Farming System, Cultivar, and Sampling Date Shape the Phenolic and Mineral Profiles of Olive Leaves

Olive leaves are a by-product of olive farming and have a high phenolic content. However, the effect of farming system, cultivar, and harvest time on their content is poorly studied. In June, August, and October 2022 (108 samples), leaves of trees ‘Istarska bjelica’, ‘Leccino’, and ‘Oblica’ within an organic and integrated grove were sampled. From these samples, the seven main phenolic compounds and twelve mineral elements were quantified. Data were analysed by three-way ANOVA with tree as a random effect. All 19 compounds and elements showed at least one significant interaction and 14 elements and compounds showed a significant three-way interaction; thus, no factor could be interpreted in isolation. Eight of the ten elements with a three-way interaction showed a reversal of system difference with cultivar–date combinations. Five of the six phenolics with a farming system effect were 26% to 65% higher with organic management; in contrast, apigenin-7-O-glucoside was 37% lower. Sampling date predominantly affected the secoiridoids: oleuropein and oleacein increased 5.9- and 15.6-fold, respectively. Phenolic compounds and mineral elements were correlated: boron correlated negatively with oleuropein (r = −0.76), whereas silicon correlated positively with oleacein (r = 0.46). To standardise olive leaves as a bioactive source, cultivar, harvest date, and farming system must be specified.