Combined application of ZnO-nanoparticles and melatonin improves tea resilience to drought stress by enhancing photosynthesis, antioxidant defense, and secondary metabolite synthesis

Background Drought stress (DS) reduces crop yield, and quality, which makes it essential to create effective strategies to address this challenge. Zinc oxide nanoparticles (ZnO-NPs) and melatonin (MT) are known to enhance crop growth. Nevertheless, little research has examined how these two factors alleviate DS and enhance the yield and quality under DS. Methods This study assessed the separate and combined impacts of ZnO-NPs (biogenic) and MT on reducing the harmful effects caused by DS. The study contained these treatments: control, DS (50%), DS + ZnO-NPs (100 mg kg⁻ 1 ), DS + MT (100 µM L⁻ 1 ), and DS + ZnO-NPs (100 mg kg⁻ 1 ) + MT (100 µM L⁻ 1 ). Results Drought stress significantly decreased growth by increasing membrane damage (malondialdehyde, MDA) and oxidative damage (hydrogen peroxide, H 2 O 2 ), and by reducing nutrient uptake, soluble proteins, photosynthetic pigments, leaf photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), and intercellular CO 2 concentration (Ci). The combined treatment of ZnO-NPs and MT showed better performance in improving plant growth compared with individual treatments. Specifically, the combination of ZnO-NPs and MT promoted growth by increasing chlorophyll content, Pn (56.47%), Tr (89.13%), Gs (122%), antioxidant activities (average 63.84%), melatonin synthesis, and nutrient uptake, while reducing MDA and H 2 O 2 production. Notably, this combined treatment up-regulated the expression of CsAPX , CsCAT , CsPOD , and CsSOD , which helped mitigate drought-induced oxidative damage. Additionally, combined treatment of ZnO-NPs and MT improved tea quality by increasing the concentrations of caffeine (16.32%), catechin (12.43%), polyphenols (54.35%), and theanine (53.70%). Conclusion These findings highlight that combined ZnO-NPs and MT treatment is an important practice to enhance tea resilience to DS and offer a sustainable option to improve crop productivity under changing climate conditions and increasing drought stress.