Effects of SiO2 Nanoparticles on the Yield and Quality of Sophora tonkinensis Under Drought Stress

This study investigates the novel application of silicon nanoparticles (SiO2 NPs) to enhance drought tolerance and medicinal quality in the threatened medicinal plant Sophora tonkinensis, providing technical support for its conservation and cultivation. Six treatments were applied: control (CK), CK + 100 mg/L SiO2 NPs, CK + 200 mg/L SiO2 NPs, drought stress (SD), SD + 100 mg/L SiO2 NPs, SD + 200 mg/L SiO2 NPs. After 21 days of foliar application, we assessed biomass, physio–biochemical parameters (including soluble protein, soluble sugar, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), malondialdehyde (MDA), superoxide anion (O2.-), and hydrogen peroxide (H2O2)), as well as concentrations of matrine, oxymatrine, genistin, genistein, total alkaloids, and total flavonoids. Under drought stress, the application of 100 mg/L SiO2 NPs was the most effective treatment for enhancing biomass accumulation and eliciting a coordinated physio–biochemical response. This was demonstrated by a significant increase in leaf soluble protein content and root SOD activity, along with a decrease in oxidative stress markers (H2O2 and O2.-). Furthermore, SiO2 NPs application under both normal and drought conditions selectively enhanced the accumulation of bioactive compounds in the roots, with the optimal concentration being compound-specific. Notably, under drought conditions, the application of 200 mg/L SiO2 NPs proved optimal for enhancing the biosynthesis of several key medicinal compounds in the roots. Specifically, this treatment significantly maximized the content of matrine (214.15 μg/g), genistin (4.06 μg/g), genistein (48.56 μg/g), total alkaloids (9.96 mg/g), and total flavonoids (11.44 mg/g) compared to the drought-stressed control (SD). These results demonstrate that SiO2 NPs significantly improve yield and key medicinal components of S. tonkinensis under drought stress, with a differential efficiency depending on the concentration, plant organ, and target compound.