Optimising Olive Leaf Phenolic Compounds: Cultivar and Temporal Interactions

All olive (<i>Olea europaea</i> L.) plant tissues have a high phenolic content. However, the effects of the cultivar and sampling period on the tissue phenolic content remain almost unknown; in addition, the interactions between nutrient uptake and leaf phenol concentrations have not been clarified. This study sampled olive leaves to explore how the cultivar, sampling period, and their interaction affect leaf phenol and nutrient concentrations. Leaves were collected from six cultivars during three seasonal periods: harvest (October; SP1), dormancy (January; SP2), and pruning (March; SP3). Five were Istrian cultivars ('Bova', 'Buža muška', 'Buža puntoža', 'Istarska bjelica', 'Rošinjola'), and one was the Italian cultivar 'Leccino'. Phenolic profiles in olive leaves were correlated with potassium (K), phosphorus (P), and copper (Cu) concentrations. However, significant correlations between these nutrients and oleuropein, verbascoside, and total phenolic content (TPC) were determined only for 'Rošinjola'. Oleuropein was the most abundant phenolic compound, while among genotypes, 'Buža muška' showed the highest oleuropein levels across all sampling periods, indicating its potential source of oleuropein in olive leaves. Seasonal variations in olive leaf phenolic compounds appear to be strongly influenced by phenological phase, nutrient dynamics, and weather conditions, as confirmed by multivariate analysis across sampling periods and cultivars. The findings emphasise the importance of selecting both an appropriate cultivar and sampling period to maximise the accumulation of olive leaf phenolic compounds. Nevertheless, long-term experimentation on cultivars with a high leaf phenolic potential, like 'Buža muška' and 'Rošinjola', is necessary in order to develop appropriate farming strategies for maximising phenolic compounds with human or plant health benefits.