Abstract: The aim of the present study was to compare the accuracy and reproducibility of six statistical models for the calculation of olive (Olea europeae L.) heat requirements to trigger the onset of flowering in three Portuguese regions: Reguengos de Monsaraz, Valença do Douro, and Braga. Other aims were to ascertain the date on which the heat-accumulation period started and the threshold temperatures above which the development of reproductive structures starts in olives. The starting and peak dates for the regional O. europeae flowering season were estimated by monitoring airborne pollen from 1998 to 2004 using “Cour”-type samplers. The threshold temperature values calculated for the three regions were very similar (9.0 °C for Valença do Douro, 9.2 °C for Reguengos de Monsaraz, and 9.7 °C for Braga). The accumulated daily mean temperature model had less interannual and inter-regional variation, showing best predictive results for 2004, with absolute differences between the observed and predicted dates of 4 d in Reguengos de Monsaraz and 2 d in Valença do Douro and Braga for the onset of flowering date and of 2 d in Reguengos de Monsaraz, 7 d in Valença do Douro, and 4 d in Braga for peak flowering dates. This model was the most accurate, reproducible, and operational to calculate heat requirements for olives to flower, with an average mean temperature accumulation of 1 446 °C in Reguengos, 1 642 °C in Valença do Douro, and 1 703 °C in Braga to reach the onset of flowering. The best initial date for this accumulation was 1 January.(Author for correspondence. Tel: +351 22 607 4900; Fax: +351 22 609 9157; E-mail:inoronha@ibmc.up.pt)

Key words: flowering season, heat requirements, olives, pollen, Portugal.