Extending the Vase Life of Vanda Orchid Cut Flowers Using Plasma Technology

Abstract

Flower senescence during transport is a major concern for exporters, as physiological disorders reduce quality and price. Extending vase life is crucial, and while 1-MCP is widely used, it requires low temperatures and is less effective in disease control. Cold plasma generated by dielectric barrier discharge produces reactive oxygen and nitrogen species (RONS), offering an alternative method for preserving cut flowers. This study compared the effectiveness of cold plasma and 1-MCP treatments on the vase life of Vanda ‘Pachara Blue’ orchids. Flowers were treated with T1 (control at 25 °C), T2 (1-MCP), and T3 (cold plasma). Both 1-MCP and cold plasma significantly reduced ethylene production (26.15 and 25.20 µL C₂H₄/kg/hr, respectively) and respiration rate (63.92 and 57.44 mg CO₂/kg/hr, respectively) compared to the control (40.93 µL C₂H₄/kg/hr and 118.21 mg CO₂/kg/hr). Vase life was extended to 19.33 days in both treatments, an 87.12% increase over the control (10.33 days). Additionally, cold plasma slightly improved water uptake and reduced petal discoloration. These findings indicate that cold plasma is a promising alternative to 1-MCP, offering effective flower preservation without the need for low-temperature conditions and potential additional benefits in floral quality.

1. Introduction

Nonthermal plasma (NTP) technology, often referred to as a chaotic state, comprises numerous radical species like O, O₂, O₃, OH, NO, NO₂, and NO_x, along with electric fields, ultraviolet radiation (UV), free radicals, and high-energy particles [1]. Generally, glow discharge, corona discharge, radiofrequency discharge, dielectric barrier discharge (DBD), microwave discharge, and plasma jet can be generated in various methods by nonthermal plasma [2]. The reactive oxygen and nitrogen species (RONS), i.e., NO, OH, and N₂, are generally known as beneficial in the postharvest process. For example, the application of exogenous NO gas resulted in the postponement of respiration and ethylene synthesis peaks in some plants, such as mangoes [3]. Furthermore, oxygen species (ROS) such as hydrogen peroxide (H₂O₂), hydroxyl radical (OH), and ozone (O₃), as well as nitrogen species (RNS) like nitric oxide radical (NO), generated by DBD, are primary antimicrobial agents responsible for microbial inactivation. They achieve this by causing damage to proteins and DNA [4]. However, such radicals, i.e., NO, exhibit a brief half-life in gaseous form; thus, they are usually reported by NO₂⁻ and NO₃⁻ because NO is their precursor [5,6,7,8]. In addition, the OH radical is extremely sensitive to an extended time in the gas phase, making it difficult to detect. Conversely, the long-lived H₂O₂ is relatively less impacted by these factors [9]. The RONS obtained from those processes play an important role in microorganism defection and the breakdown of chemical residues in products; this contributes to enhancing food security and health outcomes [10,11]. Moreover, NTP technology is widely applied in fields such as medicine, electronics, material science, and agricultural science [12]. Examples of research in these fields include studies showing that NTP can improve skin wound healing rates in murine models by approximately 17.39% compared to the untreated group. Additionally, the surface properties of polymers, including wettability, moisture resistance, printability, dye uptake, and adhesion resistance to other polymers or materials, can also be enhanced by NTP [13,14]. In the agricultural sector, NTP has become popular for various applications such as microbial inactivation, stimulating seed germination, and enhancing plant growth [15,16,17,18,19,20,21,22]. Moreover, it has the potential to modify metabolic characteristics and enhance the endurance capacity of fresh agricultural products, helping them withstand senescence and adverse environmental conditions. Yi et al. [23] found that fresh-cut mango cubes treated by dielectric barrier discharge (DBD) at 75 kV for 3 min have the potential to improve quality and extend their shelf life. Also, Jin et al. [24] investigated the shelf-life extension and fruit qualities affected by cold atmospheric plasma (CAP), and their findings indicated that CAP significantly delayed ripening and that the total aerobic bacterial count in jujubes treated with CAP was consistently reduced. As a result, this NTP treatment contributes to maintaining the physiological quality of the products and extending their shelf life [25]. In addition, previous studies have reported that reactive species such as NO can significantly delay senescence in flowers such as lilies, gladiolus, gerberas, and white prosperity by enhancing protein levels. NO also helps maintain optimal turgor pressure by improving water uptake and reducing water loss—key factors in preventing head drooping and wilting [26,27].

