Metabolic Responsiveness of Olive (Olea europaea L.) Cultivars Following Foliar Silicon Application: A Comparative LC–MS/MS Study

Foliar silicon (Si) application can affect phenolic compounds in olive leaves, but little is known about the onset and duration of this response or the extent to which it depends on cultivar. Therefore, this study examined short-term changes in secondary metabolism in ‘Istarska Bjelica’ and ‘Leccino’ at the onset of spring vegetative growth. Four biological replicates were analysed per cultivar × treatment × sampling period combination, and the data were evaluated by three-way factorial ANOVA. Thirty-four phenolic compounds were analysed at 1, 8 and 15 days after treatment. One day after treatment, oleuropein concentration was 30.1% higher compared with the control during the same sampling period and the sum of all other identified phenolic compounds (TP-Ole) by 12.5% irrespective of sampling period. Overall, ‘Istarska Bjelica’ contained 16.5% more TP-Ole than ‘Leccino’. The oleuropein response did not persist beyond day 8. The treatment, considered as a main factor, was generally associated with lower concentrations of several low-abundance phenolic compounds, including apigenin, luteolin, 4-hydroxybenzoic acid, and oleacein, as well as the more abundant apigenin-7-glucoside in ‘Istarska Bjelica’. By contrast, verbascoside showed a nominally significant cultivar-dependent response, with its concentration increasing following the treatment only in ‘Istarska Bjelica’ but this interaction was not retained after correction. Such responses were observed in fewer compounds than previously reported, which may reflect the applied element and the duration of the post-treatment period, which were not compared experimentally in this research. Nominally significant cultivar × treatment × sampling period interactions were found for three flavonoids only, and none was retained after correction. Multivariate analysis ranked cultivar as the dominant source of variation in the phenolic profile (35.2%), followed by sampling period (8.3%) and treatment (5.0%). A single foliar treatment was therefore associated with selective and temporary changes in the phenolic profile, without uniformly increasing all phenolic groups.