## 1 Introduction  
Nanotechnology is the branch of science that studies different types of nanoparticles (NPs), ranging in size from 1 to 100 nm [1]. It is also the most progressing field of science, providing researchers with new tools to apply NPs to plants. Application of nanotechnology include improvement of agricultural production with bioconjugated NPs, the cultivation of pest-resistant plant varieties, and the use of foliar fertilizer to improve crop growth [2]. In recent years, NPs have been extensively applied in commercial applications and are gradually being developed into the agriculture [3]. Different types of NPs such as zinc oxide nanoparticles (ZnO-NPs), gold nanoparticles (Au-NPs), copper nanoparticles (CuO-NPs), silver nanoparticles (AgNO\({}_{3}\)-NPs), and titanium dioxide nanoparticles (TiO\({}_{2}\)-NPs) have been used by researchers for the growth and development of plants [4].  
Among these NPs, ZnO-NPs have received much more attention. It is estimated that 33,400 tons of ZnO-NPs are produced globally, making them the third most universally used NPs [5]. The levels of ZnO-NPs in environment range from 3.1 to 31 \(\upmu\)g\(\cdot\)kg\({}^{-1}\) in soil and 76-760 ug\(\cdot\)kg\({}^{-1}\) in water [6]. ZnO-NPs are considered to be a bio-safe material with photo-oxidative and photo-catalytic activities towards biological species [7]. In addition, ZnO-NPsalso contributed to plant growth and development. The growth performance and dry weight of _Cucumis sativus_ increased significantly by using ZnO-NPs [8]. ZnO-NPs enhanced the germination and growth of different crops [9]. ZnO-NPs positively augmented the growth and yield parameters along with pod number in _Arachis hypogaea_[10]. The rate of germination in peanut plant also increased through the application of ZnO-NPs [11]. The exogenous application of ZnO-NPs was conducive to penetration into seeds and enhanced water uptake, thus promoting seed germination [12]. The deficiency of Zn probably enhanced the level of reactive oxygen species (ROS), while the application of ZnO-NPs could relieve the damage caused by ROS because it increased the activity of antioxidative enzymes [13].  
Tea (_Camellia sinensis_) is widely consumed around the world for its good taste and health-promoting properties [14; 15]. The chemical composition of tea mainly consists of alkaloids, amino acids, polysaccharides, vitamins, volatile compounds, mineral elements, and polyphenols, all of which have unique sensory properties and healthy features [14]. For example, one of the main components of tea polyphenols is flavonoid, and in addition, tea polyphenols have been associated with anti-cancer, anti-microbial, anti-inflammatory, and anti-atherosclerosis properties. A major part of the health benefits of tea is attributed to tea polyphenols [16]. Total polyphenols and amino acids affect the bitterness and astringency of the green tea, while total soluble sugar could improve the sweetness and mellow taste [17]. As green tea is produced without strong oxidation, its quality depends more on the inherent chemical composition of the fresh leaves [18]. Tea quality is a cumulative property that manifests itself as a combination of a series of metabolites. Meanwhile, tea quality is also affected by the production process. Considering the various advantages of tea, it is helpful to study the changes in physiological indexes and regulation measures during its growth and development.  
As a foliar fertilizer, ZnO-NPs have been studied on other plants, but there are few studies on the effect of ZnO-NPs on the physiological characteristics of tea. Thus, we adopted different concentrations of ZnO-NPs to spray tea leaves to fill this gap. The objectives of this study are to (1) investigate how ZnO-NPs influence tea growth by modulating antioxidant activity, (2) explore the influence of ZnO-NPs on secondary metabolites of the tea leaves, and (3) examine the interacting effect of ZnO-NPs on aromatic components in the tea leaves.