Summer Stress Mitigation in Rainfed Olive Trees Across Multiple Sites: Comparative Effects on Yield and Oil Quality of Glycine Betaine, Kaolin, and Calcium Carbonate in “Koroneiki” and “Lianolia Kerkyras” Cultivars

Olive tree (Olea europaea L.) is a major Mediterranean crop, valued for both fruit yield and high-quality oil, yet extreme summer stress, including high temperature, intense irradiance, and water limitation, can substantially reduce productivity and affect oil composition. The objective of the present study was to evaluate the mitigating efficacy of foliar applications of glycine betaine (GB), kaolin (K), and calcium carbonate (CC) under rainfed conditions across three Greek sites on “Koroneiki” (in two sites) and “Lianolia Kerkyras” (in one site) cultivars. Treatments were applied during the summer, and effects on fruit yield, oil content per fruit, oil yield per tree, and key oil quality parameters—including total phenols, flavonoids, antioxidant capacity, and fatty acid composition—were assessed. GB significantly enhanced fruit yield and oil production for “Koroneiki” at the site with the harshest environmental conditions (24.37 kg fruits per tree and 4.69 kg of oil per tree compared to 19.16 kg fruits per tree and 3.48 kg of oil per tree in control). In contrast, K proved most effective at the other two sites for both cultivars (43% and 52.8% increase in fruit yield and oil mass per tree in “Koroneiki” respectively and 30% as well as 34% increase in yield and oil mass per tree in “Lianolia Kerkyras”, respectively. CC exhibited limited impact on both productivity and quality. Under all treatments, the oils produced could be classified as extra virgin olive oils, with the products exhibiting minor effects on the functional properties of the oils. These findings indicate that the efficacy of stress-alleviating foliar treatments is strongly influenced by both environmental conditions and cultivar. Overall, K was the most effective treatment, followed by GB. Tailored application of these treatments represents a sustainable approach to maintaining olive productivity and preserving oil quality in the context of climate change.