## 1 Introduction  
Volatile organic compounds (VOCs) are involved in a wide range of functions in ornamental plants [1, 2]. For example, they attract pollinators such as insects, which play an important role in the reproduction of plants and enable the development and growth of fruits and seeds [3, 4]. VOCs contribute to protection against herbivores, microbial growth, and abiotic stress [5, 6]. In some plants, VOCs have allebotathic effects on surrounding plants, protecting the plant and reducing damage from the outside world when they are injured [7, 8]. In addition, these compounds improve the aesthetic and economic value of plants, which are important indicators of the ornamental value of cut flowers and plants [9]. Due to the importance of VOCs emitted from plants, research on the floral substances of ornamental plants has gradually attracted attention. This mainly involves the identification of plant volatiles, analyses of the main floral components, and the effects of volatile components on insect pollination behavior [10; 11; 12]. In addition, with the development of molecular biology, the anabolic pathways of floral substances have been gradually explored, and new methods have been developed for achieving genetic improvements of floral substances [13; 14; 15]. The main secretory organs of flower fragrances in plants are odorant glands, which are generally distributed in the whole infloresence of plants, including petals, anthers, calyx, and pedicels, and some volatiles of plants are produced by specific odorant glands [16; 17; 18]. The odorous glands of plants are usually shaped like whips, brushes, rods, or papillae on the epidermal cells, and they are often shaped like ridges, folds, cones, glandular hairs, or papillae [19]. Scanning electron microscopy (SEM) is often used to observe the structures of plant aroma secretion glands, and transmission electron microscopy (TEM) is used to observe the changes in internal substances in aroma secretion tissues to study the release mechanisms for plant aroma substances [20; 21]. Observations of the structures of petal fragments during flowering of _Michelia laba_ showed that the granular contents in the petals might be the main precursors to fragrance release in the organs [22]. Studies on the flowers of _Jasminum sambac_ have found that there are a large number of granular substances in the upper and lower epidermal cells of its petals. The number of these particles was significantly reduced during opening of the flower, and the particles finally disintegrated. It is speculated that these granular substances were related to the formation of the jasmine fragrance [23]. The aromatic substances in plants are mainly derived from secondary metabolites [24]. At present, more than 1000 floral compounds have been identified in plants, and they cover more than 60 families [25]. Common aromatic substances include linalool, benzyl alcohol, benzaldehyde, basil, phenethyl alcohol, and so on, which are found in many plants [26; 27]. It was found that there are great differences in floral compositions, relative contents, and release amounts among different species and even for the same species in different environments [28]. For example, comparing the volatile compositions between the tropical water lily and the cold water lily, researchers detected 79 kinds of volatile compounds in the tropical water lily and 71 compounds in the cold water lily. Among these volatile compounds, 31 were aromatic substances. Compared with the other two types of water lities, the amounts of volatile substances released by tropical water liles were higher than those of cold-resistant water lilies [29]. _Nymphaea_ L. is a perennial ornamental aquatic flower belonging to the Nymphaea family, which is distributed among tropical, subtropical, and temperate regions [30]. There are approximately 46 species worldwide, which can be divided into two categories: tropical water lities and cold-resistant water lilies [31]. Among them, _N.