## 3 Results and Discussion
### Screening of Characteristic Volatile Compounds of Ta Tee Leaves Under Magnesium Regulation  
There are many types of volatile compounds in tea; however, the formation of its special odor characteristics depends on a small number of characteristic volatile compounds [30]. [PERSON] et al. [31] found that the volatile compounds in An tea mainly consisted of esters, acids, and hydrocarbons, which accounted for more than 50% of the total volatile compounds, of which seven volatile compounds played a key role in the odor characteristics. [PERSON] et al. [32] found that the key aroma active compounds in Wuyi rock tea were mainly 11 aromatic volatile compounds, especially linalool and geraniol. It can be seen that characteristic volatile compounds play an important role in the formation of special odor characteristics of tea. Accordingly, in this study, based on the determination of volatile compounds, firstly, multi-group volcano plots and K-means clustering analysis were used to screen for significant changes in differential volatile compounds in tea tree leaves at different magnesium concentrations. The results showed (Figure 2A) that a total of 357 volatile compounds significantly changed in tea tree leaves with increasing magnesium concentration, and subsequent K-means cluster analysis of these 357 volatile compounds revealed that 197 volatile compounds showed a significant upward trend and 160 showed a significant downward trend with increasing magnesium concentration. Based on the above significantly changed volatile compounds, OPLS-DA models of ML, MM, and MH were constructed to screen for key differential volatile compounds, and the results showed (Figure 2B) that the constructed OPLS-DA models effectively distinguished the three different treatments in different regions. The fit (R2Y = 0.990) and predictability (Q2 = 0.799) of the models reached a significant level (\(p<\) 0.005) and could be used to screen for key differential volatile compounds. From the S-Plot of the model, 130 key differential volatile compounds (VIP \(>\) 1) were derived that significantly differentiated ML, MM, and MH. Further analysis of the classification and content of key volatile compounds revealed (Figure 2C) that the 130 key volatile compounds could be classified into 15 groups, namely, acids, alcohols, aldehydes, amines, aromatics, esters, ethers, halogenated hydrocarbons, heterocyclic compounds, hydrocarbons, ketones, nitrogen compounds, phenols, sulfur compounds, and terpenoids, of which only the content of nitrogen compounds showed a decreasing trend as the magnesium concentration increased, while the content of the other 14 groups of volatile compounds showed an increasing trend. Key volatile compounds were analyzed using a bubble feature map, and 35 characteristic volatile compounds were screened and obtained (Figure 3A). The 35 characteristic volatile compounds could be mainly classified into 10 groups (Figure 3B), which were terpenoids (28.57%), ester (14.29%), heterocyclic compounds (14.29%), acids (8.57%), alcohols (8.57%), aldehydes (8.57%), hydrocarbons (5.71%), ketones (5.71%), amines (2.86%), and aromatics (2.86%), and in short, terpenoids accounted for the largest proportion, followed by esters and heterocyclic compounds. The transformation relationship between compounds is extremely complex. For example, ketones, hydrocarbons, and aldehydes can be converted into alcohols, while alcohols and acids can form esters after esterification and may also be dehydrogenated to form isoprenoid structures, which in turn leads to the synthesis of terpenoids [33]. The conduct of these reactions involves the participation of a multitude of enzymes in a complex process [34]. As reported, terpenoids, esters, and heterocyclic compounds are the main components of volatile compounds in tea tree leaves, and they are also important volatile compounds for the formation of the special odor characteristics of tea, and their content directly affects the intensity of odor characteristics of tea [35; 36].