Effect of Drought and High-Light Stress on Volatile Compounds and Quality of Welsh Onion (Allium fistulosum L.)

Abstract

Welsh onion (Allium fistulosum L.) is a globally significant culinary vegetable with extensive cultivation and high application value. In China, Welsh onion is vulnerable to drought and strong-light stress in summer production, resulting in growth inhibition and quality decline. This study utilized LED-intelligent spectral-customized lamps to simulate high-light stress and a 10% PEG-6000 Hoagland solution to simulate drought stress. The effects of different stress treatments on the nutritional quality, volatile compounds, and mineral element composition of the edible portions were systematically analyzed. The results demonstrated that drought stress significantly promoted the accumulation of alcoholic compounds in leaf tissues while reducing the content of sulfur-containing compounds. High-light stress markedly increased the levels of hydrocarbon compounds in leaves. Sulfur-containing compounds in leaf tissues were predominantly disulfides, but under combined drought and high-light stress, their content decreased, while the proportion of trisulfides significantly increased. Volatile compounds in pseudostems were primarily composed of sulfur-containing and aldehyde compounds, yet their levels markedly declined under combined stress. Additionally, combined stress led to reductions in pyruvic acid, soluble sugars, and soluble protein content in the edible portions, while the crude fiber content increased, thereby significantly impairing nutritional quality. This study provides a scientific basis for understanding the abiotic stress response mechanisms of Welsh onion and offers valuable insights for cultivation management and quality regulation.

1. Introduction

Plants can synthesize hundreds of thousands of primary and secondary metabolites with distinct flavors and aromas. Vegetables are an essential component of our daily diet, and their primary metabolites consist of sugars, acids, salts, bitter compounds, and volatile substances, among others. The unique flavor of vegetables is often determined by the volatile substances they contain. Allium plants, one of the largest plant genera in the world, are widely cultivated for their flavor, medicinal, and nutritional properties [1,2]. Nowadays, the consumption of these vegetables is increasing worldwide, partly due to consumer awareness of their potential to enhance health and reduce the risk of diseases [2]. The active components, such as organic sulfur compounds, in Allium plants show broad-spectrum prevention potential for metabolic syndrome, cardiovascular disease, cancer, and infectious diseases through multiple pathways, such as anti-oxidation, anti-inflammation, regulation of metabolism, anti-cancer, and immune regulation, and have important health promotion value [3,4,5]. Some studies have shown that Allium crops’ characteristic flavor and health function are closely related to volatile flavor components [6,7]. The volatile flavor compounds mainly consist of sulfur compounds, alcohols, aldehydes, esters, etc., among which sulfur compounds are the source of the distinctive flavor and largely determine the quality of Allium crops [8].

As the critical component of the characteristic aroma of Allium, sulfur compounds are produced by the degradation of its precursor S-alk (En)-cysteine sulfoxide (ACSO) [8,9]. ACSO accumulates in the cytoplasm of stored mesophyll cells in intact Allium cells. In contrast, alliinase accumulates in the vacuoles of vascular bundle sheath cells, and alliinase is released from vascular sheath cells [10]. It hydrolyzes ACSO to form a series of reactive organosulfur compounds with unique flavors and significant biological activity [11]. For example, onions are flavorful and tear-inducing because they contain a high proportion of 1-PECSO [12,13]; ACSO dominates garlic [13,14]; and ACSO and MCSO dominate leeks [15]. It is well known that different varieties of Allium vary in flavor and intensity [16]. Allium plants grown in various regions may produce different flavor intensities depending on the environmental conditions under which they are grown [17,18]. The pungency of Allium plants increases with higher sulfate levels in the soil, but the sulfate content must be regulated within an appropriate range to achieve the desired flavor intensity [19,20]. The effect of soil sulfate on flavor intensity reaches a saturation point, beyond which additional sulfate does not significantly enhance pungency [21]. Furthermore, previous studies showed that a high nitrogen supply affected the flavor intensity and quality of onions. MCSO and PCSO increased with nitrogen availability, while 1-PECSO increased first and then decreased under high-nitrogen treatment [22,23]. Ling-jun, et al. [24] showed that the sulfide content in Welsh onion increased significantly with N and S application. Still, the effect of S on the distribution of mineral elements in different organs of Welsh onion was far less than that of N.

