumulation (GDD) of 5 commercial apricot cultivars, was carried out in a fruit area in North-west of Italy. Since autumn 1999 to spring 2001, climatic data were recorded by meteorological stations installed in the experimental orchards. Hourly temperature data were measured since the end of summer till full-bloom. Phenological data were weekly determined in the orchard, following Fleckinger stages (INRA). Ten twigs of each variety were weekly collected since November to February and put under controlled conditions, in order to determine the date of endodormancy breaking. To calculate WCR some methods were compared (Hutchins, Weinberger-Eggert, Utah, North Carolina). GDH and GDD were calculated using the base temperature of 4.4°C. In addition ten vegetative buds of three varieties ('San Castrese', 'Tonda di Costigliole' and 'Laycot') were weekly collected and used to observe morphological, histochemical and histoenzymatic changes using an histological method pointed out to study the physiological mechanisms involved in dormancy and bud breaking. Results show different amounts of chilling units and growing degree for the five cultivars; the histological method is effective to put in evidence variations of cell components, as different amount of starch and different state of nucleus activity that could be correlated with the evolution of vegetative buds from completely dormant to metabolically active.