Co-Application of Silicon with Selenium, Sulphur, Zinc, and Iron in Plants: Mechanisms of Stress Tolerance, Nutrient Homeostasis and Secondary Metabolism.

While individual Si-nutrient interactions have been reviewed separately, a comparative analysis of multiple Si-element interactions remains lacking. This review compares current knowledge on co-application of Si with selenium (Se), sulphur (S), zinc (Zn) and iron (Fe), focusing on stress tolerance, nutrient homeostasis, physiological responses, secondary metabolism and agronomic relevance. The evidence indicates that combining Si with these elements helps maintain reactive oxygen species (ROS) homeostasis, strengthen antioxidant defenses, stabilize photosynthetic function and improve nutrient uptake, translocation and use efficiency. Responses depend on plant species, nutrient form, application strategy and environmental conditions; at the metabolic level, Si-based combinations affect the synthesis of phenolic compounds, amino acids and sulphur-containing metabolites. Si-Se and Si-Fe proved most effective under heavy-metal stress, through regulation of metal transport, detoxification and sequestration, and Si-S and Si-Zn under drought, salinity and nutrient-deficient conditions, by enhancing osmotic adjustment, nutrient-use efficiency, ionic homeostasis and photosynthetic performance. Agronomically, these interactions can increase crop productivity, nutritional value and biofortification potential, and mitigate toxic-element accumulation in edible parts. Knowledge gaps remain regarding molecular regulation, variability among species and environments, and the long-term effectiveness of nanoparticle formulations. Since most evidence comes from hydroponic, pot and greenhouse studies, standardized field experiments are needed to assess agronomic relevance.