Morphophysiological, biochemical, and nutrient response of spinach (Spinacia oleracea L.) by foliar CeO2 nanoparticles under elevated CO2

Nanomaterials offer considerable benefits in improving plant growth and nutritional status owing to their inherent stability, and efficiency in essential nutrient absorption and delivery. Cerium oxide nanoparticles (CeO₂ NPs) at optimum concentration could significantly influence plant morpho-physiology and nutritional status. However, it remains unclear how elevated CO₂ and CeO₂ NPs interactively affect plant growth and quality. Accordingly, the ultimate goal was to reveal whether CeO₂ NPs could alter the impact of elevated CO₂ on the nutrient composition of spinach. For this purpose, spinach plant morpho-physiological, biochemical traits, and nutritional contents were evaluated. Spinach was exposed to different foliar concentrations of CeO₂ NPs (0, 25, 50, 100 mg/L) in open-top chambers (400 and 600 CO₂ μmol/mol). Results showed that elevated CO₂ enhanced spinach growth by increasing photosynthetic pigments, as evidenced by a higher photosynthetic rate (Pn). However, the maximum growth and photosynthetic pigments were observed at the highest concentration of CeO₂ NPs (100 mg/L) under elevated CO₂. Elevated CO₂ resulted in a decreased stomatal conductance (gs) and transpiration rate (Tr), whereas CeO₂ NPs enhanced these parameters. No significant changes were observed in any of the measured biochemical parameters due to increased levels of CO₂. However, an increase in antioxidant enzymes, particularly in catalase (CAT; 14.37%) and ascorbate peroxidase (APX; 10.66%) activities, was observed in high CeO₂ NPs (100 mg/L) treatment under elevated CO₂ levels. Regarding plant nutrient content, elevated CO₂ significantly decreases spinach roots and leaves macro and micronutrients as compared to ambient CO₂ levels. CeO₂ NPs, in a dose-dependent manner, with the highest increase observed in 100 mg/L CeO₂ NPs treatment and increased roots and shoots magnesium (211.62-215.49%), iron (256.68-322.77%), zinc (225.89-181.49%), copper (21.99-138.09%), potassium (121.46-138.89%), calcium (118.22-91.32%), manganese (133.15-195.02%) under elevated CO₂. Overall, CeO₂ NPs improved spinach growth and biomass and reverted the adverse effects of elevated CO₂ on its nutritional quality. These findings indicated that CeO₂ NPs could be used as an effective approach to increase vegetable growth and nutritional values to ensure food security under future climatic conditions.