Estimation of chilling and heat accumulation periods of the olive tree in a warm and sub-humid climate, using the partial least squares (PLS) regression

Abstract

Olea europaea L. is an anemophilous species of the Mediterranean basin and one of the most widespread fruit trees with notable agricultural and economic values. However, its pollen is recognized as one of the main causes of allergic reactions. Temperature and the rates of chilling and heat accumulations are vital for the olive reproductive cycle and determine the timing and the intensity of pollen season. Variations in its reproductive phenology can be a meaningful bio-indicator regarding the response to climatic changes.

In this study, recent phenological observations enabled the determination of flowering start dates for olive trees in Tétouan. These were compared with pollination dates defined according to different criteria, to determine which best approximates the observed dates. This study also aims to delineate chilling and heat requirements periods using a partial least squares (PLS) regression and estimate thermal requirements for the start date of pollination in Tétouan (NW of Morocco). The PLS analysis clearly delineated three discontinuous chilling accumulation periods spanned from September to the first week of December and one heat accumulation period from 14th February to 18th April. The average of chilling requirements and heat requirements was 4.89 chill portions (CP) and 17,191.71 growing degree hours (GDH), respectively. Linear regression revealed that forcing period had a closer relationship with timing and was then the main driver of the beginning of the olive pollen season. Defining the thermal accumulation periods and estimating chilling and heat requirements could provide evidence of future climatic changes impacts on tree phenology.