Orchids are classified as tropical ornamental plants, which are crucial to agricultural economics in Thailand. The most famous Thai orchids are Cattleya, Vanda, Rhynchostylis, Phalaenopsis, and Dendrobium [28]. The top three orchid exporters are the Netherlands, followed by Thailand and China, indicating that Thailand is the 2nd largest orchid exporter in the world [29]. In the past five years, the export price of orchids has seen a continuous increase. In 2019, the quantity of orchids exported from Thailand was more than 23,000 tons, and the value was around USD 74.2 million, with USD 3.0 per kilogram. Moreover, the estimated wholesale price of Thailand orchids is approximately USD 1.34–5.36 per kilogram in 2024 [30,31].

Currently, the global fresh-cut flowers market has a value of over 1093 billion baht in 2022 and will reach 1852 billion baht by 2031, and orchids are one of the major fresh-cut flowers [32]. The Vanda orchid is one of the most popular cut flowers due to its color, especially the blue color with big florets, cold tolerance, the uniqueness of the flower shape characteristic, and vase life of about 11–12 days [33,34,35]. However, during the senescence phase of orchid flowers, there is an automatic rise in ethylene production and respiration rates. This natural increase in ethylene and respiration occurs as the flowers age, leading to their eventual decay [36,37,38]. Typically, in the later stages of flower senescence, common symptoms are wilting, petal fading, flower abscission, and reduction in fresh weight [39,40]. Physiological and morphological plant responses, such as wilting caused by the limitation of water uptake by the stem due to air blockage, microbial growth, and physiological occulting, contribute to a decrease in the fresh-cut flower’s vase life [41,42,43]. Thus, extending vase life during transportation to customers presents a significant challenge in the post-harvest handling process [44,45].

Maintaining quality is an influential aspect to consider when evaluating the quality of cut flowers for both the domestic and export markets [46]. Some conventional methods for shelf-life prolongation, such as 1-methylcyclopropene (1-MCP) and modified atmosphere packaging (MAP), are widely used in agricultural industries, but there are still some limitations. For example, neither 1-MCP nor MAP can efficiently manage fungal diseases [47,48]. Although other shelf-life extension chemicals, such as sulfur dioxide (SO₂), commonly used for grape preservatives, can inhibit microbial infection and eliminate various pathogenic bacteria, when used in high concentrations, they stimulate berry abscission [49,50,51,52,53].

Accordingly, the aim of this study was to utilize cold plasma treatment to prolong the shelf life of Vanda orchids. Our research will offer theoretical backing for recommending cold plasma as a replacement for traditional methods for extending vanda orchids’ vase life. The changes in plant physiological and chemical traits were compared with traditional methods (i.e., storage at room temperature (25 °C) and 1-MCP).

2. Materials and Methods

2.1. Atmospheric Cold Plasma System Preparation

A dielectric barrier discharge (DBD) is a type of electrical discharge, which was used in this experiment. The DBD plasma unit comprised a high-voltage (HV) electrode with a honeycomb pattern measuring 4 × 9 cm², a full-plate ground electrode, and a 1 mm thick alumina ceramic (Al₂O₃) dielectric barrier placed between the two electrodes. Plasma was generated from the surrounding air by applying a sinusoidal high voltage with a peak-to-peak value of 6.2 kV at a frequency of 18 kHz. The DBD plate (Figure 1A) was plugged in with a portable air conditioner (Figure 1B). The DBD portable air conditioner properties were a power supply of 220 to 240 V/50 Hz with an airflow of 320 m³/h and 6.87 sq m as a room area that was used in plasma treatment.

The reactive oxygen and nitrogen species (RONS) emitted from DBD were evaluated by using an optical emission spectrometer (OES) device fitted with an Exemplar LS-Smart CCD spectrometer operating at a resolution of 1.5 nm. The air-discharged plasma was gathered using a 2.5 mm diameter SMA-905 optical fiber, and data were collected across the UV–visible spectrum, ranging from 200 to 800 nm wavelength, with an integration time of 300 ms. It was found that the dominant emission bands of the nitric oxide (NO), hydroxide (OH), and nitrogen (N₂) are clearly shown (Figure 2).

