Budburst marks the start of flower development. Unlike most deciduous fruit trees, where budburst coincides with the onset of flowering, in olive trees, there is a gap of several months between budburst and flowering. Flowering is fundamental to determining the yield of the olive crop. Both genetic and environmental factors influence the timing and quality of flowering. Pollination, the transfer of pollen between flowers, takes place during the flowering phase. In general, olive trees are considered allogamous and exhibit autoincompatibility. This means that pollen from different cultivars is required to fertilise the ovaries of the flowers. Although there is a large overlap in the flowering time for most of the cultivars in the Mediterranean climate, differences in flowering phenology between cultivars (Table 1) should be considered for plantations in areas outside of the Mediterranean basin and/or without nearby olive trees. It has been reported that each year there is a 22-day gap in flowering between the first and last cultivar (Figure 4). Therefore, it is essential to combine different cultivars to facilitate cross-pollination. Environmental factors, particularly temperature, have a significant impact on phenology, resulting in a variation of almost one month between the earliest and latest years (Figure 4). High temperatures accelerate and shorten the flowering period, while cooler temperatures delay and prolong it. In addition, adverse weather conditions, such as extreme temperatures or high humidity, can disrupt the pollination process and result in significantly reduced fruit set. 

Winter dormancy allows olive trees to withstand low temperatures in winter and later to bud when chilling requirements are met and when warmer temperatures favourable to growth and development occur. The chilling requirement is the amount of accumulated chill required by a variety to release dormant buds from dormancy. Chilling accumulation is therefore closely related to temperature. A wide range of temperatures, from 7ºC to 15ºC, contributes to chilling accumulation, with the optimum temperature being 12.5ºC. Temperatures exceeding 16ºC do not allow the accumulation of chilling, and higher temperatures can negate accumulated chilling. Empirical or statistical methods can be used to estimate the dormancy period. Recent studies on the chilling requirements of cultivars have proposed a new methodological approach using a leafy explant cutting method under forced growth conditions to better understand the release from dormancy. This method has shown that olive cultivars have different ranges of chilling requirements (Table 1). To meet the challenge of climate change, cultivar adaptation needs to be assessed in experimental trials in different regions. Recent studies have shown that high winter temperatures may prevent the accumulated chilling requirements from being sufficiently met, resulting in asynchronous and prolonged reproductive budburst, flowering and fruit ripening, thus hampering olive cultivation (Figure 3).

Different olive cultivars have a different pattern of shoot growth that defines the canopy volume, shape and density. Those traits are essential to define the suitability of a cultivar for the different olive growing systems and their pruning needs.

In this section

1.1.1. Vigour

1.1.2. Cultivar suitability for different olive growing systems

Canopy volume was evaluated in the Olive Germplasm Bank of Córdoba (IFAPA) in 4-year-old trees before any pruning was applied. Therefore, the canopy values were an estimation of the vigour of the cultivar (Table 1). Most of the cultivars of the list were considered to have medium vigour. However, a high variability in canopy volume was observed in the full set of cultivars of the Germplasm Bank (Figure 1). Besides, the canopy surface was tightly correlated to canopy volume, so the categorisation for both traits was the same. It is noteworthy that this characterisation was done using an unmanned aerial vehicle (drone) that showed to be very efficient and accurate in estimating canopy volume.

The characterisation presented here was done in the climatic conditions of Córdoba, southern Spain. It is quite possible that in other climatic conditions the characterisation could vary significantly. For example, climatic conditions allowing a shorter or longer growing period could produce, respectively, a decrease or increase in canopy volume in some of the cultivars. Therefore, it is highly recommended to perform local evaluation of the cultivars of interest to accurately determine their vigour in a given climatic condition. 

Some of the data could be surprising, such as the fact that the Arbequina variety has a higher canopy volume than Picual. It is necessary to stress the fact that those data were taken in 4-year-old unpruned trees. This initial vigour could be modified in subsequent years by different factors, for example crop yield, as is the case of Arbequina.

