The study focused on ten fragrant Phalaenopsis orchids (see Fig. 1) selected from about 500 most popular Phalaenopsis varieties in the Chinese flower market, including three pink varieties: Phalaenopsis schilleriana (S), ‘Orange Beauty’ (O), and ‘SX098’ (SX); two rose-red varieties: ‘H026’ (H) and ‘SK16’ (SK); two orange varieties: ‘Brother Sara Gold’ (B) and ‘SH51’ (SH); two yellow varieties: ‘Zhi Duo Xing’ (Z) and ‘Yan Die Huang Fei’ (Y); and one purple variety: ‘Purple Martin’ (P). All were sourced from Xiamen HEMING Flower Technology Co., Ltd. (China) and cultivated for two years in a greenhouse equipped with a fan water curtain cooling system, internal and external sunshade system, and heating system at the Yantai Academy of Agricultural Sciences (Shandong Province, China). The temperature was 25°C–30°C, the light intensity was 10,000 lx, and the humidity was 70%~85%. The growing medium was Chilean-imported sphagnum moss, with fertilizer for 2000 to 3000 times the concentration of N, P, and K content of 20-20-20. Watering was carried out on sunny days to ensure that the leaves did not collect water at night. Yellow and blue armyworm boards were used to control pests. 4-leaves\1-heart 2a newly matured seedlings with healthy, pest-free, and similar growing conditions were selected for the experiment.

Flowers were collected on the second day in full opening and placed against a white background under the same lighting conditions. The color parameters of the flowers were then measured using a colorimeter (NF555, Nippon Denshoku Industries Co. Ltd., Tokyo, Japan) to obtain the values for L* (lightness, ranging from black to white, 0–100), C* (saturation, calculated as C* = (a*2 + b*2)1/2) and h (hue angle, calculated as h = tan⁻¹(b*/a*)) were also determined [15].

After the experimental materials bloomed, ten plants were sampled and placed in a room with the same environmental conditions (room temperature at 25°C). The flower colors were measured using the Royal Horticultural Society (RHS) color chart, and the floral fragrance was assessed through olfactory sensory evaluation. The Phalaenopsis flowers were selected on the second after they bloomed. A group of 20 people (10 males and 10 females) evaluated and scored the fragrance of each cultivar. Scoring ranges from 0 for no fragrance, 1 for a light scent, 2 for a moderate fragrance, and 3 for a strong scent. The average score was calculated, and the fragrance characteristics were described after scoring [16,17].

Flower samples, including sepals, petals, and one gynandrium, were taken for analysis. A 2 g samples were weighed and placed in a sampling vial, then sealed and allowed to equilibrate for 10 min. The solid-phase microextraction method was employed, placing the sampling vial on a ceramic heating plate at 45°C (Corning PC-4200, Corning Inc., Corning, NY, USA) for extraction by using a 100 μm polydimethylsiloxane (PDMS) extraction fiber (Beijing Huayi Instrument Technology Co., Ltd., Beijing, China) for 30 min. The fragrance components were measured using Headspace Gas Chromatography-Mass Spectrometry (GC-MS) [18]. A Shimadzu GCMS-QP2010 plus gas chromatography-mass spectrometry system (Shimadzu Corporation, Kyoto, Japan) was used. The chromatographic conditions were as follows: the chromatography column was Rtx-5MS (60 m × 0.25 mm × 0.25 mm); the injection volume was 1 μL; helium was used as the carrier gas, pressure at 112.0 kPa; the total flow rate was 8.0 mL/min, with a column flow rate of 1 mL/min and a split ratio of 5:1. The temperature program was as follows: the injector temperature was set to 250°C, the initial column temperature was 40°C (held for 2 min), then increased to 180°C at a speed of 8°C/min, followed by a further increase to 250°C at a rate of 15°C/min, held for 5 min. The mass spectrometry conditions include an interface temperature of 230°C; the ionization method was electron ionization (EI) source; the ion source temperature was set to 200°C; and the detector voltage was 0.8 kV. The fiber head was inserted into the injector for analysis over 2 min. Each experiment was repeated three times parallelly. The sampled mass spectra were analyzed using the NIST library and WILEY library in series for compound identification, combined with manual spectral analysis to confirm the chemical constituents of the volatiles. The relative percentage of each component in the sample gas was calculated by using normalized peak area, with the formula: Content (%) = (the peak area of the specific substance/sum of all gas peak areas in the sample) × 100 [19]. By referring to the characteristics of chemical compounds and relevant literature, the volatile components of each cultivar were analyzed, allowing for the selection of non-fragrant and fragrant gas components, ultimately identifying the main fragrant components responsible for the Phalaenopsis floral scent.

