# New Insights into the Mechanism of Spatiotemporal Scent Accumulation in Orchid Flowers
## 3 Discussion  
Monoterpens, important VOCs contributing to floral scents, have been found in many plants [20; 22; 26; 27; 28; 29]. Additionally, monoterpens, including linalool, geraniol, and several others, have been suggested to be the main VOCs emitted by _Phalanopsis_ spp. [10; 12; 13], _D. officinal_[19], and _Cymbidium ensifolium_[30]. Among the VOCs emitted by KOVA flowers, only monoterpens, including linalool, _beta_-Ocimene, _cis_-allo-Ocimene, _cis_-Citral, _c_itral, and geraniol, increased quantitatively from the scentless stage D7 to the scented stage D8, while the levels of benzaldehyde, benzyl alcohol, and _beta_-Farnesene showed a tendency  
Figure 6: Phylogenetic relationships between RcMTPSs and MTPSs from other plants. All amino acid sequences without repeats from RcMTPs from KOVA and MTPs from other plants were used to establish a phylogenetic tree based on the neighbor-joining method. These data were employed to analyze the relationship between the MTPs. Protein abbreviations are as follows: FhLin, _Freesia hybrid_ linalool synthase; TPS-CIN, _Arabidopsis thaliana_ terpene synthase-like sequence-1,8-cineole; SITPSS38, _Solanum Lyopersicum_ terpene synthase 38; QH5, (3R)-linalool synthase; OCS, _trans_-beta-Ocimene synthase; VvaTerp, _Vitis vinifera_ (-)-alpha-terpineol synthase; AAC37366, 4S-limonene synthase; FES, fenchol synthase; AAM53944, (R)-limonene synthase; PT1, _Pinus head_ (-)-alpha-pinene synthase. to decline (Figure 2), which was not consistent with the change in the intensity of the floral scent based on perception by the human nose between the two stages. Although relatively high contents of benzaldehyde, benzyl alcohol, and _beta_-Farnesene were detected in the KOVA flower at stage D7, they could not reach the threshold value for perception by the human nose, which might cause the KOVA flower at D7 to be scentless to humans. Therefore, monoterpens are the main VOCs that contribute to the scent of KOVA flowers. Additionally, at the D8 stage of bloom, the contents of several monoterpens, including linalool, geraniol, nerol, D-limonene, and (-)_-trans_-Isopiperitenol, were higher in the tissues of the hypochile region of the lip than in the other flower parts, and the linalool and geraniol accounted for the highest proportions of the total monoterpene contents in all flower parts, respectively, at stage D8 (Figure 4). There were some differences in the compositions of emitted and endogenous scents (Figures 2 and 3), which is a common phenomenon and can be explained by the different detection methods used [31; 32; 33]. These results suggested that linalool and geraniol were the main contributors to KOVA floral scents, and the highest production of monoterpens in the hypochile caused this flower part to be more fragrant than the other parts at full bloom (Figures 2-4). floral scents usually help plants to attract and guide pollinators [34; 35], and the lip in orchid flowers acts as an attractant and landing stage for pollinators via its visual and tactile cues and fragrance [36; 37; 38]. The hypochile is the basal part of the lip and near the gynostemium (Figure 1), and this suggests that the hypochile, which is the most fragrant flower part due to the increased production of monoterpens, may facilitate the attraction of effective pollinators in the wild. MEP pathway structural genes encoding enzymes play an important role in terpene production in flowers [15]. DXS, as the rate-limiting enzyme in the MEP pathway, can control the total content of terpen in plants [39]. The expression levels of _Rhyncholololocattleya DXSs_ (_RCDXS_s) were upregulated in four parts of the KOVA flowers at D8 (Figure 5). It is at this stage that most monoterpens are biosynthesized and the total monoterpene content is the highest (Figures 2 and 3). Additionally, the expressional level of another important MEP pathway gene, _Rhyncholololocattleya HDS_ (_RCHDS_), was obviously higher in all KOVA flower parts at stage D8 than at stage D7 (Figure 5).