Traditional olive orchards are planted at wide distances among trees (8-9 m) and are trained to form canopies with large volumes. In those orchards, vigorous cultivars are suitable to rapidly establish sufficient tree size to have commercial yields. Most of the olive cultivars existing today were selected by the growers to fit in this growing system.

However, a significant trend to intensification has been evidenced in the olive orchards planted in the last decades. This intensification consisted of an increase in plant density and a trend towards mechanising the different tasks, especially, harvesting. This new trend needs trees that can be trained to form low volume canopies that can also bear fruits on a regular basis. Therefore, medium-and low-canopy volume cultivars (Table 1) will be better adapted to the new olive orchards, especially those planted in a hedgerow system. Besides, the lower the canopy, the less costly are the pruning operations, which is one of the most labour-consuming tasks in olive growing. Obviously, other characteristics, such as branching habits, branching flexibility, productivity, oil content and composition, among others, should also be considered to select the best cultivar for any given environmental and management conditions.

In this section

1.2.1. Fruit yield

1.2.2. Oil yield

The olive is a subtropical evergreen species that is well adapted to the mild Mediterranean winter climate. The Mediterranean region is considered one of the most vulnerable areas to the effects of climate change in the near future. In particular, the olive tree is highly sensitive to climate change, which can have profound effects on phenology, yield, pest and disease patterns, among other factors. There is even a risk of the crop disappearing in certain areas due to possible associated effects of climate change. In addition, the expansion of olive cultivation to regions of the world that are very different from the Mediterranean climate area has made it essential to enhance our understanding of olive phenology. All phenological phases (budburst, flowering and ripening) are influenced by weather conditions, making the study of phenology crucial for understanding the impact of climate change and identifying ways to mitigate its effects.

In this section

1.3.1. Phenological Monitoring

1.3.2. Chilling Requirement

1.3.3. Budbursting

1.3.4. Flowering

1.3.5. Ripening

For monitoring the different phenological stages, a triangle diagram (Figure 1) is used to record the most delayed, common and advanced phenological stages by applying the BBCH scale (Figure 2A: Budburst, 2B: Flowering, and 2C: Ripening). A minimum average flower or fruit load should be present on the assessed tree to ensure a correct evaluation.

                  Most common stage

 Most delayed stage          Most advanced stage

Figure 1: Phenological triangle

During the growing season, shoots develop and buds emerge from the axillary nodes of olive leaves. These buds remain dormant throughout winter until spring, when budburst occurs. Before budburst, it is impossible to distinguish between vegetative and reproductive buds. The expression of the flowering locus T (FT) in olive leaves increases during winter, in conjunction with chilling accumulation. This fact implies that the induction of olive flowering may occur just before budburst. Once chilling requirements are met, budburst starts if the temperature is suitable. In regions where chilling requirements are not met, the number of reproductive buds and their period is irregular (prolonged and staggered; Figure 3). Table 1 shows the budburst dates for different cultivars.

Figure 3: Asynchronous flowering due to insufficient chilling causes the presence of different phenological stages within the same tree (from Medina-Alonso et al., 2020).

Table 1: Classification of chilling requirements, flowering  and date of budburst in the World Olive Germplasm Bank of Córdoba (Spain) and Marrakech (Morocco).

Ripening is the process that precedes fruit senescence and its eventual decline. During ripening, significant metabolic and biochemical transformations alter the colour, composition, weight, oil content, respiration and fruit retention force. These changes are influenced by factors such as the cultivar, environmental conditions, agronomic practices like irrigation, and the fruit load.

Table 1 shows a classification of ripening timing for cultivars in Spain, while Figure 5 shows the variations between years. For most cultivars, the colour of the fruit changes from intense green to yellowish-green, partially purple, purple, and eventually to violet-black, as shown in Figure 2. The timing of olive ripening is closely linked to the harvesting season, which in turn depends on the intended use of the olives. Olive cultivars suitable for "Sevillano style" table olives are usually harvested when they have reached a yellowish-green colour, while naturally black "Greek style" table olives are harvested when the fruit has turned completely black. When olive fruits are used for oil production, early harvesting tends to result in higher-quality oils due to their higher polyphenol content, among other characteristics. 

