## 1 Introduction  
Terpenoids are probably the most widespread group of volatile secondary metabolites produced by plants [1]. They are derived from two independent pathways leading to the formation of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the two basic building blocks. Monoterpenes are C\({}_{10}\) compounds synthesized from geranyl diphosphate (GPP) via the MEP (2C-methyl-d-erythritol 4-phosphate) pathway in plastids, while sesquiterpenoids (C\({}_{15}\)) are produced from farnesyl diphosphate (FPP) via the mevalonate (MVA) pathway in the cytosol [2; 3; 4; 5]. A small group of volatiles and their profiles are responsible for unique flavors for individual food [6]. Different plants and their different tissues emit unique aromas, scents, flavors and fragrances, due to the presence of one or, in most cases, a mixture of several volatile compounds [7; 8].  
Citrus belongs to a large family, Rutaceae, members of which are cultivated worldwide as fruit crops. Aroma compounds are important for citrus, not only as a critical attribute of fruit quality, but also as valuable commercial products used extensively in cosmetics, food and household items for fragrance [9; 10], as well as for medicinal purposes, such as prevention of tumor growth [11] and Gram-positive bacterial infection [12]. Citrus leaves have increasingly been used in volatile studies, because of their richness in volatile compounds, rapid growth and large biomass, and because they are available throughout the year [13], they have been used for characterizing citrus cultivars and also for evaluating the impact of the environment on volatiles [13; 14]. Leaf oil composition is more diverse than in fruit and is not over dominated by limonene or limonene/\(\gamma\)-terpinene, which commonly constitute over 70% of total volatiles in fruit peel [15; 16; 17]. In general, however, the number of citrus leaf volatile studies is limited; in particular, there is a lack of information for the comparative study of young and mature leaf volatiles from different citrus cultivars. Although volatile changes during the opening of leaf buds and development from young to mature leaves have been reported previously, the study was limited to grapefruit and lemon only [18; 19].  
Historically, leaf volatiles were analyzed by hydrodistillation [14; 20; 21; 22; 23] and solvent extraction [24], which takes a long time for analysis. Recently, solid phase microextraction (SPME) integrated with GC-MS has been shown to be much more sensitive, reproducible and efficient for metabolomics studies of volatiles, has been widely used in plant research [25; 26], including some studies on citrus fruit volatiles analysis [27; 28; 29], and has become established as a suitable substitute for gas chromatography analysis of essential oils. However, to our knowledge, there is only one study of citrus fresh leaf volatile profiling with SPME coupled with GC-MS [30], and this utilized polydimethylsiloxane (PDMS) fiber, rather than a complex fiber mixture of divinylbenzene/carboxen/ polydimethylsiloxane (DVB/CAR/PDMS), which is more widely used, because of its ability to extract a larger number of volatiles, rather than other fibers [27; 31; 32].  
In this study, by using HS-SPME coupled with GC-MS and application of a DVB/CAR/PDMS complex fiber mixture, volatile profiles of young and mature leaves of nine cultivars from five citrus types were characterized. The study revealed that monoterenes are the major components from theleaves of nine citrus cultivars, and differences among cultivars, as well as two developmental stages were observed.