ZnO nanoparticles enhance growth and essential oil content and composition in German chamomile by modulating antioxidant activity

German chamomile ( Matricaria chamomilla L.) is a key medicinal and aromatic plant valued for its bioactive compounds across pharmaceutical, cosmetic, and food industries. Optimizing the synthesis of these metabolites requires efficient nutrient management. Although zinc is an essential micronutrient involved in plant metabolism, the comparative effects of bulk zinc oxide (ZnO) and zinc oxide nanoparticles (ZnONPs) on chamomile growth and phytochemical accumulation remain unclear. This study examined the effects of foliar-applied ZnO and ZnONPs (0, 20, 40, 80, and 160 mg L⁻¹) on growth traits, physiological responses, and essential oil yield and composition. Zinc treatments significantly shifted secondary metabolite profiles, and several compounds absent in the control emerged following Zn application. ZnONPs at 80 mg L⁻¹ markedly increased bisabolol oxide A and artemisia ketone, while 160 mg L⁻¹ enhanced spathulenol, α-bisabolol oxide B, α-trans-bergamotenol, and (E)-β-famesene—compounds with known antioxidant and antimicrobial properties. Overall, ZnONPs outperformed bulk ZnO, with 80 mg L⁻¹ producing the greatest improvements in growth, phenolics, flavonoids, anthocyanins, and essential oil content, while both 80 and 160 mg L⁻¹ optimized essential oil composition. These results demonstrate the strong potential of ZnONPs as a targeted strategy to enhance growth and high-value secondary metabolites in the plant. • ZnO nanoparticles improved physiological traits and floral productivity. • Secondary metabolism was stimulated, enhancing plant bioactive compounds. • Essential oil profile of chamomile shifted under ZnO nanoparticle treatment.