Concentration-Dependent Effects of Foliar Zinc Oxide Nanoparticles on Growth, Photosynthesis, Antioxidant Defence and Yield in Brassica juncea L.

Nanotechnology offers approaches to improve crop productivity through nutrient delivery. This study investigated the concentration-dependent effects of foliar-applied zinc oxide nanoparticles (ZnO-NPs) on growth, photosynthesis, antioxidant defense, redox homeostasis, and yield of mustard (Brassica juncea L.) ZnO-NPs were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy, confirming a crystalline hexagonal wurtzite structure with a crystallite size of 29.3 nm and an aggregated morphology. Plants were treated with 25, 50, 75, or 100 mg/L ZnO-NPs and evaluated at 30 and 45 days after sowing, with yield assessed at harvest. Responses were concentration-dependent, with 50 mg/L producing the greatest benefits. This treatment enhanced growth, biomass, SPAD index, leaf area, gas exchange, PSII efficiency, carbohydrate and protein accumulation, antioxidant enzyme activities, leaf elemental composition, and yield traits. It also reduced reactive oxygen species, malondialdehyde, and electrolyte leakage while increasing proline and glutathione, indicating improved redox balance and membrane stability. Among the concentrations tested, 50 mg/L ZnO-NPs produced the most consistent beneficial response under the present experimental conditions, whereas the magnitude of these effects declined at 75 and 100 mg/L. These findings highlight the significance of concentration optimization while considering the use of ZnO-NPs as foliar treatment in B. juncea for Zn fertilization. Nevertheless, further comparative studies using conventional Zn fertilizers are necessary to determine whether the observed responses are specifically associated with the nanoscale properties of ZnO-NPs or are primarily attributable to Zn nutrition.