## 4 Discussion  
ZnO-NPs have positive effects on plants as they could alter the physical, chemical, and biological characteristics, as well as the catalytic properties of plants, thereby affecting their physiological and biochemical activities. In our study, ZnO-NPs sprayed on tea trees improved the chlorophyll, soluble sugar, soluble protein, tea polyphenol, and MDA contents and POD and SOD activities (Figures 1-3). Chlorophyll is the primary site of photosynthesis in green plants, and its content is a significant index reflecting the performance of plant photosynthesis. [PERSON] et al. [10] pointed out that exogenous ZnO-NPs increased the chlorophyll content of guar (_Cyamopsis tetragonoloba_ L.). [PERSON] and [PERSON] [28] found that chlorophyll content of tomato (_Lycopression esculentum_) was significantly increased after spraying various concentrations of ZnO-NPs on the leaves, which was similar to our result. [PERSON] et al. [29] expounded that the application of ZnO-NPs promoted the nutritional growth of tomato seedlings, and their total chlorophyll content increased with the increase in the concentration of ZnO-NPs. In this study, ZnO-NPs increased the content of chlorophyll in a concentration-dependent manner, with plants treated with 150 mg\(\cdot\)L\({}^{-1}\) having the highest chlorophyll content and samples treated with other concentrations (10, 50, 100, or 200 mg\(\cdot\)L\({}^{-1}\)) having lower levels of chlorophyll content. The chlorophyll content increased due to the fact that ZnO-NPs may induce the translation and/or transcription of the enzyme genes involved in the biosynthesis of chlorophyll. Similar increases in chlorophyll content by the application of other NPs have also been documented [30; 31; 32]. The plant photosynthetic capacity could be represented by the chlorophyll content. This study revealed that the foliar spray of ZnO-NPs boosted the accumulation of chlorophyll to its maximum in the tea tree leaves. The chlorophyll content measured in the fresh leaves was linearly correlated with the ZnO-NPs concentration, and the chlorophyll content increased generally with the increasing ZnO-NPs concentrations, but chlorophyll synthesis was inhibited when the ZnO-NPs concentration was too high. Chlorophyll a and total chlorophyll in the fresh leaves of tea trees reached the maximum when the concentration of ZnO-NPs was 150 mg\(\cdot\)L\({}^{-1}\), with a lower chlorophyll content under other treatments (10, 50, 100, and 200 mg\(\cdot\)L\({}^{-1}\)), and the chlorophyll b content did not change significantly. The possible reason is that ZnO-NPs can enter the chlorophyll biosynthesis process, resulting in an increase in the rate of translation or transcription of chlorophyll-related genes and an increased pigment content in the leaves [33]. It may also be explained by the presence of ZnO-NPs affecting the activity of related enzymes in the chlorophyll biosynthesis process or decoding the related proteins affecting chlorophyll synthesis, which accelerates the rate of chlorophyll synthesis and increases the chlorophyll content in the leaves. Soluble protein is a crucial osmoregulatory substance in plant leaves and one of the most essential energy sources for plant growth and development [26]. In this study, it was observed that foliar spraying of ZnO-NPs on the tea trees could increase the soluble protein content in the fresh leaves. This may be because ZnO-NPs can affect the structure or protein activity via binding proteins and sterols directly, thus regulating the activity of proteins and other enzymes in the membrane, maintaining protein structure and regulating the enzymes responsible for protein synthesis [34]. The same conclusion was reached by [PERSON] and [PERSON] [35] and [PERSON] et al. [36]. Soluble sugar is a vital substance for plants to maintain the concentration of cellular fluid in the body and improve water retention. They are also factors that affect the flavor of tea leaves.