In the present study, we measured the availability of RONS through NO₂⁻, NO₃⁻ and H₂O₂ analyses to represent the occurrence of RONS emissions by DBD. Nine plastic dishes containing 30 mL of deionized water were placed in several room positions to evaluate the dispersion of NO₂⁻, NO₃⁻, and H₂O₂ radicals and their concentrations (Figure 3). Using a commercially available kit comprising Griess reagents (Cayman Chemicals, Ann Arbor, MI, USA) to determine the emergence of NO₂⁻ and NO₃⁻, UV/Vis spectrophotometric (Shimadzu UV–1800, Kyoto, Japan) absorbances at 372 and 507 nm were used, respectively. The iodometric titration method was used to analyze the appearance of H₂O₂ [54]. Also, the samples of solution were added to 1 mL of 2% potassium iodide solution and 2 M HCl. The mixture was left in darkness for 15 min, causing it to change to a yellow hue. Subsequently, 0.1 M sodium thiosulfate solution was gradually added until the yellow color lightened. Upon adding a starch indicator, the solution transitioned to a blue color. Titration was then performed with a 0.1 M sodium thiosulfate solution until the blue solution became clear. First, the discharge time was fixed at 60 min to evaluate the distribution of RONS in the treatment room. The findings found that the distribution of those RONS was uniform throughout the room. Then, we investigated the concentration of RONS in each discharge time (minutes) combined with standing time (minutes) as follows: 30 × 30, 30 × 60, 60 × 30, 60 × 60, 90 × 30, and 90 × 60. The temperature and humidity during discharge were 25 °C and 70%, respectively. The concentrations of NO₂⁻, NO₃⁻, and H₂O₂ in each condition are shown in

Table 1. The effect of discharge time combined with standing time on the concentration of RONS in each condition under room temperature is 25 °C and air humidity is 70% RH.

Discharge Time × Standing Time	The RONS Concentration (ppm)
	NO2−	NO3−	H2O2
30 × 30	0.31 ± 0.51 b	9.16 ± 1.21 b	0.57 ± 0.00 b
30 × 60	0.36 ± 0.20 b	10.41 ± 0.98 b	0.95 ± 0.00 ab
60 × 30	0.38 ± 0.11 b	11.31 ± 1.11 b	1.13 ± 0.03 ab
60 × 60	0.46 ± 0.39 b	16.79 ± 2.39 ab	0.50 ± 0.00 ab
90 × 30	1.38 ± 0.26 a	21.25 ± 0.65 a	0.76 ± 0.01 ab
90 × 60	1.42 ± 0.67 a	20.75 ± 1.06 a	1.32 ± 0.00 a
Sig. (LSD < 0.05)	*	*	*

* Denotes that the result is statistically significant (p < 0.05); a, b = means followed by the same letters in the same column are not statistically significantly different from each other (p < 0.05).

.

In addition, we found that the concentration of NO₂⁻, NO₃⁻, and H₂O₂ in discharge time and standing time at 90 × 30 and 90 × 60 trended higher than other conditions (Table 1). Hence, the discharge time at 90 min combined with standing time at 30 min was substantial enough to be used further because it took less time than the discharge time at 90 min combined with standing time at 60 min, and the RONS concentration was not significantly different.

2.2. Plant Materials

The Vanda ‘Pachara Blue’ (V. Pachara Delight × V. coerulea) used in this study was obtained from Nakhon Pathom Province, Thailand, in February 2024. Orchid plants with flower buds were selected and transported to Plasma Vertical Farming, located in Science and Technology Park, Chiang Mai University. Consequently, the plants were nursed until the flowers had fully bloomed. For the experiment, Vanda inflorescences with 6 to 7 open florets and peduncle lengths of approximately 20 to 25 cm were chosen. After cutting off the peduncle from the orchid’s stem, soak the peduncle in deionized water within 1 h and re-cut it in water again before the treatments were started.

2.3. Experimental Design

Completely Randomized Design (CRD) was an experimental setup for this study. Vanda cut flowers were treated with different treatments, as described in

Table 2. The methodology for Vanda’s vase-life prolongation in each treatment.