The data was analyzed, and charts were created using Microsoft Excel 2016, while the correlation coefficients between flower color and fragrance were calculated using IBM SPSS Statistics 24 software.

The petal colors of 10 Phalaenopsis orchids were measured (lightness L*, chroma C*, and hue angle h), as shown in Fig. 2. L* indicates the brightness and the value range is 0 to 100. The higher the value is, the lighter the color is. The C* value represents the saturation of the color. The higher the value is, the brighter the color is. h is the hue angle and the way colors are positioned in the color system, ranging from 0 to 360°. The closer the h value is to 0° (360°), the redder the color is; Closer to 90°, the more yellow the color is; Closer to 180°, the greener the color is; Closer to 270 degrees, the bluer the color is. The results indicated that pink and yellow Phalaenopsis series had higher L* values, lower C* values, and lower h values, while magenta and purple Phalaenopsis cultivars had lower L* values, higher C* values, and higher h values. The L* value of flower color was negatively correlated with C* and h values. The color distribution of these 10 Phalaenopsis cultivars is quite broad, with h values ranging from 25° to 325°. The flower colors are diverse but lack blue-green hues class. In China, red represents festivity and is used for festivals, and white and yellow represent grief and are used for funerals, so the magenta and orange cultivars with higher C* and h values are much more popular than other colors in the market.

The red color in plant petals primarily originates from anthocyanins, while yellow is mainly synthesized by carotenoids; The mix of these two pigments creates an orange color. Phalaenopsis orchids exhibits a rich variety of flower colors, encompassing nearly all color spectra except for blue-green, and the flower color pattern exhibit multiple complex models such as spots-shape, stripes-shape, and halo-shape. Wang et al. used the ISCC-NBS color nomenclature system to categorize the flower colors of Phalaenopsis orchids into seven major color groups: yellow, brown, red, violet, pink, purple, and white [20]. A negative correlation between the brightness and chroma of flower colors they observed. The results of our experiment were confirmed and consistent with those of previous studies [21], indicating that the brightness (L* value) of Phalaenopsis orchid flower colors is significantly negatively correlated with chroma (C* value) and hue angle (h). Specifically, as the depth of flower color increases, the L* value decreases, while the C* value increases.

The sensory evaluation was conducted on the fragrance of 10 Phalaenopsis orchid cultivars, and the results are shown in Table 1. The cultivars ‘Sara Golden’, ‘Purple Martin’, ‘SK16’, ‘SX098’, ‘SH51’, ‘Zhiduoxing’, and ‘Yandie Huangfei’ had relatively strong fragrances. Among them, ‘SH16’ had the strongest and most pungent smell. The yellow and orange cultivars, including ‘Sara Golden’, ‘SH51’, ‘Zhiduoxing’, and ‘Yandie Huangfei’, were characterized by a fruity sweet fragrance.

In total, over 60 gas components were identified in 10 fragrant Phalaenopsis orchids, which were mainly classified by their chemical structure into alcohols, esters, alkenes, and benzenoid compounds. Among them, ‘SK098’ and ‘Yandi Huangfei’ were primarily composed of alcohol, reaching 53.62% and 89.60%, respectively. ‘SK16’ and ‘Zhidaoxing’ had the highest proportions of ester components, at 58.05% and 49.22%. ‘Sara Huangjin’ and ‘SH51’ were dominated by alkenes, at 54.22% and 45.04%, respectively. ‘H026’ was primarily composed of esters and alkenes, accounting for 31.15% and 32.25%. ‘Zima Ding’ had 36.70% aromatic compounds and 24.76% alcohols, while the Tiger-striped Phalaenopsis orchid was mainly comprised of aromatic compounds, esters, and alcohols, accounting for 29.79%, 25.62%, and 18.88%, respectively. ‘Xiang Cheng Mei Ren’ was primarily made up of esters and alkenes, at 32.73% and 35.59%.

According to Table 2, the volatile components of 10 fragrant Phalaenopsis orchids were classified and analyzed. The results, categorized in descending order, showed that the components ranked as follows: alcohols, alkenes, esters, and aromatic compounds. Within each category, the contribution rates of components, ordered from highest to lowest, are as follows. Alcohols: linalool, citronellol, eucalyptol, n-hexanol, nerol, dodecanol. Alkenes: ocimene, cedrene, β-laurolene, α-pinene, γ-terpinene, pinene. Esters: hexyl acetate, neryl acetate, di-n-butylphthalate, geranyl acetate, isopropyl myristate, diisobutyl phthalate. Benzenoid: fluorenes, 1,4-dimethylbenzene, dibenzofuran, indole.

From the perspective of flower color classification, the main fragrance components of yellow Phalaenopsis orchids are dominated by esters (hexyl acetate), alcohols (linalool and eucalyptol), and alkenes (cedrene and basilene). In contrast, the purple-red and pink orchids primarily contain alcohols (geraniol, n-hexanol, and nerol) and aromatic compounds (1,4-dimethylbenzene and fluorenes).

