Drought and the olive tree in a changing climate: a multi-level response characterisation to explore and valorise Italian cultivars

The Mediterranean climate faces significant warming and reduced rainfall due to climate change. This will make droughts more frequent and severe, affecting agriculture. Higher temperatures can damage crops while lower rainfall increases the need for irrigation. Despite that irrigation has sustained agriculture in arid regions, competition for water is increasing. To adapt to current challenges, improvements in irrigation practices, adoption of agro-ecological techniques and use of drought-tolerant crops are essential to stabilise crop performance. The olive tree (Olea europaea L.) is one of the most widely cultivated crops in the Mediterranean. Although it is considered a drought-tolerant species, different cultivars do not respond equally to water stress. In this study, 18-month-old plants of three olive cultivars (Giarraffa, Leccino and Maurino) were grown in a growth chamber and monitored during one month of drought stress. ‘Maurino’ and ‘Leccino’ are mainly grown in central Italy and are derived from local oleasters, while ‘Giarraffa’ is locally adapted to Sicily and probably introduced from Spain or Morocco. Given their long-term adaptation to different environments, it was hypothesised to use them as representatives of different response patterns of the olive plant with the aim of evaluating and differentiating their drought tolerance. A multi-level approach was used to evaluate physiological changes, metabolic profiles, biochemical responses and anatomical observations. All cultivars showed significant physiological responses within the first two weeks of stress, while metabolomic and biochemical responses were more pronounced at the end of the stress. In particular, the cultivars exhibited different coping mechanisms: ‘Giarraffa’ adopted a "drought avoidance" strategy with early stomatal closure and investment in stem osmoregulation. In contrast, ‘Maurino’ adopted a “water-consuming” strategy with high initial water use and high investment in phenolic antioxidant compounds, which did not prevent a dramatic increase in electrolyte leakage. ‘Leccino’ shared the responses of the other two cultivars, maintaining a basal carbon fixation and investing in osmoprotectant molecules which confer drought tolerance. These findings highlight the importance of considering the timing and integration of observed responses within the plant. This study provides valuable knowledge on drought tolerance mechanisms in olive cultivars, supporting future breeding programs and water management strategies. Future research should investigate mature trees in field settings during the reproductive stage to assess impacts on fruit and oil quality.