## 3 Results and Discussion  
### Effect of Magnesium Regulation on Volatile Compounds of Tea Tree Leaves  
Volatile compounds, which are mainly derived from secondary metabolites of tea trees, are the main compounds that form the aroma characteristics of tea [23]. Elemental supply plays an important role in the synthesis of volatile compounds in tea trees; however, there are significant differences between different elements in regulating the synthesis of volatile compounds in tea trees [24]. For example, Al and Pb are toxic and detrimental to the growth of tea trees, reducing their ability to synthesize volatile compounds [25; 26], while Se and Mo are beneficial in increasing tea yield, volatile compound content, and tea aroma [27; 28]. In this study, the volatile compounds of tea tree leaves were measured after treatment with different magnesium concentrations (Figure 1A) and it was found that a total of 772 volatile compounds was detected (Table S1), which could be categorized into 15 groups, namely, terpenoids (19.04%), esters (17.23%), ketones (12.44%), heterocyclic compounds (11.27%), alcohols (10.88%), aldehydes (5.83%), acids (5.18%), amines (3.63%), aromatics (3.37%), hydrocarbons (3.37%), phenols (2.85%), ethers (2.20%), nitrogen compounds (1.55%), halogenated hydrocarbons (0.65%), and sulfur compounds (0.52%), with the highest number of terpenoid compounds. Analysis of total volatile compounds after treatment with different magnesium concentrations revealed (Figure 1B) that ML, MM, and MH did not differ significantly (\(p>0.05\)) in the total content of volatile compounds. The principal component analysis (Figure 1C) showed that principal component 1 and principal component 2 could effectively distinguish ML, MM, and MH in different regions, with an overall contribution of 68.35%. It can be seen that although there was no significant difference in the total content of volatile compounds in tea leaves after treatment with different magnesium concentrations, different groups of volatile compounds underwent significant changes in their content. Further analysis revealed (Figure 1D) that the volatile compounds most relevant to ML were mainly ketones, acids, halogenated hydrocarbons, alcohols, and aldehydes, while phenols were relevant to MM, and terpenoids, sulfur compounds, aromatics, and nitrogen compounds were relevant to MH. [PERSON] et al. [29] analyzed the effect of different amounts of applied nitrogen on the volatile compound content in tea leaves and found that an increase in nitrogen dosage had no significant effect on the total content of volatile compounds in tea leaves, but the terpenoids content was significantly increased. [PERSON] et al. [10] analyzed the effect of magnesium fertilizer on the quality of tea leaves in the field and also found that an increase in the amount of magnesium had a lesser effect on the total content of volatile compounds in tea leaves. A similar phenomenon was also found in this study, which showed that magnesium regulation changed the content of different types of volatile compounds in tea tree leaves, although the effect on the total amount of volatile compounds in tea tree leaves was small.  
Figure 1: Effect of magnesium regulation on the type and content of volatile compounds in tea tree leaves. Note: ML: Mg\({}^{2+}\) concentration 0 mmol/L; MM: Mg\({}^{2+}\) concentration 0.4 mmol/L; MH: Mg\({}^{2+}\) concentration 0.8 mmol/L; (**A**) Types of volatile compounds of tea tree leaves and their percentage of quantity; (**B**) Effect of magnesium regulation on the volatile metabolite content of tea tree leaves; (**C**) Principal component analysis of different samples under magnesium regulation; (**D**) Principal component analysis of volatile compounds of different samples after categorization under magnesium regulation.