Olive Olea europaea

“Abstract Olive trees require a period of chilling temperatures, either diurnally fluctuating, or at a constant—but definite—level before inflorescences develop. In this study, bud and leaf samples were taken at weekly intervals during this critical period from trees receiving temperatures favorable for flowering and from trees under temperature conditions known to prevent flowering.”

“Phenology was studied between 1992 and 1994 at different elevations (between 80 and 820 m above sea level) and on 2 different host species (apple, Malus silvestris Miller, and pear, Pyrus communis L.). The association between accumulation of degree-days (DD) and rate of oviposition and egg hatch was analyzed with simple linear regression using 10°C as the lower developmental threshold temperature and 15 February as the starting date. Temperature-driven rates of oviposition and egg hatch were similar for the 3 yr, all elevations, and the 2 tree species.”

“For each CLC reference year, GDD was computed for olive cultivation (Tbase= 7 °C, January–May) and viticulture (Tbase = 10 °C, April– October) exclusively over registered cultivation pixels, and per-country means were subjected to linear regression trend analysis (p < 0.05). For olive cultivation across 11 Mediterranean countries, statistically significant positive GDD trends were detected in 7 countries, with long-term country means ranging from 476.2 in France to 1214.3 in Cyprus — values that consistently and substantially exceed the canonical 700 GDD suitability threshold, indicating that heat availability is no longer the primary thermal constraint for Mediterranean olive growers. The first appearance of olive cultivation in Slovenia’s 2012 CLC dataset, with a median of 546.5, provides land-use-mapped evidence of a poleward displacement of cultivation boundaries.”

“Positive CU begin to accumulate just after the day in the fall when the largest negative accumulation is experienced. The chilling period ends for Olea when 825 units are reached 67 , while for Quercus , it ends at 790 units, assuming that value from the calculated unit range of Richardson (1974) 141 . CU provides a detailed understanding of how different temperature ranges contribute to breaking dormancy.”

“Temperatures later on in spring are translated into Growth Degree Days (GDD) and affect the speed at which anthesis occurs, with warmer temperatures promoting earlier flowering.”

“Recent experimental evaluations of such warm spells indicate that they have the potential to drastically reduce flowering intensity [ 8 ]. When the number of chilling units was higher (1200 CU) in 2019, no response of flowering intensity to warming was observed. Additionally, the trees were one year older in 2019 (i.e., three years old), which cannot be discarded as a contributing factor to flowering intensity in young trees.”

“The thermal requirement varies between 1251 and 1381 °C d.”

“Chilling hours were accumulated for temperatures between 0 and 7.2 °C using the simple chilling-hours model [33]; periods with fewer than 200 h were recorded as adverse.”

“The average chilling accumulation from the first day in which positive chilling units were detected to the peak of chilling date was 1,246 chilling units in Perugia and 1,034 chilling units in Brindisi, which are notably higher than the 130 and 27 chilling units recorded in Menzel and Zarzis, respectively. In the Spanish areas, this accumulation value was 606 chilling units for Córdoba and 767 chilling units for Jaén. The heat accumulation start dates were significantly different between these olive growing areas (F=5.825; P=0.001).”

“A mean temperature variable was introduced to consider the current conditions during flowering (from the first pollen concentrations to the maximum). The best fitting model was obtained considering the maximum pollen concentration date and the modified chilling unit formula with a cut‐off temperature of 12°C. The validity of this model was tested using data from 1999 to 2002 (not included in the model parameterisation).”

“Partial least squares (PLS) regression with leave-one-year-out stability screening isolated two robust windows: late November–mid-January (chilling) and late February–late March (forcing). Mean chilling requirements were similar across cultivars (~69 Chill Portions (CP); Coefficient of Variation (CV) ~10–13 %), whereas forcing demand differed (mean GDH ≈ 7864 for Haouzia, 8524 for Menara, 8967 for Picholine Marocaine). Sentinel-2 normalised difference vegetation index (NDVI) (moderate resolution imaging spectroradiometer (MODIS), fallback) showed that the spring NDVI peak occurs ~1 month before BBCH 65 and therefore does not directly indicate full bloom timing in evergreen olive canopies.”

“Abstract Controlled low temperature chilling caused flower initiation in container-grown olive ( Olea europaea L.) trees at any time of the year. Optimum inflorescence production under controlled conditions occurred after exposure for 70 to 80 days to a diurnal sine wave temperature pattern, with a 2°C minimum and a 15°C maximum. These same temperatures, but changing abruptly from one to another failed to cause inflorescence formation.”

“The trigonometric method was used to quantify the number of chill hours, considering base temperatures (Tb) of 7.0, 9.5, and 13°C (high, medium, and low chill, respectively), and was applied to present climate (1983-2012) and to two future climate (2041-2070 and 2071-2100) conditions. Th”

“It is generally thought that olives (Olea europaea) require several days (over 80 days) of chilling temperatures (7.2°C) for flower induction; minimum nighttime temperatures of 2-4°C and maximum daytime temperature of 15.5-19°C are considered optimum for flower and fruit production. Environmental chamber studies were conducted on potted olive trees for the purpose of defining flowering conditions for ‘Arbequina’.”