Drought stress is one of the major abiotic factors limiting crop production. It not only affects plant growth and yield, but also reshapes secondary metabolic networks, thereby altering the flavor quality of crops [25,26]. Studies have shown that moderate drought stress can significantly enhance the biosynthesis of the flavor precursor alliin in garlic bulbs and increase the activity of allinase. This coordinated upregulation of metabolic pathways ultimately leads to a marked increase in characteristic pungent flavor compounds such as allicin [18]. Furthermore, drought stress impacts the flow of primary metabolism by inhibiting photosynthetic carbon assimilation and altering nitrogen metabolism, thereby providing substrates and energy for the synthesis of flavor substances [27,28]. Under adverse conditions, plants allocate more of their limited carbon and nitrogen resources to the synthesis of secondary metabolites with defensive functions [29]. This resource allocation alters the accumulation patterns of basic nutritional components such as soluble sugars and free amino acids, which, in turn, reshapes the overall flavor profile of the product, for example, potentially resulting in reduced sweetness and increased bitterness [30].

Light is a key environmental factor regulating plant growth and development that directly affects the synthesis of metabolites and the formation of quality [31,32]. In Allium plants, light intensity, light quality (spectral composition), and photoperiod significantly affect plant growth, nutrient accumulation, and flavor synthesis by regulating light-dependent metabolic pathways. Studies have shown that different light quality combinations can effectively increase leaf pigment content, vitamin C, soluble sugar, organic acids, and antioxidant activity [33,34]. Supplementing blue light on the basis of white LED can significantly affect the photosynthetic characteristics and growth response of Welsh onion [35]. In addition, continuous light may lead to down-regulation of onion photosynthesis, thereby inhibiting its biomass accumulation and distribution [36].

Under natural conditions, plants are often exposed to two or more combinations of stresses simultaneously or continuously. With the recent climate changes, high radiation levels and drought are considered more serious abiotic stress factors. Both bloom and water scarcity trigger morphological, physiological, biochemical, and molecular changes that adversely affect plant growth, health, and productivity [37]. Welsh onion is a perennial herbaceous plant of the genus Allium in the family Liliaceae. The summer production of Welsh onion in China is often affected by drought and intense light, which leads to poor growth and lower yield and quality [38,39]. It is worth noting that research on the synthesis of volatile compounds in allium plants under stress is still relatively limited. Therefore, this study aimed to investigate the effects of drought and bloom stress on the volatile compounds synthesis in Welsh onion and to provide a scientific basis for the production and quality improvement of Welsh onion.

2. Materials and Methods

2.1. Plant Material and Cultivation

The experiment was conducted in a lightroom at the College of Horticulture Science and Engineering, Shandong Agricultural University, Tai’an, China (longitude: 117.12° E; latitude: 36.19° N) in March 2021. ‘Zhangqiudacong’, from Zhangqiu City in Shandong Province, is a Chinese National Geographic Indication product and is highly favored by consumers for its unique quality. So, we used the Welsh onion variety ‘Zhangqiudacong’, sourced initially from the Tai’an Denghai Wuyue Taishan Seed Industry Co., Ltd. (Tai’an, China).

The seeds were planted in 32-hole trays, and the cultivation substrate was a 6:3:1 mixture of charcoal, perlite, and vermiculite. The seedlings were watered with 1/2 Hoagland nutrient solution every three days after sowing. When the seedlings grew to 4–5 leaves and the plant height was approximately 20 cm, the seedlings were transported into dark plastic containers (area: 0.036 m²; 20 plants/container) for hydroponic cultivation. The electrical conductivity (EC) and pH of the nutrient solution were maintained at 2.2~2.5 ms·cm⁻¹ and 6.8~7.0, respectively. The nutrient solution was ventilated with an air pump every 2 h and replaced every 2~3 d.

