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.

This study investigates the effect of combined sulfur (S) and nitrogen (N) foliar fertilization on leaf S and N concentration, as well as on the growth of olive fruit and on the quantity and quality of olive oil, obtained from two olive cultivars ‘Istarska bjelica’ and ‘Leccino’ in two consecutive years. S and N are some of the most important nutrients, and both play a crucial role in plant oil production. The here-reported fertilization program significantly increased S concentration in leaves without affecting N concentration, which led to an increase in fruit yield and improvement of all fruit morphological parameters. The best oil yield per tree was obtained under the treatment with the highest S/N dose. Oil quality was not affected by S and N supply, and this allowed us to classify all our oil samples as extra virgin (EVOO). Regarding the content of total phenols (TPC) and composition of fatty acid methyl esters (FAME), they remained unaltered under the applied treatments. All investigated fruit morphological parameters, as well as fruit and oil yield, were highly cultivar-dependent. ‘Istarska bjelica’ was characterized as a cultivar with higher fruit mass and pulp percentage, while its stone parameters were lower than those of ‘Leccino’. Consequently, the extraction oil yield obtained from ‘Istarska bjelica’ fruits was much higher. Moreover, environmental conditions had a great impact on fruit and oil quantity. The here-obtained results led us to the conclusion that supply of S and N can enhance oil production without affecting its nutritional quality, a finding that could generate large long-term effects on economic growth in the olive oil sector.

Up to date, there are no reports on the effects of combined sulfur (S) and nitrogen (N) foliar application in olive orchards on volatile compounds and sensory characteristics of virgin olive oil (VOO). In this work, the effects of increasing the fertilizer S and N concentration on volatile compound composition and odor and taste attributes of monovarietal VOOs of Istarska bjelica and Leccino cultivars were investigated. The volatile compounds were analyzed by gas chromatography with flame ionization and mass spectrometric detection after isolation by solid-phase microextraction, while sensory attributes were assessed by a professional panel. In all the investigated VOO samples, aldehydes were the most represented group of compounds, with (E)-2-hexenal as the most abundant, followed by (Z)-3-hexenal and 1-penten-3-one. Significant differences in the volatile profiles and sensory attributes were observed between VOOs from different treatments, mainly attributable to the interactions between all the factors that were investigated: treatment, cultivar, and year. Although significant interactions were noted for sensory attributes as well, the score for overall quality was generally lower for Leccino VOOs and for VOOs from both cultivars that were obtained after the highest SN dose treatment. All the samples were graded by overall sensory scores that were higher than eight and were characterized by well-pronounced fruitiness and the absence of sensory defects. The results of this study showed that foliar application of S and N significantly affects the quality of VOO, confirming that such a practice can modulate the characteristic olive oil odor and taste attributes and thus possibly influence its acceptability and preference among consumers.

The must used to make sparkling wine has a low pH value and moderate sugar content, and its potassium content can have a strong influence. An excess of potassium often leads to an insufficient supply of magnesium, since potassium has a strong antagonistic effect on magnesium, and, consequently, to poorer photosynthesis and a poorer quality of the must. The aim of this study was to determine whether the application of foliar fertilizers based on magnesium, phosphorus, and amino acids could reduce leaf potassium content, affecting the reductions in sugar content and pH value and increasing the total acidity of the must. A fertilizer trial with three replicates was conducted on the cultivar Istrian Malvasia (TCtrl—NPK, TMg—NPK + Agromag (6% MgO), TMgP—NPK + Agromag + Fosforo (30% P₂O₅) i TMgPBS—NPK + Agromag + Fosforo + Bio Prot) in a random complete block design. The NPK fertilizer was applied in the autumn. Foliar fertilization was applied three times during the growing season (31 May, 7 July, and 22 August 2014), and leaf samples were collected for leaf analysis before each application. The results show that foliar fertilization significantly reduced leaf potassium content, especially when treated with magnesium alone (treatment TMg). In addition, foliar fertilization significantly lowered the pH and increased the sugar content of the must. The results obtained in this research give a scientific contribution to the creation of fertilizer treatments for vines with a positive effect on the basic chemical composition of the base wine and provide a good basis for further research in reducing the use of certain enological practices during production.