“The analysis was implemented on cloud computing, allowing PBDM models to be run from any computer connected to the internet, to interface with state-of-the-art climatic drivers, and to scale efficiently with increasing computational loads and user requests. Findings include that chilling required for olive blooming will decrease in large areas of the Andalusian provinces of Jaen, Cordoba, and Sevilla, with some areas not meeting the minimum chilling threshold and some accumulating no chilling by the end of the century under the high greenhouse gas (GHG) emission scenario. Olive blooming will occur up to five weeks earlier in the Jaen, Cordoba, Sevilla, and Granada provinces, but olive yield is expected to increase or remain stable.”

“There was no significant difference among cultivars in fresh weight fruit yield averaged over 2 years (2013 and 2014), ranging from 2.14 to 2.45 kg/tree. During December to March, calculation of chilling hours below 12.5 °C was 141 hours during 2012–13 and 161 hours during 2013–14. The other seven cultivars did not flower and fruit during these 2 years of growth at Lalamilo, perhaps due to a greater requirement for chilling hours.”

“Genotypic (cultivar) effects were dominant in determining 24 out of the 39 studied variables. Concerning cultivars, inflorescence beginning (BBCH51) was between 57.35–81.57 days of year (DOY), needing an amount of chilling of 21.47–23.46 CP and heat ranged in 198.96–344.38 GDD (growing degree-days) and 4,915.77–8,494.11 GDH (growing degree-hours). Flowering beginning (BBCH61) was between 111.29–132.66 DOY and required 538.37–764.59 GDD (12,538.3–17,884.7 GDH).”

“A series of experiments was carried out over three consecutive fall-winter periods, with one-node and three-node leafy and defoliated explants from shoots sampled from nonbearing (OFF) and bearing (ON) trees and forced in a growth chamber at different constant temperatures (12.5, 15, 20 and 30 °C). Buds acquired reproductive budburst capacity after a period of chilling accumulation on the tree for both OFF and ON explants, with percentage of reproductive budburst substantially higher in the OFF explants. Reproductive budburst occurred on explants sampled from early January to the second half of February, with slight variation in dates among the different experiments and seasons.”

“The important phenological stage 6 (flowering) was the shortest stage. An average of 704 Chilling Hours (CH) with a threshold of 2.5 °C was required to overcome the chilling period, 1139 Growing Degree Days (GDD) for the beginning of flowering, and 4463 GDD for harvest. The pollen production per anther was 82589 grains (± 14084 pollen grains), with a rate of 81% viability and 12% pollen tube germination.”

“The results revealed clear inter-cultivar differences in endodormancy and ecodormancy durations, thermal requirements and flowering dates. Arbequina exhibited the earliest dormancy release with relatively low chill requirements, whereas Picholine Marocaine and Dahbia flowered later and required higher chill accumulation. In all cultivars, bud growth activity increased near the time of dormancy release, indicated by water content exceeding 30 %, with only minor genotypic variation in the transition between dormancy phases.”

“The olive wide genetic variability can be used to overcome some of the challenges faced by olive growing, including climate warming. One effect of climate warming in olive is the difficulty to fulfill the chilling requirements for flowering due to high winter temperatures. In the present work, we evaluate seven olive cultivars for their adaptation to high winter temperatures by comparing their flowering phenology in the standard Mediterranean climate of Cordoba, Southern Iberian Peninsula, with the Subtropical climate of Tenerife, Canary Islands.”

“A strong correlation was observed between total quantity olive pollen collected over the season and rainfall recorded during the second fortnight in February. According to the proposed model, an average of 680 Chilling Hours (CH) are necessary to overcome the chill period and break the state of bud dormancy, and 481 Growth Degree Days (GDD) °C are needed to induce flowering. Models for predicting daily mean pollen concentrations combine temperature and the previous days' pollen concentrations as predictor variables to provide a high level of prediction.”

“Winter temperature has a direct influence on phenology and cold accumulation during the winter dormancy period [31]. In a study under controlled conditions, it was identified that the olive tree enters the winter dormancy period when the temperature drops below 14.4 ◦C [32]. Paniagua et al. [33] use this threshold temperature to calculate the growing period of the olive tree in regions of Spain.”

“Until 2004, the trees accumulated similar GDD values. With rising temperatures, earlier flowering occurred at fixed GDD values of 650 and 750. Subsequently, the trees began accumulating higher GDD values, stabilizing the flowering date and preventing further advances.”

“It is worth noting that. under the RCP8.5 projection for the far future, the results showed that in areas located at the lowest elevation, i.e., Gazi, Chill Portions are expected to practically nullify for some years (Fig.[5e](https://link.springer.com/article/10.1007/s10584-022-03462-4#Fig5), [i](https://link.springer.com/article/10.1007/s10584-022-03462-4#Fig5)). The day of the year of peak Chill Portion accumulation did not vary considerably in comparison to the reference period.”

