## 3 Discussion  
### Complexity and Diverse of Herbaceous Peony Floral Aroma Compounds  
Based on the sensory evaluation of 87 herbaceous peony cultivars, the small proportion of aromatic ones in the three cultivar groups was found. Only 16 cultivars released strong fragrance, and most of the 87 cultivars had no/light fragrance, accounting for 50.57% of all varieties. The cultivation of aromatic herbaceous peony is indispensable. The mixture of volatile compounds produces a variety of flower fragrances [48]. Aromatic herbaceous peony also has differences in volatile components to give people different olfactory experiences. The development of sensitive analytical methods, such as SPME, DHS, and GC/MS, has made it easy to collect and identify volatile compounds. These methods have been used to study the aromas of tree peony [3,20,42], grape [51], rose [43,52], and lily [17]. In this study, 68 volatile components of 17 cultivars (16 strong fragrance ones and the Joker with no fragrance as the control) were collected through SPEM and analyzed by GC/MS, of which 32 were reported for the first time. The differences may be caused by the different varieties and detection methods. Terpenes, benzenoids/phenylpropanoids, and fatty acid derivatives were the main chemical categories. These strong-scented varieties could be used as breeding parents to produce aromatic generations and have high economic value as essences, spices, or teas, which can be used in the development of flavor food and other fields using aromas. A total of 26 compounds with a relative content more than 100 ng/g (93.84% of the total flower fragrance) were further analyzed. These compounds were considered to be the most important volatile compounds in the composition of herbaceous peony fragrance. Monoterpenes were the main substances released by most cultivars, while a few cultivars, 'Alexander Fleming', 'Angel Cheeks', and 'Fen Yu Lou', were dominated by benzene/phenylpropanoids and fatty acid derivatives (Figure 4A). The order of total volatile release of each cultivar is basically consistent with the results of aroma evaluation, indicating that sensory perception is closely related to volatile compound contents. The characteristic aroma components play a determining role in the aroma of plants. The most commonly used method to identify the characteristic aroma components is the flavor threshold method, which is mainly used in the aroma analysis of edible products, and also in plants [53, 54, 55]. The characteristic aroma components of _chrysanthemum_ 'jinba' were isocycloctiral, eucalyptus alcohol, \(\alpha\)-pinene, \(\beta\)-farmesene, and caryophyllene [56]. Cineole, \(\beta\)-myrcene, and \(\beta\)-ocimene were the characteristic aroma components of _Rhododend fortune_[57]. B-Myrcene, D-limonene, \(\beta\)-ocimene, linalool, and 2-PE were the characteristic aromas of Begonia [55]. The greater the OAV, the higher the contribution of this compound to aroma. In this study, seventeen characteristic aroma components were identified, among which four have very high OAVs (>80) and were recognized as the important characteristic aroma components of herbaceous peony flower fragrance, including linalool, citronellol, geraniol, and 2-PE (Table S5). Fatty acid derivatives were not considered as characteristic substances due to the low OAVs and the small difference in their total content among different sensory cultivars. The diversity of herbaceous peony flower fragrance is mainly due to the different release of monoterpenes and benzene/phenylpropanoids. Although monoterpenes and benzene/phenylpropanoids have been reported to be the main compounds released from herbaceous peony [36; 37; 38; 39], the characteristic aroma substances are still unclear. This study identified the main contribution substances and analyzed the main reasons for the sensory differences of herbaceous peony flower fragrance and found that a large amount of released fatty acid derivatives were not the main contribution volatiles.