Studying Parameters Affecting Accumulation of Chilling Units Required for Olive Winter Flower Induction

Abstract

With global warming, mean winter temperatures are predicted to increase. Therefore, understanding how warmer winters will affect the levels of olive flower induction is essential for predicting the future sustainability of olive oil production under different climactic scenarios. Here, we studied the effect of fruit load, forced drought in winter, and different winter temperature regimes on olive flower induction using several cultivars. We show the necessity of studying trees with no previous fruit load as well as provide evidence that soil water content during winter does not significantly affect the expression of an FT-encoding gene in leaves and the subsequent rate of flower induction. We collected yearly flowering data for 5 cultivars for 9 to 11 winters, altogether 48 data sets. Analyzing hourly temperatures from these winters, we made initial attempts to provide an efficient method to calculate accumulated chill units that are then correlated with the level of flower induction in olives. While the new models tested here appear to predict the positive contribution of cold temperatures, they lack in accurately predicting the reduction in cold units caused by warm temperatures occurring during winter.

Olive Flowering dependence on winter temperatures - linking empirical results to a dynamic model

“…Using this data, we attempted to develop a method for calculating CUs that correlated with the level of flower induction observed. We We found that all three models were successful in predicting the empirical flowering levels obtained under cold winters yielding high 𝑖, but much less effective at predicting flowering levels under winters with warm periods in which low to moderate flowering levels were observed (Engelen et al, 2023). We attributed this lack of success under conditions marginal for flowering to the inherent limitation of this class of models, disregarding the dynamic process of temperature change throughout the winter.…”

“…Both the production rate 𝐾 0 (𝑇) and the decay rate 𝐾 𝑥 (𝑇) of 𝑥 are temperature-dependent. To ensure a bell-shaped temperature dependence of chilling accumulation, based on previous experiments (Engelen et al, 2023, Hartmann & Porlingis, 1957, we defined 𝐾 0 (𝑇) the production rate of 𝑥 to be a Gaussian function with a maximum at the temperature known to be optimal for olive flowering (11 C)…”

“…To fit the remaining 5 parameter values: 𝐴, 𝐵, 𝐶 determining the functional shape of 𝐾 𝑥 (𝑇) the decay rate of 𝑥, and the two Hill function parameters 𝑍 𝑠𝑐𝑎𝑙𝑒 and 𝑝 we used the flowering and temperature data we collected. In the fitting procedure (Figure 5) we used as the model input data multiple sets of experiments in which 𝑖 values of several trees were determined in spring and the corresponding temperature profiles throughout the winter months were monitored (Engelen et al, 2023) (see Table S2 for the list of experiments used). To start the optimization procedure, we arbitrarily initialized the model parameter values.…”

“…We have previously collected data on 𝑖 levels in five different olive cultivars and monitored the hourly temperatures throughout nine to eleven winters in several locations in Israel exhibiting a variety of weather conditions (Engelen et al, 2023). Using this data, we attempted to develop a method for calculating CUs that correlated with the level of flower induction observed.…”

“…In the current work, we employ a multi-faceted approach to study the impact of realistic temperature profiles and specifically heat spells on olive flowering. We began by experimentally testing the effect of heat spells of different durations on olive flowering and gene expression, adding that to our previous database (Engelen et al, 2023). We then proceeded by constructing a new dynamic model that captures the effects of winter temperatures on olive flower induction, inspired by the dynamic model of Fishman et al for release from dormancy.…”