Comparative Effects of Silicon and Silicon Nanoparticles on Photosynthetic Performance, Antioxidant Capacity, and Na/K Homeostasis in Cucumber under Salt Stress

Soil salinization severely restricts plant growth and threatens food safety. Silicon nanoparticles (SiNPs) can alleviate salt stresses, while the mechanisms remain unclear. Aiming for targeted application of SiNPs under soil salinization, silicate (Si) was selected as reference Si source, and the effects of Si and SiNPs on plant growth, photosynthetic performance, antioxidant performance, and ionic homeostasis in cucumber under salt stress were comprehensively investigated with the usage of principal component analysis (PCA) and transcriptome sequencing. In general, salt stress induced significant growth inhibition in cucumber, with decreased biomass and root morphological parameters, while both Si and SiNPs alleviated growth inhibition in cucumber. In addition, Si and SiNPs application at high concentration more effectively enhanced Na/K homeostasis, and low concentration application of Si and SiNPs induced a more significant regulation on antioxidant enzyme activities. In addition, SiNPs induced a more extensive regulation on gene expression network than Si in response to salt stress in cucumber, which was supported by the results of enrichment analysis. Moreover, correlation analysis between key modules of weighted gene co-expression network analysis (WGCNA) and physiological traits implied that genes related to chloroplast functions, redox balance, and membrane transport processes could be the key targets of SiNPs. Overall, the results showed that Si and SiNPs promoted salt stress resistance in cucumber with a dose-dependent pattern. SiNPs application more effectively promoted cucumber biomass and regulated gene expression network than silicate under salt stress, while several key target genes of SiNPs in modulating salt stress in cucumber were also identified. The results would provide a basis of targeted application of SiNPs under soil salinization.