## 2 Results and Discussion
### Variation in Volatile Constituents from Nine Citrus Cultivars  
As indicated in Table 2, citrus leaf volatiles were characterized by a high percentage of monoterene hydrocarbons, predominantly consisting of limonene, \(\beta\)-terpinene, \(\gamma\)-terpinene, E-ocimene and \(\beta\)-pinene. The main oxygenated monoterenes were linalool, \(\beta\)-cital, citronella and \(\alpha\)-cital, while other compounds identified in appreciable abundance were \(\alpha\)-terpineol and _cis_-geraniol. The aldehydes were comprised of high percentages of 2-hexenal and hexanal. In addition, the major sesquiterpene hydrocarbons were \(\beta\)-elemene, cyanophyllene and \(\gamma\)-elemene, while substantial amounts of \(\alpha\)-amorphene, \(\beta\)-farnesene and \(\alpha\)-farnesene were also accumulated. The principle ester compound  
Figure 2: Relative composition (%) of major chemical groups from young (Y) and mature (M) leaves of five citrus types: mandarins (Ponkan, Satsuma), sweet oranges (Liubencheng, Qingjia), pomelos (Yuhuanyou, Zaoxiangyou), hybrids (Huyou, Hongshigan) and lemon (Eureka). from leaf volatiles was nerol acetate and geranyl acetate (moderate to 5.27%), while only trace amounts of ketones, aromatic hydrocarbons, phenolics and alcohols were found (Table 2). A wide variation in leaf volatile compounds was observed among the different citrus types and cultivars (Table 2). In order to rank the major volatiles, the average percentage of individual volatiles, in young or mature leaves, respectively, was calculated, and the data for the top 20, on average, for all nine citrus, are shown in Tables 3 and 4. For young leaf volatiles, it was found that linalool ranked first, on average, for all samples, as well as for four individual cultivars, but ranked eleventh or thirty-third for two pomelo cultivars; limonene ranked first in Eureka lemon and ranged between fourth to seventh in most cultivars, but was absent in Satsuma mandarin; E-ocimene and \(\beta\)-pinene were identified as the top two constituents from pomelos, but were absent in sweet oranges and Ponkan; and, similarly, \(\beta\)-terpinene ranked second in three cultivars, but was absent in another three (Table 3). Among the mature leaf volatiles, limonene ranked first, on average, for all samples, being highest in Eureka lemon and ranging to seventh in Ponkan. E-ocimene and \(\beta\)-pinene were again the top two volatiles from pomelos and were absent from sweet oranges and Ponkan, whereas \(\beta\)-pinene ranked first in Hongshigan. \(\beta\)-terpinene was the most abundant in sweet oranges and Ponkan, but was not found in Satsuma mandarin and pomelos (Table 4). Interestingly, differences in accumulation of some volatiles were observed between cultivars from the same group. For example, \(\beta\)-pinene, \(\beta\)-elemene and \(p\)-cymene were identified from leaf volatiles of Satsuma mandarin, but were absent in Ponkan. Similarly, \(\beta\)-terpinene was observed in Ponkan and Yuhuanyou, but was not found in Satsuma and Zaoxiangyou. \(\gamma\)-terpinene, on the other hand, was high in leaf volatiles of Huyou and Satsuma, but low in pomelos and Eureka. The fact that monoterene derivatives, such as linalool and limonene, were the main volatiles found to be present at high concentrations in young and mature leaves is perhaps not surprising, since they are known to play an important role as defense metabolites against herbivores and pathogens [19,30,42-44]. Taken together, each cultivar generates a different profile of volatiles that contribute to its distinct aroma attributes (Figure S1). We found linalool, which has already been reported as a major constituent from leaf volatiles of mandarin [21,30,45] and leaf oils of sour orange [41], to be a major compound in young leaves of most cultivars. Limonene, which was predominantly found in lemon, has been reported previously [13,17], as has \(\gamma\)-Terpinene, which is a major component in leaf oil of Satsuma mandarin [20].