AEROBIOLOGIA 8 (1992),415 -418 The role of air temperature in determining release and flowering of Corylus avellana L. dormancy GIUSEPPE FRENGUELLI, EMMA BRICCHI, BRUNO ROMANO, GIANFRANCO MINCIGRUCCI FRANCESCO FERRANTI, EVASIO ANTOGNOZZI SUMMARY. 7 years of airborne pollen monitoring in Perugia (central Italy) were used to determine the temperature requirements to break dormancy and to resume growth and bloom of Corylus avellana L.. Corylus needs 1000 chill-units to complete its dormancy and this vahte, in the Perugian area, is met by the end of December or the first days of January. Moreover Corylus trees require 220 growth degree hours before they are able to flower. If air temperature is high, this value can be achieved in only 10 days, but if the tem- perature remains too low, the heat accumulation can require up to 35 days. With these parameters it is possible to buiM a model to predict the date of the beginning ofCorylus avellana pollen season. Key words: Corylus, dormancy, forecast. Giuseppe Frenguelli, Emma Bricchi, Bruno Romano, Gianfranco Minci- grucci, Francesco Ferranti, Dep. Plant Biology, University of Perugia, 1-06100 Perugia, Italy. Evasio Antognozzi, Agronomic Institute of Tree Crops, University of Pe- rugia, 1-06100 Perugia, Italy. INTRODUCTION Most woody plants, both coniferous and di- cotyledonous species, show, during their annual cycle, an active growth in spring and summer, and a dormancy phase in autumn and winter, together with the development of resting buds (Wareing and Phillips, 1978). When seasonal growth ceases in late summer, the plants enter a reversible phase of predormancy, during which the range of conditions under which growth oc- curs narrows. In autumn, trees enter a true dor- mant phase and growth cannot occur even under favourable conditions. At the end of this period, a transition to postdormancy occurs and the range of environmental conditions under which the plants resume growth widens (Kramer and Koziowski, 1979). One of the most important factors determin- ing the induction of dormancy in many woody plants is day lenght. In most species, long days promote vegetative growth, and a shortening of day lenght, in late summer, brings the cessation of active growth, and resting buds are formed. Bud dormancy involves an interaction between the influence of substances inhibiting growth, 8 (1992) 4]5