## 4 Discussion
Furthermore, the application of foliar Si significantly increased the Si content in leaves, which is in agreement with recent findings [47]. According to [PERSON] [48], other minerals were at optimum levels. For some elements, such as calcium and iodine, genotype differences were more pronounced than the effect of Si foliar application. The concentration of copper in Istarska Bjelica leaves was found to be higher than that in the Leccino cultivar, as previously shown by [PERSON] et al. [37]. The mineral content variability of different genotypes has been previously confirmed in various studies [37; 38; 49]. Oleuropein, as a major polyphenolic constituent in olive leaves, is known to have various pharmacological functions [50]. Human cervical cancer cells (HeLa) were shown to be sensitive to oleuropein as it stopped their cell cycle at the G2/M phase in oleuropein concentrations of 150-200 \(\upmu\)M and induced apoptosis through mitochondrial apoptotic cascade [51]. OL extract (OLE), rich in polyphenols, successfully reduced the viability of HeLa cells with a decrease in cyclin D1 gene expression and an increase in p21 expression, suggesting its involvement in the intrinsic apoptosis pathway [52]. Here, extracts from both cultivars were effective in inhibiting the growth of HeLa cells with an IC\({}_{50}\) that differs between the cultivars and the Si treatment. Two of the other most abundant polyphenols in olive leaves in our research were luteolin and oleanolic acid, and both have been shown to inhibit cancer cell growth. Luteolin has been shown to induce autophagy in U2 OS cells by sensitising doxorubicin (DOX)-mediated autophagy signalling. Luteolin alone also led to a cell death mechanism, but less efficiently [53]. A study by [PERSON] and [PERSON] [54] showed a significant cytotoxic effect of luteolin on HCT-15 adenocarcinoma cells with increased ROS production, leading to apoptosis and DNA fragmentation. Oleanolic acid, which was the second most abundant polyphenolic species in this research, was shown to cause dose-dependent HCT116 colon cancer cell death and inhibited tumour growth in colon cancer xenograft mice [55]. Oleanolic acid was found to be cytotoxic to several cancer cell lines, MCF-7, DU145 (prostate cancer) and U87 (glioblastoma). It increased the gene expression of p53, cytochrome c, Bax, PARP1, and Cas-3, indicating its ability to activate apoptosis ([PERSON] et al., 2018) [56]. OL extract with a high terpenoid content and a high concentration of oleuropein showed a dose-dependent inhibition of HCT116 cell growth with IC\({}_{50}\) values of 97.06 \(\upmu\)g/mL, according to [PERSON]. [PERSON] et al. 2021 [57]. The highest inhibitory effect of extracts from Istarska Bjelica is surprising, as most polyphenols in the leaves of this cultivar had lower concentrations. However, the higher concentrations of rutin and oleanolic acid, which have known anti-cancer properties [58; 59], could explain this phenomenon. The inhibition of HeLa, HCT116, and U2 OS cell growth in the present study was concentration-dependent, as Si3 treatment showed higher inhibition rates and significantly higher concentrations of various polyphenols (including the most abundant ones). These results suggest that foliar application of Si could be used to increase the production of certain polyphenols in olive leaves, improving the already high inhibitory effect of OL extracts on cancer cells. Further research could investigate the synergistic effects of OL extracts with systemic therapies.