# Volatile Metabolome and Aroma Differences of Six Cultivars of _Prunus mume_ Blossoms
## 2 Results and Discussion
### Analysis of Aroma Differences of Six Cultivars of P. mume Blossoms  
According to the previous analysis, contents of methyl benzoate, benzaldehyde, benzyl benzoate, eugenol, 4-allyphenol, unknown19, and benzyl alcohol were found to be high in all six _P. mume_ species, and these seven main compounds in _P. mume_ also belonged to the fifteen differential compounds that were screened. This might indicate that these major components were also responsible for the aroma of different plum blossoms. However, due to the different olfactory thresholds of the various compounds, it is the odor activity values (OAVs) of the compounds, rather than their contents that actually determine the contribution to the aroma. Based on the above analysis of the differences in volatile metabolites in different cultivars of _P. mume_ blossoms, OAV and aroma descriptions were further investigated for differences in the aroma characteristics of these six _P. mume_ species. The OAV is defined as the ratio of the actual concentration of a compound in the sample to the olfactory threshold of the compound, reflecting the actual odor intensity of the compound in the sample [18], and is widely used for aroma analysis of various fruits [18; 19], flowers [7], and foods [20; 21; 22]. The OAVs of sixty-five identified volatile metabolites in six cultivars of _P. mume_ blossoms were listed in Table S2. A Supervised PLS-DA was performed based on the OAV data, as presented in Figure 4 (R\({}^{2}\)X = 0.998, R\({}^{2}\)Y = 0.922, Q\({}^{2}\) = 0.861). It was found that the OAV data of each cultivar blossom were not clustered, as well as that of the contents of volatile compounds (Figure 4A). This is probably due to the absence of the OAV of some unknown compounds, leading to insufficient information. Nevertheless, it can still be similarly observed that the OAV data of JM and LE, GLH and GF are respectively closer to each other, while that of YD and DFZS are somewhat further away from others. The cross-validation results of 100 permutation tests indicated that this PLS-DA model was reliable without overfitting (intercepts of R\({}^{2}\) and Q\({}^{2}\) were 0.145 and \(-\)0.462, respectively; Q\({}^{2}\) and R\({}^{2}\) of the permutation tests models were both lower than the current model) (Figure 4B). Based on this PLS-DA analysis of OAVs, nine aroma differential volatile metabolites with VIP \(>\) 1 were screened in six cultivars of _P. mume_ blossoms, as seen in Figure 4C, and their aroma details are listed in Table 3[23; 24], including benzaldehyde, 2-nonenal, methyl benzoate, benzyl acetate, 3,4-dimethoxytoluene, trans-\(\beta\)-Ionone, eugenol, (E)-cinnamic alcohol, and 4-allyphenol. These compounds greatly contribute to the aroma differences of the six cultivars of _P. mume_ blossom. Six of them are in the list of fifteen differential compounds based on contents, except for 2-nonenal, dimethoxytoluene, and trans-\(\beta\)-ionone. It is noteworthy that trans-\(\beta\)-ionone, a compound with an extremely low Olfactory Threshold of 0.000007 \(\mathrm{\SIUnitSymbolMicro g}\), is screened out with the second highest OAVs, just lower than that of methyl benzoate. Trans-\(\beta\)-Ionone is a natural plant volatile compound, and it is the 9,10 and 9\({}^{\prime}\),10\({}^{\prime}\) cleavage product of \(\beta\)-carotene by the carotenoid cleavage dioxygenase [25]. The higher Trans-\(\beta\)-Ionone OAV in YD and JM gives these two cultivars a more intense violet and raspberry flavor, which may be weaker in DFZS. Figure 4: PLS-DA analysis of volatile compounds based on their OAVs in six varieties of _P. mume_ blossom samples. (**A**) PLS-DA score plot; (**B**) cross-validation plot of the PLS-DA model with 100 permutation tests; (**C**) loading plot of PLS-DA (red dot represents the most differential compounds, VIP \(>\) 1). \"\" indicates that the compound may be other isomers. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**No.