High temperatures

Temperature plays multiple roles in the survival, vegetative growth, flowering and fruit production of olive trees. Before establishing a new plantation, we should study the meteorological history of the area to ensure that there is no frequent temperature drop below 0ºC. This condition will minimise the risk of frost damage to our plants and fruit. An upper temperature limit that we should consider when selecting the planting site would be roughly 40ºC. An area with frequent incidents of higher temperature would make an olive plantation prone to several problems. 

Olive trees need a certain amount of mildly low temperature hours during winter to induce bud dormancy break and produce satisfactory flowering in the subsequent spring. The effective temperature range is 5-12ºC and the number of these chilling hours varies among olive varieties and should be checked for the specific one we intend to plant. In general, olive varieties grown in cold climates and known for their cold tolerance require a greater accumulation of chilling for decent flowering. On the other hand, varieties grown in hot environments have minimal chilling needs and can have a satisfactory flowering even after very mild winters. For example, Koroneiki and Arbequina have low chilling requirements and can produce decent flowering even in warm winter areas. Similarly, the olive varieties originating from North African countries and the Middle East are considered to have low chilling requirements. 

Flowers are the most sensitive plant parts to heat stress. Air temperature over 30ºC decreases pollen germination and fruit set. When temperature exceeds 35ºC, we may observe complete flower death and crop failure. Koroneiki, Kalamon, Levantinka and Pendolino are relatively tolerant, however, their flowers are also damaged over 35ºC. Chemlali Sfax followed by Meski are also tolerant to heat stress. Moreover, a drastic loss of pollen viability occurs when pollen is subjected to a combination of high humidity and high temperature, again with cultivar differences for this trait. 

When fruit set has been completed, olive trees are more resilient in high temperatures. However, a range of 20-25ºC can be considered as ideal for their vegetative growth and fruit development. Over 35ºC photosynthesis is hindered and overall plant performance is reduced. At temperatures over 40ºC or during prolonged heat incidents, leaf and fruit drop occurs. High temperatures during the summer increase irrigation needs and reduce the effectiveness of olive fruit fly control measures. Barnea is relatively heat tolerant during fruit growth. High temperatures affect morphological, anatomical and physiological responses with fewer changes -and therefore greater heat tolerance- observed in cultivars such as Biancolilla compared to Cerasuola and Nocellara del Belice. 

Temperature also affects fruit growth processes and chemical transformations. High temperature during fruit maturation leads to reduced fruit weight, oil concentration and oil quality. Barnea, Coratina and Picholine are tolerant in terms of dry fruit weight and final oil concentration, while Souri, Peranzana and Maurino are more tolerant in terms of the quality of the oil produced. Cultivar by temperature interactions have been observed for fatty acid composition, for instance, with decreasing oleic acid and increasing linoleic acid content in Arbequina compared to no response in Coratina. Similarly, different groups of cultivars have been identified according to their oleic acid content stability, from high (Frantoio, Picual), intermediate (Carrasqueño, Ocal) to low (Arbequina, Koroneiki and Arbosana) stability. Table 2 presents an overall classification of olive varieties in terms of their tolernce to high-temperature stress

Table 2. Tolerance of olive varieties to high temperature.

Figure x. Flowering damage in olive due to heat.