Foliar-applied silicon nanoparticles mitigate cadmium stress through physio-chemical changes to improve growth, antioxidant capacity, and essential oil profile of summer savory (Satureja hortensis L.)

The use of nanofertilizers is a promising method for enhancing plant productivity and mitigating environmental pollution. The aim of this study was to show the silicon nanoparticles (Si-NPs) effects on growth, water content, phenolic and flavonoid content, and essential oil (EO) profile in aerial parts of summer savory (Satureja hortensis L.) in cadmium (Cd)-contaminated soil. The pot experiment was conducted with Cd concentration (0, 10, and 20 mg Cd kg−1 soil) and foliar application of Si-NPs (0, 0.75, 1.5 and 2.25 mM of SiO2). The results revealed severe Cd stress (20 mg Cd kg−1 soil) significantly decreased root and shoot weight, relative water content (RWC) but increased Cd accumulation in roots and shoots and proline concentration. Moderate Cd level (10 mg Cd kg−1 soil) improved total phenolic content (TPC), total flavonoid content (TFC), and EO content. Under Cd stress, the 1.5 or 2.25 mM Si-NPs were highly effective concentrations to improve the growth and EO yield. The main EO constitutes were carvacrol (43.12–57.6%), γ-terpinene (20.56–25.6%), p-cymene (5.53–11.3%), and thymol (1.65–8.2%) with changes in their concentrations under Cd and Si-NPs. Heat map analysis (HMA) showed fresh weight, dry weight, and EO yield with a higher variation during these treatments had the most significant impact on distinguishing the clusters. The present study recommended 1.5–2.25 mM Si-NPs in improving the physio-biochemical status of summer savory plants to cope with Cd stress.