Nanoceria versus bulk cerium oxide: differential effects on growth, antioxidants, pigments, and essential oil of Dracocephalum kotschyi Boiss.

Abstract Background Dracocephalum kotschy Boiss., an endemic and vulnerable herb native to Iran, is highly valued for its distinctive botanical features and wide range of pharmacological properties. However, its growth and metabolite production can be affected by environmental conditions, necessitating strategies to improve agronomic performance and phytochemical quality. This study aimed to investigate the effects of cerium oxide nanoparticles (CeO 2 NPs) and bulk CeO 2 at different concentrations (0, 50, 100, and 200 mg L −1 ) on growth parameters, antioxidant defense systems (both enzymatic and non-enzymatic), photosynthetic pigments, and essential oil production in D. kotschyi, using a completely randomized design (CRD) with three replicates. Results Bulk CeO 2 at 100 and 200 mg L −1 and CeO 2 NPs at 100 mg L −1 significantly increased inflorescence length, plant height, and leaf length. Conversely, all CeO 2 NP treatments, along with the higher concentrations of bulk CeO 2 , reduced leaf and stem dry weights, except for 200 mg L −1 bulk CeO 2 , where biomass remained comparable to the control. CeO 2 NPs, particularly at 100 mg L −1 , markedly enhanced the activity of antioxidant enzymes and modulated oxidative signaling, as indicated by elevated H 2 O 2 levels. Both forms of CeO 2 also improved carotenoid content and non-enzymatic antioxidant capacity, with the most pronounced effects at 100 mg L −1 CeO 2 NPs, which additionally increased chlorophyll b content. Essential oil content was significantly enhanced by 50 and 100 mg L −1 CeO 2 NPs and 200 mg L −1 bulk CeO 2 , with the highest essential oil yield recorded at 100 mg L −1 CeO 2 NPs and 200 mg L −1 bulk CeO 2 . Quantitatively, foliar application of 100 mg L −1 CeO 2 NPs increased the essential oil percentage by 1.56-fold compared to the control, while APX and CAT activities rose by 3.29- and 2.88-fold, respectively. These quantitative comparisons highlight the pronounced stimulatory effects of CeO 2 treatments on both phytochemical characteristics and antioxidant enzyme activities in D. kotschyi . Conclusions Overall, 100 mg L −1 CeO 2 NPs emerged as the most effective treatment, improving antioxidant capacity, pigment content, and essential oil yield, despite a slight reduction in biomass. These findings highlight the potential of CeO 2 NPs as a nanotechnological approach for enhancing the agronomic performance and phytochemical quality of D. kotschyi . Thus, while CeO 2 NPs show promise for improving agronomic and phytochemical traits, their potential pro-oxidant risks at higher doses warrant careful ecological consideration in defining safe application levels.