Long-Term Metabolic Responses of Olive to Bacterial and Fungal Inoculation Differ Between Cultivars

Olive leaves represent a metabolically active tissue that plays an important role in plant responses to biotic stress. The present study comprised two independent experiments investigating biochemical responses of olive leaves to bacterial and fungal challenge under controlled conditions. Changes in primary metabolites (sugars, organic acids and free amino acids), phenolic compounds and lipid peroxidation were analyzed using chromatographic and spectrophotometric methods. In the bacterial experiment, pronounced differences were observed in primary metabolism. Tryptophan concentrations ranged from approximately 50 mg kg−1 DW to more than 360 mg kg−1 DW in ‘Istarska bjelica’, while sucrose concentrations reached up to 87 g kg−1 DW, demonstrating cultivar-dependent differences in carbohydrate metabolism. Phenolic profiling showed that secoiridoids were the dominant phenolic class, with oleuropein concentrations exceeding 27 g kg−1 DW across bacterial treatments. In the fungal experiment, amino acids showed greater variability than sugars and phenolic compounds, whereas MDA concentrations ranged from approximately 190 to 300 nmol g−1 DW but did not differ significantly among pathogen treatments. Overall, the two experiments showed distinct patterns of metabolite variation associated with bacterial and fungal challenge. These findings contribute to a better understanding of cultivar-dependent metabolic responses and provide a basis for future studies of olive–microbe interactions.