Comparative Metabolic Fingerprinting of Olive (Olea europaea L.) Cultivars Under Boron Foliar Fertilisation

Olive (Olea europaea L.) cultivars often exhibit genotype-specific responses to micronutrient management. In this study, we investigated the metabolic leaf fingerprinting of three cultivars ‘Rošinjola’, ‘Leccino’, and ‘Istarska bjelica’ at two sampling periods (SP-I = 64 days after treatment (DAT) and SP-II = 118 DAT), following boron foliar fertilisation (+B = 41.62 mM B; −B = 0 mM B) applied 50 days after anthesis. To our knowledge, this is the first study to provide such a detailed evaluation of boron-induced shifts in phenolic metabolism in olive leaves. At harvest (SP-II), all three cultivars showed higher concentrations of total identified phenolic compounds in +B plants compared with the −B controls. Notably, the concentration of verbascoside at harvest was higher in +B plants of ‘Istarska bjelica’ and ‘Leccino’, but not in ‘Rošinjola’. Oleuropein content increased in +B plants at harvest to a level higher than 4870 mg/100 g DW, irrespective of cultivar. Conversely, apigenin-7-glucoside declined from SP-I to SP-II in ‘Leccino’ regardless of treatment, whereas in ‘Istarska bjelica’, this decrease occurred only in control plants, with boron preventing the seasonal decline. These findings confirm the prolonged effect of boron foliar fertilisation on phenolic metabolism in olive leaves and highlight cultivar-specific differences in metabolic responses. Further research is needed to clarify how these metabolic shifts relate to primary plant metabolism and how they influence olive oil quality traits among cultivars grown under Croatian conditions.