Drought

The origins of the olive tree date back to the eastern Mediterranean region more than 5000 years ago, extending towards North Africa, the Iberian Peninsula and the rest of southern Europe. These regions are characterised by hot summers and low rainfall, with olives originally cultivated on shallow soils and sloping land, in areas unsuitable for other crops due to the lack of water resources and soil limitations. The domestication of the olive tree in environments with very limited water resources has made this crop particularly well adapted to drought conditions.

Olive trees affected by water stress reduce the water content and water potential of their tissues, establishing a particularly high potential gradient between leaves and roots, and stopping canopy growth, but not photosynthetic activity and transpiration.

Olive leaves tolerate low water potential that would kill most annual and perennial crops. Olive leaves are highly adapted to drought as they are hypostomatic, meaning that the stomata are only on the underside of the leaf, while the upper side is impermeable. This means that transpiration takes place only on the underside of the leaf. In addition, the stomata are protected from excess radiation by trichomes, and the olive leaf can turn on its axis to reorient itself at a more vertical angle to avoid excess radiation.

An important component of the response to water stress in olive trees is the physiological phenomenon of the stomatal control of transpiration. The stomata close when the water potential difference between the leaves and the soil exceeds the limits required for safe xylem function. This allows the olive tree to remain sufficiently hydrated to maintain its physiological functions, including photosynthesis, even under severe drought conditions. This regulation is very effective at low to moderate water stress, but its efficacy decreases at high water stress. Moreover, when the olive tree is subjected to water deficit, the stomata open earlier each day and remain open for shorter periods.

Another characteristic of the adaptation of the olive tree to situations of limited water availability is the high capacity to transport water to the leaves. The water potential between the leaves and the roots is the driving force that allows the water to rise to the leaves. However, beyond a certain limit, air can enter through the conduit pit membranes, generating bubbles and causing embolism, which reduces hydraulic conductivity. Transpiration is then reduced to relieve tension and prevent further cavitation. The olive tree has small diameter xylem vessels, which makes this phenomenon less likely. In addition, the vessels are interconnected, allowing water to circulate through a neighboring vessel that is still functional.

The root system of the olive tree develops in superficial layers to make more efficient use of the water provided by infrequent and light rainfall. It is also highly plastic, adapting rapidly to changes in water supply by redistributing the active roots in the soil. The combination of the capacity of the olive tree to lower water potential and its extensive root system increases its ability to withstand drought by increasing the amount of extractable soil water.

Related to phenology, olive trees flower in mid to late spring under Mediterranean conditions, allowing olives to escape the negative effects of cold on flowering and fruit set, while requiring tolerance mechanisms to maintain internal water status and metabolic activity during hot and dry summers. Some observations indicate the loss of most flowers or fruits under intensive drought periods, confirming that flowering and fruit set are very sensitive to water deficit.

Moreover, the nature and pattern of fruit growth are important for drought adaptation, as timing and magnitude of stress determine the effect on cell number and size. Thus, the early stages of fruit formation tolerate moderate water stress without negative effects on future oil accumulation potential. The efficient use of limited water and carbon fixation by the fruit maintains limited growth and survival during the later stages of fruit growth in summer, and the rapid recovery of trees after autumn rains provides substantial new assimilates for fruit growth.

In addition to these mechanisms for coping with water stress and drought, the olive tree has another adaptive feature to drought, namely its rapid recovery after severe stress when rainfall or irrigation arrives. The roots of olive trees have been found to absorb water immediately, even after months of very dry soil. However, some components are affected by drought and cannot recover in such a short time. For example, water stress during flower induction (which occurs throughout the summer and early autumn) affects flowering the following year. Similarly, during episodes of severe drought, the xylem vessels lose their transport capacity, which cannot always be recovered, affecting vegetative and productive behaviour in subsequent years.

Despite the high adaptation of the olive tree to water stress, differences in the tolerance of olive varieties to drought have been observed in different experiments. In general, it seems that olive varieties native to arid regions have a greater ability to adapt to drought conditions than those from regions with a more temperate climate. Nevertheless, the identification of the traits of the more drought-adapted varieties is ambiguous, as it depends on the varieties compared in each study.

Table 1 presents an overall classification of olive varieties in terms of their tolerance to drought stress.

Table 1. Tolerance of olive varieties to drought.