Table olive cultivars show large variability in size, shape and flesh-to-stone ratio (Figures 1 and 2). Nevertheless, medium (3-5 g) to large fruits (above 5 g) are preferred by both consumers and the industry. Similarly, a flesh-to-stone ratio greater than 5:1 is considered a threshold for table olives. Among the various shapes, symmetrical rounded fruits with symmetrical stones are more suited for olive pitting in the industry. Stone detachment is also related to the roughness of the pit´s surface, being easier in cultivars with a smooth or rough surface compared to those with a knotty stone (Gordal Sevillana, Toffahi, Ascolana Tenera).

Figure 1: Variability in fruit size and morphology among table olive cultivars in the same field trial. 

Figure 2: Fruit weight and flesh-to-stone ratio of various table olive cultivars in the same field trial.

Olive fruit growth and development lasts for 4-5 months and includes five stages, the last of which is non-climacteric maturation (beginning about 180 days after anthesis). As in other fruit crops, fruit detachment force (DF) declines steadily as fruit matures.

The DF of olive fruits increases from the fruit set (30 DPA), until fruits reach their full size, around the harvest season of table olives that takes place when fruits show a green-yellowish colour. Subsequently, fruit DF begins to decrease until it drops below 100g at 270 DPA, when ripening is concluded (Figure 3).

Figure 3: Annual kinetics of changes in fruit detachment force (DF, g) after anthesis. 

In order to enable the mechanical harvest of table olives, it is also necessary to prevent external fruit colour change or bruising.

The bruising process after induced mechanical injury was characterised in various cultivars. Some cultivars are resistant to browning and do not show any brown spot three hours after application of pressure. The different response to mechanical damage shown by the cultivars could be mainly due to genetic differences. Mesocarp cells in the fruits of the sensitive cultivars are damaged and lack a cell wall as a result of the applied pressure. The cuticles of resistant cultivars are thicker compared to those of susceptible cultivars. The bruising process is enzymatic. Figure 5 shows resistant versus sensitive cultivars.

Figure 5: Photographs of 5 fruits from each of the most susceptible (left) or resistant (right) cultivars to bruising, three hours after induced mechanical pressure.

The International Olive Council (IOC) is the world’s only international, intergovernmental organisation exclusively dedicated to the olive oil and table olive sector. Founded in 1959 under the auspices of the United Nations, the IOC is headquartered in Madrid, Spain.

Since its inception, the IOC has been committed to the integrated and sustainable development of global olive cultivation. This commitment seeks to bring meaningful progress to member countries and, even more importantly, to the people whose livelihoods depend on the olive tree and its products.

According to the International Agreement on Olive Oil and Table Olives of 2015, the IOC aims, among other objectives, to promote technical cooperation and research and development in the olive sector. It encourages collaboration between national and international public and private entities and undertakes initiatives to identify, preserve, and utilise the genetic resources of olive trees.

Jaime Lillo, IOC Executive Director

The olive grove is undergoing a period of change. From the traditional rainfed olive grove, with low density and productivity, numerous local varieties and manual harvesting, there is a transition underway towards new high-density, irrigated plantations with a small number of increasingly universalised varieties. Genetic erosion and the vulnerability associated with the reduction in cultivated varieties, the incidence of devastating epidemic pests and diseases such as those caused by Verticillium dahliae and Xylella fastidiosa, as well as forecasts related to climate change have triggered great interest in conserving the cultivated biodiversity of the olive tree. Germplasm banks are collections of olive trees of varieties from a given geographical area. The objective of the Network of Germplasm Banks of the International Olive Council is the conservation, cataloguing, evaluation and free exchange of native cultivated varieties, representative of the selected biodiversity in olive-growing countries.

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