2.2. Experimental Design

The experiment included two water treatments [standard water supply (Hoagland nutrient solution) and drought stress [40,41] (Hoagland nutrient solution containing 10% PEG-6000)] and two light intensity treatments [38] [standard light intensity (800 μmol/m²/s) and high light intensity (1800 μmol/m²/s)], with a total of 4 treatments, namely, standard water supply + standard light intensity (T1), standard water supply + high light intensity (T2), drought stress + standard light intensity (T3), and drought stress + high light intensity (T4). PEG-6000 was dissolved in the nutrient solution to simulate rhizosphere drought stress. Light intensity treatment was achieved by adjusting the LED intelligent spectrum-customized light (Huizhou Kedao Intelligent Technology Co., Ltd., Huizhou, China) to achieve the required light intensity. After the seedlings grew uniformly, they were placed in the corresponding lightroom (the lighting time was 12 h/d, and the relative humidity was 60–80%). Three replicates were set up for the experiment, with each replicate consisting of 20 plants, and each treatment included 60 plants in total. On the 7th day of treatment, samples were collected to determine the nutritional quality, volatile compounds, and mineral elements.

2.3. Determination of Nutritional Quality

Five individual plants of each treatment were randomly selected. Before the measurement, the fresh pseudostems and leaves were separated, washed with distilled water, chopped, and ground to determine the nutritional quality immediately. An amount of 0.5 g of leaves and pseudostem samples was accurately weighed for the determination of various indicators.

The soluble sugar content was determined by the anthrone–sulfuric acid method [42]. The sample was supplemented with 10 mL of distilled water, homogenized in an ice bath, and centrifuged at 10,000× g at 4 °C for 10 min, and the supernatant was taken for determination. The content of crude fiber was determined by the concentrated sulfuric acid hydrolysis method and slightly modified [43]. The sample was digested with 60 mL of pre-cooled 60% sulfuric acid in a cold-water bath for 30 min and then diluted to 100 mL with the same sulfuric acid and filtered. The 5 mL filtrate was diluted to 100 mL, reacted with anthrone reagent, and determined at 620 nm. Cellulose was used as the standard. The soluble protein content was determined by the Bradford method [43]. The samples were homogenized by an ice bath in distilled water and centrifuged at 10,000 r/min at 4 °C. The supernatant was reacted with Bradford reagent and determined at 595 nm. The standard curve was drawn with bovine serum albumin. The content of free amino acids was determined by the ninhydrin solution chromogenic method [43]. The sample was ground to 100 mL with 5 mL of 10% acetic acid. The extract was reacted with the ninhydrin reagent, and the absorbance was measured at 570 nm. Leucine was used as the standard for quantification. The content of pyruvic acid was determined according to the 2,4-dinitrophenylhydrazine colorimetric method [44]. The sample was ground with 8% trichloroacetic acid in an ice bath, and the supernatant was taken after standing and centrifugation. The supernatant was supplemented with 2,4-dinitrophenylhydrazine and NaOH for color development. The determination was carried out at 520 nm, and the standard curve was drawn with sodium pyruvate.

2.4. Determination of Volatile Compounds

The content of volatile compounds in edible parts of Welsh onion was determined and analyzed by headspace solid-phase microextraction–gas chromatography–mass spectrometry (SPME-GC-MS), as described previously, with minor modifications [45]. Briefly, five fresh plants were randomly selected for each treatment, the middle area of pseudostems and leaves was taken, respectively, and they were mixed and sampled by quarter methods. Immediately, 2 g fresh samples were weighed accurately and quickly, mixed with 4 mL of 90% methanol, and put in a 20 mL headspace bottle with 2-octanol (internal standard; 0.819 μg/mL). After sealing the headspace bottle containing the sample, it was gently shaken a few times and placed in a 45 °C water bath. Then, the aged solid-phase microextraction needle (DVB/CAR/PDMS, 50/30 μm, Millipore-Sigma, Burlington, MA, USA) was inserted vertically into the headspace bottle, pushed out the fiber head, and extracted for 30 min. After the extraction was completed, the extraction needle was vertically inserted into the injection port of the gas chromatography–mass spectrometer (GC-MS QP-2010, Shimadzu, Kyoto, Japan) for quantitative analysis.

