Introduction: Zinc oxide nanoparticles (ZnO-NPs) have attracted considerable attention due to their antioxidant properties and agricultural applications. Green synthesis using plant extracts provides an eco-friendly approach that reduces toxic compounds and improves nanoparticle stability. This study aimed to synthesize ZnO-NPs using Valeriana officinalis extract and evaluate their effects on the morphological and biochemical characteristics of safflower (Carthamus tinctorius).

Materials and Methods: ZnO-NPs were synthesized using an aqueous extract of V. officinalis and characterized by particle size analysis (PSA) and atomic force microscopy (AFM). Foliar applications at 0, 10, 100, 500, and 1000 mg/L were performed in a randomized complete block design. Morphological traits, photosynthetic pigments, and defensive enzyme activities were subsequently evaluated.

Results: Spherical nanoparticles with sizes ranging from 50 to 100 nm were successfully formed. Stem and leaf lengths increased over five weeks; however, differences in stem, leaf, and root lengths compared with the control were not significant (P>0.05). Fresh weight decreased significantly by 27.8% at 100 mg/L (P<0.05). The 1000 mg/L treatment significantly reduced chlorophyll a, chlorophyll b, and carotenoid contents (P<0.05). Guaiacol peroxidase activity decreased significantly at 100 and 500 mg/L (P<0.01), while dehydrogenase activity declined at 500 mg/L (P<0.05). Changes in polyphenol oxidase activity and malondialdehyde content were not significant.

Conclusion: Safflower responses to green-synthesized ZnO-NPs were concentration-dependent. Higher concentrations inhibited photosynthetic pigments, whereas intermediate concentrations affected specific enzymatic pathways. Therefore, determining an optimal concentration is essential for the safe and effective application of ZnO-NPs in sustainable agriculture.