## 2 Results and Discussion  
### Analysis of Citrus Leaf Volatiles by HS-SPME-GC-MS  
In the present study, profiling of young and mature leaf volatiles of nine citrus cultivars from five types (Table 1) was investigated using HS-SPME-GC-MS. The percentage and retention time of the identified compounds are listed in Table 2. A total of 123 compounds were detected and separated on the basis of their chemical structures and were grouped into nine classes, _i.e._, nine aldehydes, 19 monoterene hydrocarbons, 27 oxygenated monoterpenes, 43 sesquiterpene hydrocarbons, eight oxygenated sesquiterpenes, two ketones, six esters and nine miscellaneous (Table S1). Forty-eight compounds (three aldehydes, one monoterene hydrocarbons, 12 oxygenated monoterpenes, 18 sesquiterpene hydrocarbons, two oxygenated sesquiterpenes, two ketones, three esters, two aromatic hydrocarbons, two phenolics, one alcohol and two unclassified) out of 123 were identified for the first time from citrus leaves (Table S1), while the remaining volatiles have been described previously in citrus leaf oil [13, 14, 18, 19, 20, 22, 30, 33].  
Most of the newly identified compounds were present in low amounts; nevertheless, 12 major ones, which accounting for over 1% of total volatiles, were observed in at least one citrus leaf sample. In particular, (Z)-\(\alpha\)-bergamotene, \(\gamma\)-elemene, myrtenyl acetate and \(\alpha\)-farnesene accounted for over 4% of total volatiles in some samples. This might be due to the more advanced analysis techniques or the greater number of citrus cultivars in this study.  
Currently, over 300 volatiles have been reported in citrus fruit [34]. Comparison of the leaf volatiles identified in this study with those from flowers and fruit showed that 104 leaf volatiles have been tentatively reported in other citrus volatile studies [16, 26, 27, 28, 31, 33, 35, 36], while the other 19 were reported in other plants, such as myrtenyl acetate in myrtle (_Myrtus comunis_ L.) [37], (Z)-\(\alpha\)-bergamotene from the essential oil of mulberry [38], _cis_-\(\beta\)-terpineol from _Myrtus_ species [39] and pinocarvone from the walnut tree [40]. In addition, the high percentage of aromatic hydrocarbon,  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Citrus types** & **Common name** & **Cultivars** & **Code** \\ \hline
1 & _C. reticulata_ Blanco & Mandarin & Ponkan & PK \\
2 & _C. unshiu_ Marc. & Mandarin & Satsuma & ST \\
3 & _C. sinensis_ (L.) Osbeck & Sweet orange & Liubencheng & LBC \\
4 & _C. sinensis_ (L.) Osbeck & Sweet orange & Qingjia & QJ \\
5 & _C. grandis_ (L.) Osbeck & Pómelo & Yuhuanyou & YHY \\
6 & _C. grandis_ (L.) Osbeck & Pómelo & Zaoxiangyou & ZXY \\
7 & _C. changshanensis_ Chen et. Fu & Citrus Hybrid & Huyou & HY \\  & (_C. aurantium_\(\times\)_C. grandis_) & & & \\
8 & _C. reticulate_\(\times\)_C. sinensis_ & Citrus Hybrid & Hongshigan & HSG \\
9 & _C. limon_ (L.) Burm. & Lemon & Eureka & ERK \\ \hline \end{tabular}
\end{table}
Table 1: Citrus cultivars included in the leaf volatile study.  
\(p\)-Cymene (14.35%), was only found in young leaf of Satsuma mandarin (Table 2), which has also been reported previously in _C. unshiu_ cultivars [41].