## 4 Discussion  
OL extract has been historically used as a folk remedy against fevers, malaria, and other diseases [31]. The medicinal properties of olive leaves are usually attributed to polyphenols as secondary metabolites. Many studies have shown that these OL compounds have various benefits, such as antiproliferative and cytotoxic effects on cancer cells, as well as radio-protective properties, anti-fungal activity, and cardioprotective effects [32]. Besides human health benefits, polyphenols in olive leaves serve a role in plant defence mechanisms against biotic and abiotic stressors such as _Plytophthora_ sp. [31] and _Verticillium dahliae_ [PERSON]. [33], heavy metal salts, UV-irradiation, and temperature [1,34]. The content of polyphenols in leaves tends to change; it is affected by genetic, agronomic, and environmental factors [35]. Differences in polyphenol content in different olive cultivars have been investigated in leaves [36; 37; 38] and oil [39; 40], and changes in phenolic content due to environmental stress have also been addressed [41; 42]. Plant tolerance and/or resistance to different kinds of stress could be attributed to nutritional status. Silicon is the second most abundant element in Earth's crust but still a non-essential element for plant growth. However, it is considered beneficial for plants, as it has a role in pest and disease control and enhances the deposition of phenolic compounds at infection sites [43]. Research on the relationship between silicon and polyphenol content has mainly focused on plants under biotic or abiotic stress factors. [PERSON] et al. [44] showed that Si alone did not affect phenolics status in rice plants, while in plants inoculated with _Rhizoctonia solani_, Si application significantly increased phenolic concentrations. Similarly, drought-stressed wheat supplied with Si showed an increase in total phenolics and flavonoids [20]. [PERSON] et al. [45] showed that silicon supplementation affected the phenolic content of grasses depending on tissue function (photosynthesis or stabilisation), where increased phenolic content was recorded in grass culm. Our results indicate that, in olive, Si foliar application causes a significant increase in the content of several polyphenols in leaves, namely oleuppein, tyrosol, luteolin, apigenin-7, rutin, etc. The variability of polyphenols in olives due to genotype differences has been widely researched. Among factors such as minimum and maximum temperature, humidity, sunshine hours, and mineral nutrition, olive variety was found to be the most influential on polyphenol content [46]. It was shown by [PERSON] et al. [36] that Istarska Bjelica had the lowest content of polyphenols out of three autochthonic cultivars from Istria (the other two being Buza and Rosinjola). Our results showed that Istarska Bjelica had significantly lower values of some of the detected polyphenols, e.g., 60% difference for ole-aglycone, 36% for rutin, 33% for luteolin, and 30% for tyrosol, when compared to the Leccino cultivar. However, oleanolic acid levels, as an important triterpene in OL, showed 47% higher values in Istarska Bjelica. There was no significant difference in oleuppein content between cultivars used in this research, which is consistent with previously reported results in the harvest period for the two cultivars studied [37]. The clear separation of genotypes in the PCA (Figure 2) shows the differences in polyphenic content (as well as in mineral composition) between them, but a much closer grouping of points for Istarska Bjelica shows that this cultivar is less sensitive to external factors regarding polyphenic production. In our previous research, the same observation has been reported [37]. Silicon leaf concentrations in the control of this experiment were at 123.26 (\(\pm\)6.36 SE) mg kg\({}^{-1}\), which is in accordance with levels reported by [PERSON] et al. for the Arbequina and Picaul cultivars [22].