Three cultivars of O. fragrans (‘Rixiang Gui’, ‘Yucheng Dan’ and‘Boye Yin’) were sampled at four flowering stages：the Xiangyan stage, initial flowering stage, full flowering stage, and final flowering stage. Then, the aromatic components in the three cultivarswere detected and analyzed by headspace solid-phase microextraction and gas chromatography-mass spectrometry (GC-MS). Further, the differences in the aromatic components among the three cultivars were evaluated using the multivariate statistical analysis. In addition, the perianth microstructure was observed by scanning electron microscopy and transmission electron microscopy.

The GC-MS analysis detected a total of 51 volatile aromatic components, most of which were terpenes. Among the three cultivars of O. fragrans, 29, 27 and 33 aromatic components were identified in the petals of ‘Rixiang Gui’, ‘Yucheng Dan’ and ‘Boye Yin’, respectively. According to the relative content of aromatic components, the main components in the three cultivars of O. fragrans included linalool, cis-linalool oxide, trans-linalool oxide, α-ionone, β-ionone, and 2H-β-ionone; in addition, some common aromatic substances, such as trans-β-ocimene, cis-β-ocimene, and γ?decalactone were identified. The differences in the relative content of aromatic components and the interaction of all aromatic components together resulted in the unique aroma of the different cultivars. For example, β?ionone was the main aromatic component in the petals of ‘Boye Yin’, but its relative content was lower in ‘Yucheng Dan’. Linalool and its oxides were the main aromatic components in the petals of ‘Rixiang Gui’ and ‘Yucheng Dan’, but not in the petals of ‘Boye Yin’. The results from the partial least squares-discriminate analysis showed different aromatic components in the three cultivars of O. fragrans. Moreover, 10 critical differential compounds were screened using variable importance in projection, namely, ethyl butanoate, cis-linaloloxide, trans-β-ocimene, 4-(2,6,6-trimethyl-cyclohex-1-enyl)-butan-2-ol, γ?decalactone, 3-hydroxy-2-butanone, (E)-2-hexenal, megastigma-4,6(E),8(Z)-triene, cis-β-ocimene, and β-ionone. The cluster analysis based on the relative content of the 10 key differential compounds showed that three flowering stages (the Xiangyan stage, initial flowering stage, and full flowering stage) of ‘Rixiang Gui’ (A1, A2, A3), ‘Yucheng Dan’ (B1, B2, B3), and ‘Boye Yin’ (C1, C2, C3) could be clustered together, respectively, implying that the three cultivars could be distinguished by the amounts of the 10 key aromatic substances that they possessed during these three stages. However, the final flowering stage of the three cultivars could be clustered together. This may have been because of the decreased content of volatile aromatic substances during the final flowering stage; the weakened ability of the flower to release aroma may have led to the lack of major differences in the fragrance of the cultivars. The results of the scanning electron microscopy and transmission electron microscopy revealed abundant fold structures on the cell surface of the petals. The fold structures in the petals of ‘Boye Yin’ and ‘Yucheng Dan’ were more regular than that in the petals of ‘Rixiang Gui’. The density of the structures differed among different cultivars; for example, it was higher in the petals of ‘Boye Yin’ than that in the petals of ‘Rixiang Gui’ and ‘Yucheng Dan’. In addition, we observed a large number of granular substances (osmiophilic matrix granules) attached inside the cells and on the cell surface in the petals of O. fragrans.These granules were highly volatile aggregates of aromatic substances. The density and quantity of granules in the petals of ‘Boye Yin’ and ‘Rixiang Gui’ were higher than those in the petals of ‘Yucheng Dan’. We speculated that there were no secretory structures on the surface of O. fragrans petals; instead, most cells in the petals had aromatic glands involved in the synthesis and secretion of aromatic substances. The whole petal was like a large aromatic gland where the aggregates of aromatic substances (osmiophilic matrix granules) formed in the cytoplasts, then overflowed from the cell walls, spilled out through the abundant fold structures in the epidermal cells, and quickly evaporated. We surmised that this was why O. fragrans was highly fragrant.

There were similar and differential aromatic substances among the three cultivars of O. fragrans, and the differing proportions of these aromatic substances led to the unique aroma of the three cultivars. Overall, the floral fragrance of O. fragrans mainly differed according to the cultivar but was also affected bythe flowering stages. In addition, the differences in the density of fold structures and the amount of aromatic substance (osmiophilic matrix granules) aggregated on the epidermal cells of the perianth in the three cultivars of O. fragrans may be reasons for the differences in fragrance.