With helium as the carrier gas, a Rtx-5 ms capillary column (30 m × 0.25 mm ID. × 0.25 μm; maximum temperature: 350 °C) was used, and the constant flow rate was 0.89 mL/min. The sample was desorbed for 3 min with a GC inlet at 230 °C. The temperature setting procedure was as follows: the column temperature was 40 °C for 2 min, then rose to 70 °C at a rate of 4 °C/min for 1 min, and then rose to 230 °C at a rate of 10 °C/min for 5 min. The ion source temperature of the mass spectrometer was 230 °C, and the interface temperature was 150 °C. All analyses were performed in full-spectrum scanning mode, with a mass scanning range of 45–500 m/z.

Wind analysis of volatile compounds was performed through the GC-MS reanalysis software, compared with mass spectrometry data in the National Institute of Standards and Technology (2017), and combined with artificial spectra, literature analysis, and analysis of valid RetIndex values to preliminarily identify volatile components. The peak area normalization method was used to analyze each volatile compound quantitatively.

The relative content of each component was obtained according to the peak area normalization method, and the content of each component was calculated according to [(single peak area/internal standard peak area) × internal standard peak content]/sample mass.

2.5. Determination of Mineral Elements

Welsh onion plants were randomly selected from each treatment, and the leaves and pseudostems were separated, washed thoroughly with distilled water, and blanched at 105 °C for 15 min. They were then dried at 75 °C to a constant weight and ground into a powder. An amount of 0.1 g of the dried sample was accurately weighed and placed in a 25 mL Erlenmeyer flask. An amount of 1 mL of deionized water was added to moisten, and then 5 mL of concentrated sulfuric acid was added and allowed to stand for 24 h. The mixture was then heated and digested on a 200 °C electric hot plate until white fumes appeared. H₂O₂ was added dropwise until the digest became clear. After cooling, the digest was transferred to a 50 mL volumetric flask, diluted to volume, and filtered through a 0.22 μm membrane. The total nitrogen (TN) and total phosphorus (TP) contents were determined using the Kjeldahl method [46] and the molybdenum blue colorimetric method [47], respectively.

Additionally, 0.2 g of the dried sample (pseudostem or leaf) was accurately weighed and placed in a digestion tube, and 8 mL of an HNO₃:HClO₄ (4:1, v/v) mixed acid was added. The samples were digested using a CEM high-throughput closed microwave digestion system (CEM MARS6, CEM Corporation, Matthews, NC, USA). After cooling, the digest was transferred to a 50 mL volumetric flask, diluted to volume, and then filtered through a 0.22 μm membrane. The contents of potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), sulfur (S), and iron (Fe) were determined using an inductively coupled plasma–optical emission spectrometer (ICP; iCAP 7000; Thermo Fisher Scientific, Waltham, MA, USA).

2.6. Statistical Analysis

Experimental data were collected randomly from different treatments using three independent replicates. The data are expressed as three independent experiments’ mean ± standard deviation. Principal component analysis (PCA) and redundancy analysis (RDA) analyses were performed using the R language. PCA was performed to determine the differences in volatile compounds from the samples. RDA analyzed the effects of different mineral elements on the volatile substances in various organs of the Welsh onion. All statistical analyses were performed using the DPS v 7.05 software. Calculations were performed using one-way ANOVA and Duncan’s multiple-range test. p < 0.05 was considered to be statistically significant.

