Frost

Olive trees are well-suited to warm Mediterranean climates but are susceptible to low temperatures, especially when they occur unexpectedly and out of season, potentially causing irreversible damage. Cold stress is a natural factor that restricts the growth, productivity and survival of plants, and determines their geographic distribution. Low temperatures inhibit plant growth while freezing temperatures below 0°C damage cell membranes and lead to cell death. Plants from regions with a natural habitat for cold acclimation can adapt fully, whereas those from warmer climates often struggle to withstand winter due to inadequate cold acclimation. The genetic diversity of plants is essential for global sustainability as it provides the foundation for adapting to climate change and promoting sustainable development, benefiting human populations in numerous ways.

In the context of climate change, simulation models have projected a steady rise in global average temperatures, accompanied by irregular and short-period cold spells. For instance, frost events in 2008, 2014 and 2016, over periods of up to three days, caused extensive damage to olive trees in northern parts of Iran, resulting in significant economic losses for the olive industry. Generally, frost events are highly unpredictable in terms of frequency and timing, occurring unexpectedly in late autumn or mid-winter. Symptoms of frost damage vary depending on factors such as canopy temperature, frost duration, olive tree variety, age, acclimatisation, previous crop load, water status of the soil, and plant and agricultural practices, including irrigation and pruning.

The level of acclimatisation is particularly important. In general, if plants are acclimatised, they are capable of withstanding lower temperatures. On the other hand, during the vegetative growth stage, freezing temperatures can severely damage plant tissues. Therefore, episodes of early frosts or increases in temperatures that promote exit from rest and the subsequent arrival of cold episodes can cause severe cold damage. Typically, when temperatures drop below -7°C, olive trees are susceptible to damage at different levels, with temperatures of -12°C or lower posing a significant threat, particularly in November and December when the trees are not yet adapted to the winter cold. Freezing injury in plant cells results from the formation of extracellular ice crystals, and freeze-thaw cycles can induce xylem embolism, contributing to frost damage and shoot dieback. The age of the tissue is an important factor, with younger tissues or branches being much more sensitive to frost than older tissues. Olive tissues are unable to tolerate ice crystal formation and remain super cooled to prevent ice formation at low temperatures.

To mitigate the effects of cold stress on olive trees, various strategies have been developed and studied. These strategies involve the selection of cold-resistant olive tree varieties, the implementation of appropriate cultivation practices such as pruning and mulching to protect the tree, and the provision of frost protection through methods such as irrigation and wind machines. Additionally, adaptation mechanisms in plants can help them cope with changing environments, providing a criterion for selecting plant materials that exhibit high adaptability to new environmental conditions.

It is important to note that studying cold tolerance in evergreen fruit trees is particularly challenging because freezing damage occurs in the field, but symptoms become visible at a later stage of plant growth. Previous research has revealed significant differences in frost tolerance among different olive varieties. Researchers have identified physiological responses associated with cold-induced damage in olives, and they have also established methods for rapidly assessing tolerance to detrimental temperature extremes. With the availability of this screening technique, in conjunction with the genetic diversity related to cold tolerance, it appears that the prospect of breeding for this trait is indeed feasible. The selection process for frost tolerance typically involves assessing olives in laboratory or field settings during or after frost stress and then evaluating their potential for high yields, consistent performance, and oil content. Several commercial olive cultivars, including Cornicabra, Leccino, Picual, Sourani and Zard, are generally recognised for their frost tolerance, while Frantoio, Coratina and Koroneiki are considered sensitive. Some research has focused on using artificial cold treatments to screen for cold tolerance in olive cultivars, but there have been few studies investigating naturally cold-tolerant trees in their native environments.

Although in general, wild olive trees have a lower tolerance to frost damage due to their limited adaptation to such conditions, olive-growing countries possess valuable genetic resources derived from sexual propagation. However, these resources have not been fully explored in terms of their morphological and molecular characteristics. The presence of diverse genetic resources can significantly enhance the possibility of identifying trees with high yields, quality, and stress tolerance, provided there is a comprehensive understanding of the genetic diversity within the available germplasm and their ability to adapt to changing climates. Cold tolerance in plants is a complex trait influenced by genetic characteristics, climate conditions and horticultural management practices. Identifying tolerant varieties following a natural freeze event and comparing them to commercial cultivars represents an initial step in this direction. While genetic factors play a role in cold tolerance, environmental factors also contribute to physiological and biochemical changes, such as the conversion of starch to soluble sugars, which enhance tolerance.

In general, cold-tolerant olive trees exhibit characteristics such as lower annual vegetative growth, moderate canopy density, a spreading growth habit, and intermediate internode length. Canopy structure plays a pivotal role in either reducing or increasing the possibility and severity of freezing in trees, influenced by factors like canopy height, length, density, porosity and leaf area index. Some studies have indicated that olive varieties with moderate canopy density -allowing light penetration, photosynthesis by inner shoots and sugar accumulation- exhibit good frost tolerance compared to those with denser canopies. Thus, pruning dense canopies can enhance light penetration and cold tolerance in olive trees. Table 3 presents an overall classification of olive varieties in terms of their tolerance to low-temperature stress.

Figure X. Symptoms of freezing stress in olive trees: mild (A) and intense (B) symptoms in the fruit of Arbequina and Zard, respectively; skin cracks in thin shoots of Arbequina (C), branches of Zard (D), and trunk of Arbequina (E); and complete tree death of Picual (F)

Table 3. Tolerance of olive varieties to frost