## 3 Results and Discussion
### Profiling of VOCs and Their Distribution
In statistical analysis, estragole exhibited a very strong positive correlation with the total VOCs content, with a correlation coefficient (\(r\)) of 0.9999. The data presented indicated that the total VOCs content (S2: 242.4 \(\pm\) 21.3 ng/mg; S1: 207.3 \(\pm\) 14.3 ng/mg; S4: 94.2 \(\pm\) 11.7 ng/mg; S3: 71.7 \(\pm\) 7.0 ng/mg) tended to decrease over time in tandem with the diminishing estragole content (Figure 6A). As a result, the maximized stage (S2) was considered the most efficient for obtaining estragole. EOMT (eugenol-O-methyltransferase)activity and eugenol accumulation have a significant correlation and the decrease in the transcript expression level of EOMT with leaf age [82] was expected to be responsible for the decrease in the total amount of VOCs. In MO (Table 4), 18 VOCs were identified, including aldehydes (3), monoterenes (4), sesquiterpene (1), terpene alcohols (3), phenylpropene (1), ketones (2), furan (1), ester (1), lactone (1), and fatty alcohol (1). Monoterpenes composed of isoneral, (Z)-neral (citral B), geranial (citral A), and methyl geraniale accounted for approximately 87% of the VOCs (Figure 6B) and geranial was identified as the major component of MO (Table 4). Specifically, the primary composition of VOCs in S2 stage MO, which exhibited the highest VOCs content, consisted of geranial (66.5%) and (Z)-neral (30.1%). The combined total of these two main components represented a significant percentage in all MO samples, regardless of the stages, ranging between 87.0% and 96.8% (Figure 6B). Consequently, the variation in total VOCs content across the stages was primarily attributed to changes in geranial and (Z)-neral content (as shown in Table 4), and the order from highest to lowest content was as follows: S2-1 (60 DAS; 215.6 \(\pm\) 34.1 ng/mg) > S1 (30 DAS; 166.0 \(\pm\) 4.2 ng/mg) > S2-2 (70 DAS; 35.2 \(\pm\) 12.6 ng/mg) > S2-3 (80 DAS; 20.6 \(\pm\) 11.1 ng/mg). In the case of MO, some previous reports have referred to the major components of VOCs of MO as the geranial, (Z)-neral, citronellal, (E)-caryophyllene, caryophyllene oxide, geraniol, etc. [83]. Among these diverse monoterpenes, monoterpene aldehydes (geranial and neral) were identified as the main component of MO. Although MO exhibited relatively higher levels of geranial compared to (Z)-neral, both compounds made significant contributions. These two stereoisomers of citral (mixture of geranial and neral) are also found in citrus fruits, lemongrass, and gingers to determine quality [84], and were highly quantified in the S2-1 stage and then declined. Total VOCs content was statistically concerned with geranial and (Z)-neral with correlation coefficients of 0.9866 (\(p\) < 0.005) and 0.9980 (\(p\) < 0.005), respectively. Consequently, to obtain MO with a rich VOC content, the optimal stages were S1 (30 DAS) and S2-1 (60 DAS). In contrast to the two previously discussed plant species, OBP and SO did not exhibit remarkable major VOCs. In OBP, 32 VOCs, which included aldehydes (4), monoterpenes (9), sesquiterpenes (6), terpene alcohols (8), phenylpropenes (4), and fatty alcohol (1), were found, as described in Table 5. Their distribution varied across the different growth stages, with S2-1 (60 DAS; 259.0 \(\pm\) 14.9 ng/mg) containing the highest VOCs, followed by S1 (30 DAS; 221.6 \(\pm\) 17.2 ng/mg), S3 (80 DAS; 48.5 \(\pm\) 7.3 ng/mg), and S2-2 (70 DAS; 11.1 \(\pm\) 3.8 ng/mg), respectively. Notably, the terpene alcohols and phenylpropenes displayed opposite patterns as the days progressed. The proportion of phenylpropenes steadily decreased from 93.5% to 47.9%, while the terpene alcohols' portion consistently increased from 3.1% to 37.1% over time (Figure S3). The primary aromatic compounds in OPB were eugenol, methyl eugenol, and linalool (Table 5). During the S1 (30 DAS) and S2-1 (60 DAS) stages, methyl eugenol was the major component, accounting for 58.2-74.9%, with lower amounts of eugenol (17.9%) and linalool (1.9%).