3. Results

3.1. Identification and Analysis of Volatile Compounds

Forty-one volatile compounds were detected in the leaves of the Welsh onion. The maximum number of substance components detected by the T3 treatment was 37, followed by T1, while the T2 and T4 treatments only detected 26. There were 69 kinds of volatile substances in the pseudostem of the Welsh onion. Fifty-seven species were detected by the T2 treatment, and the T4 treatment detected the least (Table S1). The volatile compounds were classified into eight categories: alcohols, ketones, hydrocarbons, sulfur compounds, aldehydes, pyrazines, esters, and others.

The volatile compounds in the edible parts of Welsh onion under different treatments were analyzed by the PCA method (Figure S1). The volatile compounds in leaves could be divided into two categories: T1 and T3, and T2 and T4, and the two categories had an overlap on the Dim1 axis, indicating that the volatile compounds in them were relatively close. The above results indicated that drought treatment had an effect on the volatile compounds in the leaves, and the effect was greater than that of high-light treatment. The volatile compounds in pseudostems could be divided into two groups by PCA analysis; T1, T2, and T3 were one group, and T4 was another.

In leaves, the total volatile compounds content was not significantly different (Figure 1A), but the alcohol content of T2 and T4 was significantly increased, while the content of the other classifications was significantly reduced, and pyrazines were not even detected (Figure 1B). Compared with T1 and T3, the alcohol content of T2 increased by 19.92% and 30.08%, and that of T4 increased by 19.1% and 28.88%, respectively. The main volatile substances of T1 and T3 were sulfurs, ketones, and hydrocarbons, while those of T2 and T4 treated leaves were mainly alcohols, ketones, and sulfurs (Figure 1C). Compared with T1, the proportion of hydrocarbon compounds in T3 increased, but the proportion of sulfur and ketones decreased. The proportion of alcohol compounds of T2 and T4 was as high as 50%, and the proportion of sulfur compounds decreased.

In pseudostems, T2 and T4, compared with T1, significantly reduced the total volatile matter of pseudostems by 29.4% and 28.8%, respectively (Figure 2A). Sulfur-containing compounds significantly contributed to the total amount of volatile compounds, of which the content of T1 was the highest, followed by T3 (Figure 2B). Generally, the main volatile substances in T1, T2, and T3 were sulfurs and aldehydes, while T4 was mainly sulfurs and ketones (Figure 2C).

3.2. Analysis of Sulfur Compounds

In leaves, the total amount of sulfur compounds of T2 and T4, compared with T1, decreased significantly by 60.09% and 52.54%, respectively (Figure 3A). Subsequently, we divided the sulfur compounds into five categories: disulfide compounds, trisulfide compounds, cyclic sulfur compounds, thiophene compounds, and other sulfur-containing compounds (Figure 3B). The content of disulfides makes a significant contribution to the total amount of sulfur compounds. Under the same light conditions, drought treatment reduced the disulfide proportion and increased the trisulfide proportion. Under the same moisture conditions, the high-light treatment increased the disulfide proportion (Figure 3C).

In pseudostems, the total amount of sulfur compounds of T2 and T4, compared with T1, was significantly reduced by 29.58% and 32.7%, respectively (Figure 4A). Under standard light conditions, the proportion of sulfur compounds decreased under drought treatment, but the proportion of disulfide compounds increased. The trend in the high-light conditions was the opposite (Figure 4B,C).

3.3. Nutritional Quality of Welsh Onion

In leaves, pyruvate, soluble protein, soluble sugar, and free amino acids in T3 were significantly lower than those in T1, which decreased by 17.26%, 17.89%, 11.30%, and 11.49%, respectively, and the crude fiber content increased by 25.37%. T4 was also significantly lower than T2, with decreases of 21.42%, 17.2%, 21.72%, and 8.91%, respectively, and the crude fiber content increased by 25.95% (

Table 1. Effects of drought and high-light conditions on the nutritional quality of the edible parts of Welsh onion.

