Application of silicon nanoparticles enhances oxidative stress tolerance in salt stressed 'Valencia' sweet orange plants

Silicon (Si) can have several beneficial effects when exogenously applied to plants, including increased tolerance against several abiotic and biotic stressors. The current study investigated the role of nanosized, monodisperse silicon particles in alleviating the harmful impact of excess sodium chloride (NaCl) on 'Valencia' sweet orange. One-year-old 'Valencia' (Citrus x sinensis L. Osbeck) plants budded onto Kuharske (Citrus sinensis L. Osbeck x Poncirus trifoliata L. Raf.) rootstock were irrigated with 0, 60 or 120 mM sodium chloride (NaCl) solution for six months. We observed an incremental reduction in plant growth with increased NaCl application. Severe foliar drop and damage were observed following 60 and 120 mM NaCl application. Silicon nanoparticles applied by direct foliar application at four concentrations (0, 200, 400 and 600 mM) to these stressed plants reduced the damage in the greenhouse. Additionally, SiO2-NP application improved root growth, increased chlorophyll content, and alleviated the osmotic effect on all NaCl-treated plants compared to the control stressed plants. Furthermore, expression of Na+ cotransporter (CsSOS1, CsSOS2, CsSOS3 and CsNHX1) and aquaporin (CsPIP1;1, CsPIP2;3, and CsTIP4;1) transcripts was enhanced in the root tissues following SiO2-NP application. These results support the positive effect of exogenous Si application at 400 mM on salt-stressed citrus plants by improving photosynthesis, water status and regulating ion content. Consequently, SiO2-NPs can be applied as part of an integrated management program to improve overall citrus plant health, especially under stressful saline conditions.