Foliar Nutrition Strategies for Enhancing Phenolic and Amino Acid Content in Olive Leaves

Studies on selenium (Se) and silicon (Si) foliar biostimulation of different plants have been shown to affect concentrations of phenolic compounds. However, their effects on olive (Olea europaea L.) primary and secondary metabolites have not been fully investigated. Therefore, the effects of foliar sprayed Si and Se and their combination on the concentration of phenols, selected metabolites involved in the phenol biosynthesis, and mineral elements concentrations were determined in olive leaves of the field-grown cultivar Leccino. During the summer period, leaves were foliar sprayed three times, after which were sampled 30 days after the corresponding application. In general, foliar treatment of Si or Se increased the concentrations of several predominant phenolic compounds, such as oleuropein, oleacein, and specific flavonoids. The effects were especially pronounced after the third application in the harvest time sampling time. Amino acids and other phenol precursors were also significantly affected. The effects were phenol-specific and depended on the treatment, sampling time, and treatment × sampling time interaction. The response of verbascoside to the applied treatments appeared to be closely linked to corresponding changes in its amino acid precursors, such as tyrosine, while its connection with tryptophan and IAA has to be cautiously considered. In contrast, for other phenolic compounds like secoiridoids, a clear interdependence with their precursors was not identified, likely due to the more complex nature of their biosynthesis. The effects on the concentrations of elements other than Se and Si were milder.