Superior efficacy of leaf over root application of nanozymes in enhancing cotton salt tolerance is associated with the maintenance of ROS homeostasis and K⁺/Na⁺ ratio

• Both foliar spray and root application of PNC&PMO nanozymes can improve cotton salt tolerance. • Foliar spray exhibits superior performance due to the higher delivery efficiency of nanomaterials. • These utilization ways maintain ROS homeostasis, regulate antioxidant enzyme activity and increase K + /Na + ratio. • Foliar spray boosts the activity of SOD and POD, and promotes the extrusion of Na + . • Root application enhances the activity of CAT and improves the uptake of K + . Plant nanobiotechnology has been widely used for enhancing crop stress tolerance. However, the comparative efficacy of foliar spray and root application of nanomaterials to plant is rarely known, which is critical for the practical application of nanotechnology in sustainable agriculture. In this work, we investigated the effects and mechanisms of two reactive oxygen species (ROS) scavenging nanozymes, i.e. polyacrylic acid coated CeO 2 and Mn 3 O 4 nanoparticles (PNC & PMO), by foliar spray or root application on improving salt tolerance in cotton. Our results showed that both foliar spray and root application could significantly alleviate salt-induced growth inhibition, as evidenced by increased plant biomass. They improved cotton salt tolerance by maintaining ROS homeostasis, regulating antioxidant enzyme activity, reducing oxidative damage, mitigating cell membrane injury, and increasing K⁺/Na⁺ ratio. Notably, foliar spray exhibited superior performance compared to root application due to the higher delivery efficiency of nanomaterials. Mechanistic differences were also observed that foliar application of PNC or PMO preferentially boosted the activity of superoxide dismutase (SOD) and peroxidase (POD), and promoted the extrusion of Na + , while root application enhanced the activity of catalase (CAT) and improved the uptake of K + . These findings elucidate the pathway-dependent mechanisms of nanozyme action and provide practical insights for optimizing nanoparticle delivery in agricultural systems.