## 4 Discussion
Tea polyphenols are the main source of bitter taste in tea, and their content determines the freshness of tea broth [37]. Studies have shown that external measures induce tea tree growth, during which tea tree genes may change, and new mRNAs and proteins are synthesized, affecting the relevant metabolic processes [38]. In this study, the soluble sugar and tea polyphenol contents in the tea leaves did not change significantly with the increasing concentration of ZnO-NPs, but were essentially lower than those in the CK. This may be because the presence of ZnO-NPs affected the expression of genes related to sugar metabolism and tea polyphenol synthesis in tea trees, which reduced the rate ofsoluble sugar metabolism as well as tea polyphenol synthesis, resulting in low soluble sugar content and tea polyphenol content in the fresh leaves. Higher level of ROS is caused by abnormal cellular metabolism, which leads to oxidative injury to macromolecules and ultimately leads to cell death [39]. The accumulation of ROS in plants indicates that lipids and proteins are seriously damaged, and the normal metabolic process in plants is destroyed, thus resulting in a decline in the plant biomass [40]. When subjected to such stress, plants activate non-enzymatic and enzymatic antioxidant systems to reduce the negative effect caused by the stress [41]. A previous study confirmed that exogenous ZnO-NPs elevated the enzymatic defense mechanisms by decreasing the MDA content, and increasing POD and SOD activities [28]. Furthermore, ZnO-NPs significantly enhanced the activity of antioxidant enzymes (POD and SOD) in the current study, especially in the case of ZnO-NPs at 100 mg\(\cdot\)L\({}^{-1}\). It is well documented that Zn plays a critical role in balancing the stability of bio-membranes and proteins by balancing the scavenging ROS production [42]. These evidences confirmed our experimental results that the application of ZnO-NPs enhanced the enzymatic antioxidant system (POD and SOD) to scavenge the excessive ROS in tea trees. Aroma components are secondary metabolites that are composed of low molecular volatiles. These components can be divided into volatile terpen (VTs), volatile fatty-acid decompositions (VFADs), and volatile phenylpropanol/benzenoid (VPBs) [43]. The aroma components in the tea leaves account for a small proportion of the dry matter of tea, but have a crucial impact on the tea quality. Differences in the types and contents of aroma components in fresh tea leaves are the basis for the formation of different tea aroma types [44]. How heavy metals affect the growth of tea trees is also well researched. A large number of studies have shown that metal elements such as copper and zinc have physiological and biochemical effects on tea trees. The application of zinc can increase the content of selenium, caffeine, and amino acids in teas and boost tea quality [45]. However, little research has been conducted on the effect of zinc fertilizer on aroma components in fresh tea leaves. [PERSON] [46] indicated that the application of zinc fertilizer could affect the content of benzyl alcohol and benzyl alcohol primrose glycosides in fresh tea leaves. One of the possible mechanisms by which tea plants respond to zinc fertilizer is the regulation of glycosyltransferase gene expression to increase the aroma glycosides in tea plants. It has been reported that the saturated and unsaturated aldehydes as well as alcohols aroma compounds are the primary sources of fresh green aroma in tea [47]. [PERSON] et al. [48] proposed that external treatment enhanced the content of terpenes and fatty-acid-derived aroma substances in fresh leaves of tea plants. In this study, when the concentration of ZnO-NPs was relatively high, fewer types of aroma substances were present in the fresh tea leaves, and only one aromatic compound was measured at 200 mg\(\cdot\)L\({}^{-1}\) of ZnO-NPs. The total number of aroma components measured in other treatments was similar, being they were mainly VTs and VFADs.