Nanoparticles as modulators of stress tolerance: Physiological and molecular insights into TiO₂ and ZnO effects in Cucumis melo L. subjected to salt shock

Nanoparticles (NPs) have emerged as innovative tools in agriculture to enhance plant productivity and stress tolerance under challenging conditions. This study aimed to evaluate the short-term effects of TiO₂ and ZnO-NPs on melon plants ( Cucumis melo L. ) under control and salt stress shock conditions. Plants were treated with NPs via root application in hydroponically system, and physiological, biochemical, and molecular responses were analysed. TiO₂-NPs increased biomass, enhanced water potential, and the maintained photosynthetic efficiency during salt shock. In contrast, ZnO-NPs did not promote growth but triggered protective responses, including reduced lipid peroxidation and improved membrane stability. Transmission electron microscopy confirmed NPs localization in root and leaves, primarily near membranes and within vacuoles, suggesting their involvement in transport and redistribution mechanisms. NPs up-regulated root aquaporins, particularly CmNIP5;1 , correlating with improved water transport and potential. TiO₂-NPs enhanced Fe redistribution in leaves, while ZnO-NPs reduced Cu levels and triggered an increase in Ca under salinity. Additionally, TiO₂-NPs promoted phenolic compound accumulation, enhancing antioxidant defences, whereas ZnO-NPs reduced these metabolites. In conclusion, TiO₂ and ZnO-NPs modulate key physiological and biochemical responses, improving stress tolerance and nutrient dynamics. These findings highlight their potential as innovative tools for sustainable agriculture and warrant further investigation into their mechanisms of action. • Melon plant responses to stress differ according to NP type. • TiO₂ and ZnO NPs applied to roots reach the leaves and are close to membranes. • The application of both NPs resulted in an improvement in water status. • TiO₂ and ZnO NPs affect nutrients and phenolics in normal and stressed plants.