**4. Spatial and Temporal Emission of Volatile Organic Compounds**

Scent is an important property of flowers and plays a vital role in the ecological, economic, and aesthetic properties of flowering plants. Each plant possesses a distinct and unique floral scent. Floral scent is composed of all the VOCs, including terpenoids, phenylpropanoids, benzenoids, fatty-acids, and their derivatives, which are emitted by floral tissues (Table 2). In *Phalaenopsis bellina*, expression analysis of the *PbGDS* (*geranyl diphosphate synthase*) gene encoding a homodimeric GDS showed that its expression is flower-specific and that maximal expression is concomitant with maximal emission of monoterpenes on Day 5 post-anthesis [23]. In *P*. *abies*, expression of *PaIDS1* (*isoprenyl diphosphate synthase 1*) encoding a bifunctional GDS and GERANYLGERANYL DIPHOSPHATE SYNTHASE (GGDS) exhibits a peak in wood where oleoresin, comprising monoterpenes and diterpenes, is accumulated [24].

Plants emit a large variety of VOCs that are actively involved in plant growth and protection. VOCs are defined as any organic compound with vapor pressures high enough under normal conditions to be vaporized into the atmosphere [53]. VOC emissions are strongly dependent on environmental conditions and developmental stages of the plant tissue. In plants, emission of VOCs is spatiotemporally regulated; a majority of VOCs are emitted from flowers compared to other plant tissues/organs, and the level of emission increases when the floral bud is close to opening and decreases as it moves to the senescence stage [61,62].

In *Vanda* Mimi Palmer orchids, various types of sesquiterpenes and benzenoids were highly expressed at the full blooming stage with the expression of floral scent genes [42,59,63]. Floral volatile emission increased bud to flowering stages in *Cymbidium goeringii*. Different types of *Maxillaria* orchids emit strong vanilla or coffee-like scents, which are responsible for pollinator attraction [64]. *M. tenuifolia* Lindl is called a "coconut orchid" due to its strong coconut-like scent, and was recognized as the best scented orchid in the 18th World Orchid Conference [65]. The sepal is a source of floral volatiles in *Maxillaria* species, and the highest level of floral volatiles separated through electronic nose and GC-MS analyses was detected at the initial flowering stage [43,66]. In addition, methyl jasmonate (mJA) emission predominantly occurs in sepals and floral parts of *C*. *ensifolium* [31]. *Phalaenopsis* is undoubtedly the most widely grown orchid in the world, and in *P. bellina*, various types of monoterpenes are produced in the full flowering stage [23,24]. Furthermore, in a comparison of fragrant and non-fragrant *Phalaenopsis* flowers, terpene compounds were found to be much more abundant with increased levels of relevant gene expression in flowers of fragrant orchids [67].

Developmental regulation of scent emission occurs at several levels, including orchestrated expression of scent biosynthetic genes [68], enzyme activities, and substrate availability [69]. Based on an evolutionary study of floral scent genes in three closely related orchid species of the genus *Gymnadenia*, it is likely that the switch from the production of one to two scent compounds evolved under relaxed purifying selection [51]. Two major volatile compounds, α-copaene and β-caryophyllene, have been identified in all floral organs of *M. tenuifolia*, with the highest levels in the petal. α-copaene and β-caryophyllene were found to be emitted in all flower developmental stages except the floral bud stage I [43]. In fact, volatile compounds of *M. tenuifolia* include α-copaene, β-caryophyllene, 1,8-cineole, limonene, β-myrcene, α-pinene, β-pinene, sabinene, and δ-decalactone, which is responsible for the typical coconut aroma. The majority of studies on *Maxillaria* fragrance reported only the chemical composition of the floral scent; however, little data are available on the spatiotemporal emission of the floral volatiles. In addition, sulfur- and nitrogen-containing volatile compounds contribute to the attraction of pollinators to flowers by mimicking food or brood sources such as carrion or dung. Besides the importance of floral scents in plant ecology, identification and functional validation of relevant genes responsible for biosynthetic and/or regulatory pathways of floral volatiles are required for a better understanding of floral scent production and for the development of novel cultivars with desirable characteristics. Transcriptomic and metabolic analyses together with genetic engineering approaches will be of great help in driving towards this goal.