“Meteorological analysis showed that the biological cycle of the olive was greatly influenced by the temperature. The relationship between an increase of temperature before the beginning of flowering and the advance of flowering was 2.5 days/°C. Another important result concerns the value of the mean temperature which by the beginning of flowering was 16°C at all monitoring stations, even if the growing degree day (GDD) requirements were different.”

“Chilling and Thermal Indices The accumulation of chilling and thermal time showed significant variation across the Mediterranean network ([Table 1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13667179/#plants-15-02980-t001); [Figure 7](https://pmc.ncbi.nlm.nih.gov/articles/PMC13667179/#plants-15-02980-f007)). Across the 16 site-years, mean chilling accumulation was 45.50 CP, 1011.77 CU 7.3, and 1586.93 CU 12.5 ([Table 1](https://pmc.ncbi.nlm.nih.gov/articles/PMC13667179/#plants-15-02980-t001); [Figure 7](https://pmc.ncbi.nlm.nih.gov/articles/PMC13667179/#plants-15-02980-f007)A–C). The relative ranking of the germplasm-bank environments varied among chilling indices.”

“The growing degree-days (GDD) approach was used to estimate olive phenology for these CC scenarios. Unchanged irrigation management with an average CIR increase up to 16% in RCP4.5 and 31% in RCP8.5 is expected. By adopting higher levels of water deficit, water savings of up to 22% can be realized.”

“Samples were modelled against Growing Degree Days (GDD), the selected climate proxy, and year, accounting for cultivar and geographic origin. Results indicate emerging trends in fatty acid composition, including a reduction in oleic acid in some cultivars and a relative increase in palmitic and linoleic acids, potentially linked to rising temperatures. However, these changes are small: fatty acid proportions remain within narrow ranges and within International Olive Council (IOC) limits, indicating that although statistically significant, they are not practically relevant to EVOO quality or oxidative stability.”

“Our results indicate that rising minimum and maximum temperatures across Istria, particularly in spring, have increased growing degree day (GDD) accumulation during the growing season. This has advanced the onset of key phenological stages and shortened the overall growth period, with the Istarska Bjelica cultivar showing the most pronounced response.”

“3.6.2. Oil Yield Response to Rainfall (120–480 GDD) The olive oil yield in year t + 1 exhibited a similarly positive and concave relationship with rainfall in the 120–480 GDD window (Figure 13). The model fits were consistently high across clusters (R2 = 0.83–0.87 for Clusters 1–2 and R2 = 0.97 for Cluster 0).”

“However, olive oil production is increasingly being affected by current climate change8–12. Recurrent warm winters, which (i) delay the fulfilment of chilling requirements, (ii) damage floral bud tissues, or (iii) reduce or entirely inhibit flowering13, along with strong heat waves (that scorch stamens) and low rainfall, have caused a sharp decline in Mediterranean yields. Spain, the world’s leading producer, has been particularly affected6.Thissituationworsenedduringthecropyears2022-2023and2023202414.”

“Methods: Analyzing climatic data (2004–2023) and conducting surveys with local farmers. A key focus was the assessment of chilling units (accumulated hourly temperatures below 7.3°C), which are crucial for olive productivity, using Excel sheets and SPSS software. Results: Findings indicate an average of 262 chill units during winter months (December–February), with extreme variability peaking at 435 units (2005) and dropping to a mere 93 units (2007).”

“It is generally assumed that lack of flowering and fruiting in olives in subtropical climates of southern Texas (e.g. Weslaco Texas area; N 26.16° Latitude 97.96° Longitude) is due to fewer chilling days (<7.2°C) during winter than most olive growing areas of the world.”

“Thus, for FFD and RD the root mean square error was around 6 and 24 days, respectively. The limited inter-cultivar variability was associated to the low average temperatures registered during winter at WOGB-IFAPA generating an early accumulation of the chilling requirements, thus homogenizing FFD of all the analyzed cultivars.Research highlights: The identification of cultivars with early FFD and late RD provides useful information for breeding programs and climate change studies for identifying adaptation strategies.”

“The predicted climatic conditions would increase evapotranspiration of vegetation and atmospheric CO2 emissions. Moreover, climate change will reduce the chill accumulation in olive groves, altering its flowering, fructification and crop yields. Thus, it is necessary to adopt management models that promote olive grove resilience in face of climate change, ensuring their socio-ecological sustainability.”

“This model [ 8 ] simulates the hourly increment of chilling that is generally gathered by commonly used models [ 21 , 22 , 23 ]. In the endodormancy release calculations, when the temperature exceeds 20.7 °C, a constant number of already accumulated chilling units are nullified, and the model does not accumulate chilling hours below 0 °C [ 8 ]. The 0 °C limit was based upon the work of Orlandi et al. [ 15 ] and the optimal temperature for chilling was considered to be 7.3 °C.”

“Prediction methods were evaluated for the earliest-flowering variety, a model variety representing the mean values for the collection, and the latest-flowering variety. The most appropriate threshold temperature for heat accumulation has been confirmed to be 12.5C; it can be used to predict the flowering time in olive.”