Climate change in recent years, especially increasing temperatures, has led to negative effects on growth, yield, and even the quality of fruits. This study investigated the impact of heat stress on key physiological and biochemical traits of olive fruit. Five olive genotypes (Ds13, Bn2, Dz4, T3, B1) and three cultivars (Amphysis, Amygdalolia, Zard) were grown throughout the fruit growth period (May-October) in 2024. A complete randomized block design with four harvest dates (May 4th, July 5th, August 5th, October 6th) was employed. Results showed a signficant decline in stomatal conductance, photosynthetic rate, sub-stomatal CO2, chlorophyll, calcium, and potassium during the summer months (July-August), followed by a recovery in October. Conversely, heat stress resulted in increased levels of electrolyte leakage (26.83%), malondialdehyde (3.88 nmol g−¹FW), sodium (0.38%), proline (50.61 µg g−¹FW), soluble carbohydrates (15.1 mg g−¹FW), total phenols (134.91 mg 100 g−¹FW), transpiration rate (peak of 3.79 mmol m2 s−¹), peroxidase (3.76 unit mg−¹) and catalase (1.54 unit mg−¹) activities, with the highest levels observed in August. Among genotypes, Bn2 displayed the strongest sensitivity to heat stress, with the highest electrolyte leakage (25.88%), malondialdehyde (3.42 nmol g−¹FW), and sodium (0.34%) levels. In contrast, Dz4 stood out for the exceptional thermotolerance, exhibiting the highest stomatal conductance (0.19 mol m2 s−¹), transpiration rate (5.54 mmol m2 s−¹), chlorophyll content (1.31 mg g−¹FW), soluble carbohydrates (13.72 mg g−¹FW), proline (38.41 µg g−¹FW), total phenols (148.23 mg 100 g−¹FW), peroxidase activity (2.97 unit mg−¹), catalase activity (1.29 unit mg−¹), and copper concentration (0.06%). These findings suggest Dz4 as a promising genotype for mitigating the detrimental effects of heat stress in olive production.
