## 1 Introduction  
The aroma released by flowers is an important aspect of the quality of ornamental plants and is an important feature of complex plant-pollinator communication [1]. Individual flowers emit volatile organic compounds (VOCs) in diverse proportions, which give flowering plants their characteristic fragames [2, 3]. Therefore, many studies have been carried out with the purpose of identifying compositions responsible for the characteristic aromas of flowering plants. The overall number of identified flower VOCs is enormous (more than 1700 in 90 different families) [4], and, interestingly, they are mainly derived from three biochemical networks (terpenoid, benzenoid/phenylpropanol, and fatty acid pathways). Previous research has shown that tissue-specific emission of floral VOCs was a typical trait of numerous species. Scent substances varied between different flower organs (petals usually release the highest amounts), which were conducive to facilitating pollinator orientation [5, 6]. In addition, emissions of flower VOCs varied over a day and usually matched the activity pattern of the respective pollinators [7].  
The genus _Lagerstroemia_ L., a member of the Loosestrife family (_Lythraceae_) comprises at least 62 species and is noted for its value as a landscape tree/shrub or a container/beddingplant because of its diverse growth habits, flower size, and flower/leaf color [8]. Therefore, most of the breeding objectives for the common craep myrtle concentrated on flower shape, flower color, and flowering time. Unfortunately, flower scent has been neglected as a significant feature in traditional breeding programs [9]. Although many _Lagerstroemia_ flowers are almost scentless, _L. caudata_ is considered to release a strong fragrance and some hybrids of _L. caudata_ and _L. indica_ emit pleasant fragrances [10; 11; 12].  
Gas chromatography-mass spectrometry (GC-MS) is currently the preferred method for the determination of volatile substances [13]. Because of the complicated compositions and low concentrations of volatile components, detection by GC-MS requires a suitable pretreatment method for the isolation and concentration of the original sample. However, after such pretreatment steps, which may be tedious and time-consuming, the VOC profile is often changed [14]. In contrast, solid-phase micro extraction (SPME) in headspace mode (HS) represents a simple, quick, and efficient alternative that combines sampling, extraction, and concentration processes. The method is also solvent-free, provides good sensitivity, and has low detection limits [15]. Recently, this technique was used to develop a powerful method for analysis of flower VOCs in numerous species [16]. However, to date, the identification of VOCs in _Lagerstroemia_ with HS-SPME-GC-MS has only been reported in one study [17].  
The main purpose of this study was to provide a theoretical framework for elucidating the chemical mechanism of flower fragrance emission and to evaluate flower scent quality. Specifically, this work aimed to identify characteristic VOCs and explore the temporal-spatial rhythmicity of scent emission of _L. caudata_ flowers by HS-SPME-GC-MS analysis. This work used the following approach: (1) optimization of sample preparation conditions (fiber coating, extraction time/temperature, sample amount, and desorption time) and concentration for subsequent GC analysis; (2) identification of VOCs and characteristic compositions; (3) investigation of the temporal pattern (day/night and through a flower's lifespan) of scent emission; (4) determination of among-organ (within flower) differences in VOC emission. It is expected that this work will assist the generation of fragment _Lagerstroemia_ cultivars with future breeding programs.