Silicon Dioxide Nanoparticles Mitigate PEG-Induced Drought Stress in Carya cathayensis by Improving Physiological Characteristics and Ultrastructure

Drought frequently threatens the yield and quality of Carya cathayensis Sarg. cultivated in mountainous regions. To search for effective drought-resistant regulators is of great significance for alleviating short-term seasonal drought in C. cathayensis during dry seasons, thereby stabilizing its yield and quality. Silicon dioxide nanoparticles (SiO2 NPs) mitigate abiotic stress in plants. To give insight into the regulatory role of SiO2 NPs in mitigating drought stress, polyethylene glycol 6000 (PEG-6000) was used to simulate varying degrees of drought conditions, and the growth phenotype, photosynthetic physiological characteristics, antioxidant defense system, and cellular ultrastructure of C. cathayensis leaves were analyzed to evaluate the impacts of foliar-applied exogenous SiO2 NPs. The results indicated that, compared with severe drought, 200 mg/L SiO2 NP application to plants under severe drought treatment significantly increased superoxide dismutase and peroxidase activities and chlorophyll and nitrogen contents, while malondialdehyde levels decreased. Furthermore, SiO2 NP application notably enhanced the net photosynthetic rate, stomatal conductance, and electron transport efficiency. This effectively alleviated both stomatal and non-stomatal limitations, thereby mitigating drought-induced photosynthetic inhibition. Additionally, Transmission electron microscopy revealed that SiO2 NPs effectively preserved the structural integrity of chloroplasts, mitochondria, and nuclei, reducing drought-induced ultrastructural damage. In conclusion, exogenous SiO2 NPs enhance drought tolerance in C. cathayensis by synergistically modulating photosynthesis, antioxidant defense, and cellular structural stability.