Treatment		Pyruvate (mg/g)	Soluble Protein (mg/g)	Soluble Sugar (%)	Free Amino Acid (mg/g)	Crude Fiber (mg/g)
Leaf	T1	0.97 ± 0.01 a	1.02 ± 0.01 a	0.84 ± 0.01 a	0.66 ± 0.02 a	0.31 ± 0.01 d
	T2	0.58 ± 0.01 c	0.61 ± 0.01 c	0.65 ± 0.01 c	0.49 ± 0.01 c	0.56 ± 0.02 b
	T3	0.81 ± 0.01 b	0.84 ± 0.01 b	0.75 ± 0.01 b	0.58 ± 0.01 b	0.39 ± 0.01 c
	T4	0.45 ± 0.03 d	0.50 ± 0.04 d	0.51 ± 0.02 d	0.45 ± 0.01 d	0.70 ± 0.02 a
Pseudo-stem	T1	0.88 ± 0.01 a	1.2 ± 0.024 a	1.39 ± 0.04 a	1.1 ± 0.01 a	0.65 ± 0.01 d
	T2	0.62 ± 0.01 c	0.81 ± 0.01 c	0.80 ± 0.01 c	0.77 ± 0.01 c	0.88 ± 0.02 b
	T3	0.83 ± 0.01 b	1.16 ± 0.01 b	1.10 ± 0.01 b	0.89 ± 0.01 b	0.78 ± 0.03 c
	T4	0.59 ± 0.01 d	0.61 ± 0.02 d	0.64 ± 0.02 d	0.63 ± 0.01 d	1.17 ± 0.02 a

The data are from three replicated experiments and are represented as means ± SDs. Different letters indicate significant differences (p < 0.05) among different treatments. T1: treatment with standard water supply and standard light intensity; T2: treatment with standard water supply and high light intensity; T3: treatment with drought stress and standard light intensity; T4: treatment with drought stress and high light intensity.

). Under the same light conditions, drought treatment significantly reduced nutritional quality.

The changing trend of each nutritional quality of the pseudostem was consistent with that of the leaves. Under the same light conditions, the contents of pyruvate, soluble protein, soluble sugar, and free amino acids in the T2 treatment were significantly lower than those in T1, which decreased by 28.74%, 34.93%, 42.35%, and 30.08%, respectively, and the crude fiber content increased by 34.80%. The contents in the T4 treatment were also significantly lower than in T3, which decreased by 5.62%, 23.94%, and 20.05%, respectively, and the crude fiber content increased by 33.30%. Under the same moisture conditions, high-light treatment reduced nutritional quality.

3.4. Analysis of Mineral Elements

The TN content in leaves was the highest in T1, which increased by 54.98%, 20.43%, and 80.91% compared with T2, T3, and T4, respectively, while in the pseudostem, it increased by 49.17%, 12.57%, and 91.66%, respectively (

Table 2. Effects of drought and high-light conditions on mineral elements in the edible parts of Welsh onion.