Different fragrant types of Phalaenopsis orchid cultivars have distinct dominant floral scent components. Current research indicates that the fragrance components of fragrant Phalaenopsis orchids are mainly terpenes, such as linalool in P. ‘Tsuei You Beauty’ [9]. P. bellina contains nerolidol, geraniol, and linalool [11,12], while P. violacea has elaeagnol, agarol, and linalool [10]. Other flower species also exhibit different dominant aromatic components among various cultivars. For instance, the fragrance components of peonies, such as Paeonia ostii and Pa. rockii, have the highest terpenes content, whereas Pa. delavayi and Pa.lutea have a relatively higher content of benzenoid compounds [22]. The results of these experiments indicate that the main determined components of floral scent in different Phalaenopsis orchid varieties differ, and the primary compounds are alcohols, alkenes, esters, and benzenoids.

A two-factor correlation analysis was conducted to examine the relationship between flower color parameters (L*, C*, h) and the contents of alcohols, alkenes, esters, and benzenoid compounds in the fragrances of 10 fragrant Phalaenopsis orchids. The Pearson correlation coefficient was calculated for each value (Table 3). The results showed that the ester content in the fragrance components depicted a highly significant positive correlation with the C* value of flower color, while the alkene content displayed a highly significant negative correlation with the h value of flower color. The saturation of flower color increased, and the content of esters in the fragrance also increased. Furthermore, a greater inclination towards the yellow color spectrum correlates with a higher content of alkenes in the fragrance.

Additionally, the alcohol content in the fragrance had a significant negative correlation with the C* value of flower color and a significant positive correlation with the L* value. This indicates that as the saturation of flower color decreases and brightness increases, the content of alcohol in the fragrance rises.

Moreover, the content of benzenoid compounds was significantly negatively correlated with L* and positively correlated with h, which meant that the darker the flower colors were, the more it leaned towards the blue-purple spectrum, the higher levels of benzenoid compounds in the fragrance were. Therefore, it is likely that there exists some correlation between the color and fragrance of Phalaenopsis orchids.

The fragrance of Phalaenopsis orchids has some correlation with their color. The main fragment components of yellow-fragrant Phalaenopsis orchids are primarily alcohols, alkenes, and esters, while the purple and pink cultivars mainly contain alcohols and benzenoid compounds. As the saturation of the flower color increases, the content of esters in the fragrance also increases synergistically. Additionally, the more the flower color leans towards the yellow spectrum, the higher the content of alkenes in the fragrance. Conversely, lower saturation and higher brightness correspond to an increased content of alcohol in the floral scent. Furthermore, darker flower colors that lean towards the blue-purple spectrum are associated with higher levels of benzenoid compounds in the fragrance.

These relationships may be linked to the synthesis of anthocyanins and benzenoid compounds in the red cultivars of Phalaenopsis orchids, both of which are produced via the phenylpropanoid metabolic pathway. The synthesis pathways of floral fragrance components (such as terpenes and phenyl/phenylpropanoids) are somewhat related to the pathways of secondary metabolites, such as pigments (anthocyanins, carotenoids, etc.) [23,24]. The mevalonate pathway is an upstream pathway for the synthesis of benzenoid compounds and anthocyanins, while phenylpropanoids serve as precursors for the synthesis of benzenoid compounds and anthocyanins. The floral fragrance and color can be regulated through the mevalonate pathway simultaneously [23,25].

The flower color and fragrance are complex traits controlled by multiple genes. Based on previous studies [5,26,27], the synthesis pathways diagram for flower color and flower fragrance were depicted (Fig. 3). The diagram illustrates that the synthesis of flower color and fragrance components is closely related. For example, the synthesis of pigments such as carotenoids, and lycopene shares the methylerythritol phosphate (MEP) pathway with fragment molecules such as alcohols like linalool, citronellol, and geraniol and alkenes such as ocimene. The synthesis of blue-purple pigments from delphinidin and cornflower blue shares the Shikimate pathway with benzenoid compounds. These reveal the reasons behind the differences in the main aromatic components between yellow and purple Phalaenopsis orchids.

Phalaenopsis orchids had varied phenotypic diversity, particularly in the variation of flower color [28]. Previous research has indicated that purple-red coloration in Phalaenopsis flowers may be controlled by a single gene [29], and floral fragrance is a heritable trait. Tong et al. conducted experiments on the fragrance components in eight newly hybridized fragrant Phalaenopsis cultivars, categorizing the scents into woody, minty, and fruity types, and identified fragrance compounds as candidate marker traits to distinguish between different fragrant flower groups [13].