Exogenous copper oxide nanoparticle treatment (seed and foliar) modifies photosynthetic rate and antioxidant enzyme activity in Brassica juncea

Nanotechnology facilitates efficient micronutrient delivery to crops, with copper oxide nanoparticles (CuO NPs) identified as promising nanofertilizers. This study investigated the effects of CuO NPs applied via seed soaking and foliar spray on the growth and physiological responses of Brassica juncea (mustard). Seeds were soaked in a 4 mg L−1 CuO NP suspension for 30 minutes prior to sowing, followed by foliar application of varying CuO NP concentrations at 25 days after sowing (DAS). Growth, photosynthetic, enzymatic, and biochemical parameters were evaluated to identify the most effective application strategy. The combination of 4 mg L−1 seed soaking and 8 mg L−1 foliar spray yielded the greatest improvements. At 45 DAS, shoot and root lengths, fresh and dry masses, leaf area, and SPAD values increased by approximately 48–51% compared to the control. Photosynthetic rate, stomatal conductance, transpiration, carbonic anhydrase and nitrate reductase activities, antioxidant enzyme activities, and proline accumulation were also significantly enhanced, suggesting improved photosynthesis, nutrient uptake, and stress tolerance. In summary, combined seed soaking and foliar application of CuO NPs substantially promoted mustard growth and physiological performance, underscoring their potential as nanofertilizers. However, further long-term field studies and environmental safety assessments are necessary before large-scale implementation.