Treatment		TN (mg/g)	TP (mg/g)	K (mg/g)	S (mg/g)	Ca (mg/g)	Mg (mg/g)	Fe (mg/g)	Na (mg/g)
Leaf	T1	31.27 ± 0.36 a	6.11 ± 0.02 a	45.32 ± 0.26 a	8.78 ± 0.06 a	10.39 ± 0.03 a	3.06 ± 0.04 a	0.20 ± 0.01 a	2.54 ± 0.05 d
	T2	20.17 ± 0.34 c	3.77 ± 0.03 c	34.60 ± 0.22 c	7.92 ± 0.08 c	7.50 ± 0.05 c	2.72 ± 0.04 c	0.15 ± 0.01 c	3.18 ± 0.14 b
	T3	25.96 ± 0.55 b	4.58 ± 0.03 b	37.03 ± 0.64 b	8.35 ± 0.08 b	8.35 ± 0.05 b	2.91 ± 0.04 b	0.18 ± 0.01 b	2.87 ± 0.08 c
	T4	17.28 ± 0.11 d	3.27 ± 0.01 d	27.13 ± 0.34 d	7.68 ± 0.03 d	6.63 ± 0.04 d	2.48 ± 0.03 d	0.14 ± 0.01 d	3.39 ± 0.01 a
Pseudo-stem	T1	18.23 ± 0.30 a	4.76 ± 0.09 a	20.32 ± 0.4 a	8.59 ± 0.03 a	8.86 ± 0.11 a	2.20 ± 0.03 a	0.30 ± 0.01 a	1.38 ± 0.06 c
	T2	12.22 ± 0.19 c	3.40 ± 0.06 c	13.41 ± 0.57 c	8.16 ± 0.05 c	5.42 ± 0.26 c	1.95 ± 0.0 bc	0.22 ± 0.01 c	1.64 ± 0.01 b
	T3	16.19 ± 0.12 b	4.40 ± 0.03 b	15.09 ± 0.2 b	8.44 ± 0.02 b	6.00 ± 0.07 b	2.04 ± 0.01 b	0.24 ± 0.01 b	1.47 ± 0.05 c
	T4	9.51 ± 0.20 d	3.05 ± 0.03 d	12.56 ± 0.21 c	7.79 ± 0.04 d	4.71 ± 0.19 d	1.84 ± 0.08 c	0.20 ± 0.01 c	1.74 ± 0.01 a

The data are from three replicated experiments and are represented as means ± SDs. Different letters indicate significant differences (p < 0.05) among different treatments. T1: treatment with standard water supply and standard light intensity; T2: treatment with standard water supply and high light intensity; T3: treatment with drought stress and standard light intensity; T4: treatment with drought stress and high light intensity.

). The S content in the leaves of T2, T3, and T4 was significantly lower than in those of T1, reduced by 9.80%, 5.43%, and 13.17%, respectively, whereas the S content in the pseudostem of T2, T3, and T4 decreased by 5.01%, 1.84%, and 9.48%, respectively, compared with T1. In general, except for Na, drought treatment reduced the mineral element contents in leaves and pseudostems of Welsh onion under the same light conditions. Still, under the same light conditions, the high-light treatment caused the mineral element contents in leaves and pseudostems to decrease.

3.5. Correlation Between Volatile Compounds, Nutritional Quality, and Mineral Elements

The positive correlation between soluble proteins and free amino acids in the leaves reached a significant level (Figure 5A). Among the flavor compounds, sulfur-containing compounds had a significant negative correlation with alcohols and ketones. Sulfur-containing compounds correlated significantly positively with soluble proteins, sugars, and free amino acids. Sulfur-containing compounds were positively correlated with Fe. There was a significant positive correlation between ketones and all mineral elements except Na and S, and the changing trends of hydrocarbon and alcohol were the same.

Among the nutritional indicators, except for crude fiber, all other indicators were positively correlated (Figure 5B). Except for ketones, there was a positive correlation between volatile compounds. In general, crude fiber positively correlated with ketones but negatively correlated with other volatile compounds and nutritional quality. Sulfur-containing compounds correlated significantly positively with pyruvate, soluble protein, and soluble sugar. Except for Na, each mineral element had a positive correlation. Sulfur-containing compounds were significantly positively correlated with TN and TP. S content had a significant positive correlation with aldehydes and lipid compounds.

4. Discussion

Different regions produced allium crops with varying flavors; even the same variety can have different flavor compounds depending on the growth environment [48]. In a controllable environment under drought and high-light conditions, we used Welsh onion (Allium fistulosum L.) as the material and analyzed the volatile compounds in the edible parts of the plant (leaves and pseudostems) using SPME-HS-GCMS. The results indicated compounds similar to those identified in previous studies, but with varying concentrations of each component [48,49,50].

