Comparison of Utah and Positive Utah models for determining of chilling and heat requirements of some olive cultivars

Determining of chilling and heat requirements of olive cultivars growing in Fars Province is important for the selection of cultivars that will exhibit satisfactory growth and development. The purposes of this study were determining the chilling and heat requirements of olive and comparison of Utah and Positive Utah Chill unit (PCU) models for determining the chilling requirements of olive cultivars. In this regard, chilling requirement of flower buds of ‘Fishomi’, ‘Dezful’, ‘Zard’, ‘Dehghan’ and ‘Shiraz’ cultivars were determined. Cuttings of these cultivars were taken during autumn when the mean temperature fell below 12ºC. Cuttings were kept at 5 ºC for periods of 0, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700 and 1800 h. Results indicated that the chilling requirements of ‘Fishomi’, ‘Dezful’, ‘Zard’, ‘Dehghan’ and ‘Shiraz’ were  900, 1000, 900-1000, 1000-1100 and 900 h, respectively. Results also showed that the PCU unit model was more efficient than the Utah for estimating chilling requirements under subtropical field conditions. Heat requirements from the end of dormancy to full bloom stage of these five olive cultivars were estimated as 199.2, 272.1, 245.2, 245.2, 321 growing degree days (GDDs), respectively. It was observed that ‘Fishomi’ had the lowest chilling requirement and ‘Dehghan’ obtained the highest one. Hence, ‘Fishomi’ can be recommended for cultivation in subtropical regions.