## 1 Introduction  
Floral fragrance has been studied over the last decade to understand the biosynthesis, emission, regulation and ecological impacts of emitted floral volatiles [1]. Flower volatiles play important roles particularly in communication between flowering plants and their pollinating agents [2,3], and repelling of herbivores [4,5]. Flower volatiles are also important due to their commercial application in food, pharmaceutical, perfume, and cosmetic industries [6,7]. Plants produce volatiles from various biosynthetic pathways such as terpenoids, benzenoids, aliphatics, aromatics and fatty acid derivatives [8,9]. terpenoids are the largest class of plant secondary metabolites, having many volatile representatives, and are important components of floral scent in a wide range of species [10]. They represent the largest and most diverse family of natural products, including over 30,000 individual compounds, half of which can be synthesized by plants [11]. Terpenoids are produced from basic C\({}_{5}\) units isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), through serial formation of geranyl diphosphate (GPP), farnesyl diphosphate (FPP), and geranyl geranyl diphosphate (GGPP) via elongation reactions [12,13], to generate the precursors of monoterpenes, sesquiterpenes, and diterpenes. The conversions of these precursors are carried out by a large family of enzymes known as terpene synthases (TPS), to produce a vast range of terpenoids [14,15]. Germplasm diversity affects agricultural development, sustainability and food security [16], and germplasm studies can provide a great deal of knowledge about the origin, development of plant material and collection of plant genetic resources. For citrus, the importance for collection, preservation, and evaluation of genetic resources for industry has been well described [17]. Citrus belongs to a large family the Rutaceae, and a diverse range of taxonomic classification systems has been proposed for the subgenus citrus [18,19], with varying numbers of citrus species. There is a large amount of variation among citrus types and cultivars because of frequent bud mutation, interspecific and intergenicic hybridization, apomixis, polyembryo and a long history of cultivation [20]. Chemical composition has also been employed in devising taxonomic classification. Studies have previously been conducted on leaf and peel essential oil for the classification of citrus cultivars [16,21,22], fruit juice [23] and blossom volatiles [24]. Chemotaxonomic studies have been useful in distinguishing the phytoplankton communities in aquatic systems, which is an economical approach that bears high advantageous for study of a large ecosystem [25]. [PERSON] _et al._ investigated the evolution and classification of bamboos, and the results derived from leaf wax n-alkanes were consistent with those from morphological characters [26]. Recently, [PERSON] _et al._ studied the classification of citrus and its related genera based on chemotaxonomic analysis from citrus leaf and peel oils, and the results were consistent with morphological and genetic characteristics [16]. These studies suggested that chemotaxonomic analysis was a reliable, revealing a powerful research tool for taxonomic studies. As far as we know, neither the evolution of chemical composition of volatiles nor their chemotaxonomic studies have been reported for flower development from various citrus types in previous studies. In general, studies concerning the citrus flower volatiles are rather scarce and substantial differences could be outlined due to genetic material, environmental conditions, analyzed stage/organ as well as sample preparation and analytic methods used for studies [27]. With these considerations in mind, the objectives of this study were to investigate the volatile profiles of flowers during blooming stages of nine citrus cultivars from five genetic types. Differences were observed both among citrus cultivars and during blooming stages.