Silicon Foliar Fertilisation Ameliorates Olive Leaves Polyphenolic Compounds Levels and Elevates Its Potential towards Different Cancer Cells

It is not yet clear how adding silicon foliar fertilisation affects olive leaf (OL) phenolics and their potential to impact different cancer cells. Thus, we conducted a field trial to study the effect of foliar Si biostimulant fertilisation on the OL phenolic content of the ‘Leccino’ (LE) and ‘Istarska Bjelica’ (IB) cultivars. The experiment compared untreated Control (C) and three distinct levels of silicon (Si1, Si2, Si3) with Si concentrations of 0.55 g/L, 1.1 g/L, and 2.2 g/L, respectively. Si3 application resulted in the highest levels of oleuropein, apigenin-7-O-glucoside, luteolin-4-O-glucoside, rutin, and tyrosol compared to the C treatment. The polyphenols showed high cytotoxic activity in three cancer cell lines tested: cervical adenocarcinoma (HeLa), colon cancer (HCT116), and osteosarcoma (U2OS). The strongest inhibition of cell growth was observed in the HCT116 cell line. All cancer cells tested were more sensitive to treatment with polyphenols isolated from plants with added Si than those without added Si. The cytotoxic activity of the extracts on the healthy cell line RPE1 was similar to that on the cancer cell line HCT116 and U2OS.