Dose-dependent alleviation of Cr(VI) stress in chili achieved by soil-applied zinc oxide nanoparticles via coordinated antioxidant and ionic regulation

Hexavalent chromium Cr(VI) contamination in agricultural soils severely limits vegetable productivity by promoting reactive oxygen species (ROS) formation, impairing photosynthesis, and disrupting nutrient homeostasis. Although zinc oxide nanoparticles (ZnO NPs) have been proposed as a stress-mitigation tool, dose-resolved evidence integrating growth, physiology, ion balance, tissue Cr partitioning, and antioxidant transcription in chili ( Capsicum annuum L.) under calcareous soil conditions remains limited. Here, we evaluated soil-applied ZnO NPs (100, 300, and 500 mg kg⁻¹) for alleviating Cr(VI) toxicity imposed using potassium dichromate (K₂Cr₂O₇; 700 µmol Cr kg⁻¹ soil). Chromium stress markedly reduced shoot/root growth, biomass, and chlorophyll pigments, while elevating H₂O₂ and malondialdehyde (MDA), indicating oxidative injury. ZnO NP supplementation mitigated these effects in a dose-dependent manner, with the intermediate dose (300 mg kg⁻¹) consistently providing the greatest improvement. At the physiological and biochemical levels, ZnO NP-treated plants showed reduced Cr accumulation in roots and shoots, restored K⁺ and Ca²⁺ levels, and strengthened antioxidant defense through increased activities of SOD, POD, CAT, and APX alongside higher relative expression of corresponding antioxidant genes (qRT-PCR). Trait association analysis showed Cr accumulation was negatively related to nutrient status and growth, while multivariate analysis confirmed clear treatment separation, with PCA explaining 98.8% of total variation. This study used supplier-designated commercial ZnO nanoparticle powder without independent physicochemical characterization. ZnO NP-only treatments were not included; therefore, the conclusions specifically reflect Cr(VI)-stress mitigation under the tested soil-applied ZnO NP system, rather than standalone nanoparticle effects or a fully resolved nanoparticle-fate mechanism. Overall, ZnO NPs—particularly at 300 mg kg⁻¹ —enhanced Cr(VI) tolerance by jointly restricting Cr uptake, improving nutrient homeostasis, stabilizing photosynthetic pigments, and reinforcing antioxidant protection, supporting their potential as a promising stress-management amendment for chili cultivation under Cr(VI)-contaminated calcareous soil conditions.