Numerous studies have demonstrated that the moisture in soil significantly impacts the flavor of vegetables [51,52,53]. There is an ongoing debate as to whether drought enhances the pungent flavor of Allium crops. It is generally believed that drought stress leads to a reduction in the size of vegetative organs in crops like onions, while the concentration of aromatic substances inside cells increases, resulting in a stronger flavor intensity [54]. However, some experiments have pointed out that there is a weak correlation between water use in onions and the metabolic pathways of their flavor precursors [55]. We speculate that differences in organ water content among various studies may be one of the reasons for the above controversy. Therefore, in this study, all indicators were measured based on the edible portions, and water content was uniformly corrected during data analysis to eliminate deviations caused by differences in water content. This approach may have caused some of our results (such as the decrease in volatile sulfur compounds under drought conditions) to be inconsistent with previous reports.

The experiment showed that the proportion of alcohol in Welsh onion leaves increased significantly under drought treatment compared with the control. Similarly, drought stress led to an increase in the release of short-chain oxygen-containing volatile compounds [56]. When subjected to a single high-light treatment, Welsh onion leaves mainly produced hydrocarbons and sulfur-containing compounds, and the total amount of volatile compounds decreased. Both low and high light intensity can reduce the release of plant volatiles. Under double stress, the total quantity of volatile compounds decreased significantly, and alcohol compounds dominated. Sulfur compounds have the lowest thresholds among various volatile compounds, but they play a crucial role in determining the flavor characteristics of crops [57]. When diluted to ppb or ppm concentrations, sulfur compounds produce fresh green onion and tropical fruit aromas [58]. The volatile compounds in leaves and pseudostems of Welsh onion were mainly disulfide compounds under different growing conditions, in which (Z)-1-propenyl propyl disulfide was the main contribution, while the contents of (e)-1-propyl propyl disulfide in pseudostems were the highest.

Enzymatic hydrolysis produces pyruvate, which is commonly used to measure the intensity of flavor [57,59]. Soluble sugars and proteins are used to describe sweetness. Drought, high-light, and double stress can reduce pyruvate, soluble sugar, and soluble protein levels in Welsh onion leaves and pseudostems while increasing crude fiber content. This can result in the deterioration of food sensitivity and a decrease in nutritional quality. Nitrogen, calcium, sulfur, and other elements affect allium crops’ flavor intensity and quality [60]. The appropriate amount of calcium can increase the spicy flavor of onion [61,62]. Suppose that plants have a limited ability to absorb sulfur. In that case, the quantity of sulfur-based flavor precursors will be significantly impacted, leading to differences in flavor precursors in different sulfur environments [63]. Unfavorable growth conditions can restrict plant roots’ uptake of mineral elements, affecting flavor precursors’ production [64]. Welsh onion leaves and pseudostems experience a reduced mineral element content (except for Na) under drought conditions, high light intensity, and a dual environment.

The analysis of volatile components, nutritional indices, and mineral elements in the edible portion revealed a significant positive correlation, consistent with previous studies’ findings [65]. It is generally believed that there is a weak positive correlation between soluble solids and spicy taste, which suggests that producing pungent compounds can also contribute to the formation of soluble solids [66,67,68]. However, it is worth noting that the biosynthetic pathways of flavor precursors and soluble carbohydrates are largely unrelated.

5. Conclusions

In conclusion, this study explains how drought and high-light stress affect Welsh onion. Drought stress increases alcohol compounds but reduces sulfur compounds in leaves. High-light stress increases hydrocarbon compounds. When both stresses occur together, sulfur compounds change, with more trisulfides and fewer disulfides. The pseudostem, an important edible part, showed a decrease in sulfur and aldehyde compounds under both stresses. There are also lower levels of pyruvic acid, sugars, and proteins, but more crude fiber, which means the nutritional quality worsens. These results help us understand how Welsh onion adapts to stress and show the potential of using special LED lights for stress studies.