Treatments	Methods for Preserving Cut Flower
1	Storage at 25 °C (control treatment)
2	The commercial 1-MCP 0.03% DP from Extico (Bangkok, Thailand) Co., Ltd. is used in a cold room with a temperature of around 15 °C for 5 h
3	Atmospheric cold plasma is discharged for 90 min combined with 30 min of standing time at a room temperature of 25 °C (Figure 4)

. Ten replicates were conducted for each treatment, with each replicate consisting of one detached inflorescence (a total of 10 inflorescences per treatment). After treatment, the plants were kept in a cold room at approximately 25 °C under full light supplementation. The peduncle of Vanda in various treatments was immersed in bottle glass containing 300 mL of deionized water and sealed with aluminum foil. For consistency and reliability, the experiment was repeated five times, with each run conducted independently.

2.4. Determination of Weight Loss and Water Uptake

The initiation of Vanda inflorescence fresh weight was weighed and monitored daily until the experiment ended. The fluctuations in fresh weight were represented as a percentage of the initial fresh weight. The results were calculated by the following equation:

$$\mathrm{Percentage} \mathrm{of} \mathrm{weight} \mathrm{loss} (\%) =100-{\displaystyle \frac{(\mathrm{daily} \mathrm{fresh} \mathrm{weight} \times 100)}{\mathrm{initial} \mathrm{fresh} \mathrm{weight}}}$$

(1)

Water uptake measurements were conducted daily by quantifying the volume of water present in the vase. The Vanda’s peduncle was also lifted above the water before each measurement, and the unit was expressed as milliliters per day (mL/d). Additionally, the top of the vase was consistently covered with aluminum foil to minimize evaporation throughout the experiment.

2.5. Determination of Floret Color

A colorimeter (CR-400 Chroma meter, KONICA MINOLTA, Tokyo, Japan) was utilized for color determination. Three groups of color charts were separated as follows: lightness (L*) ranging from black to white (0 to 100); redness (a*) ranging from green to red (−60 to 60); and blueness or yellowness (b*) ranging from blue to yellow (−60 to 60). These classifications are based on the standards set by the International Color Standardization body (Commission Internationale de I’Eclairage, CIE). The first floret from below of the inflorescence was selected for measurement, and the process was repeated three times to obtain the average values. The total color change was calculated by the following equation:

$$\Delta \mathrm{E}=\sqrt{{\left({\mathrm{L}}_0^{*}-{\mathrm{L}}^{*}\right)}^2+{\left({\mathrm{a}}_0^{*}-{\mathrm{a}}^{*}\right)}^2+{\left({\mathrm{b}}_0^{*}-{\mathrm{b}}^{*}\right)}^2}$$

(2)

2.6. Determination of Ethylene Production and Respiration Rates

Measure ethylene production and respiration rate by taking 3 inflorescences of Vanda cut flowers and placing them in a gas-tight glass jar with a volume of 1000 cubic centimeters for 2 h. Afterward, a 1-milliliter gas sample was randomly collected and injected into a gas chromatography machine (Agilent Technologies, Santa, Clara, CA, USA), model 7820A, equipped with a flame ionization detector (FID). Utilize the area under the graph to calculate the ethylene production and respiration rate using the following Equations (3) and (4):

$$\mathrm{Ethylene} \mathrm{production} \mathrm{rate} (\mu \mathrm{L}/\mathrm{kg}/\mathrm{hr})={\displaystyle \frac{\mathrm{free} \mathrm{volume} \left(\mathrm{L}\right) \times \mathrm{ppm} \mathrm{ethylene} \mathrm{measured}}{\mathrm{sample} \mathrm{wt} \left(\mathrm{kg}\right) \times \mathrm{sealed} \mathrm{time} \left(\mathrm{hr}\right)}}$$

(3)

where

• free volume (L) = Volume of glass jar—volume of Vanda cut flower;
• ppm ethylene measured = Ethylene concentration (from gas chromatography machine);
• sample wt (kg) = Sample weight;
• seal time (hr) = Duration of placing the inflorescences in a gas-tight glass jar.

$${\mathrm{CO}}_2 \mathrm{production}= {\displaystyle \frac{\mathrm{A} \times \mathrm{Y} \times 10}{\mathrm{W} \times \mathrm{T}}}$$

(4)

where

• V (mL) = Volume of glass jar;
• W (g) = Sample weight;
• V-W = Y = Gap volume;
• T (time (min)/60) = Inflorescence placement duration in a gas-tight glass jar;
• A = CO₂ production = % CO₂ − 0.03.

2.7. Evaluation of Percentage of Flower Abscission and Vanda Cut Flower Vase Life

The shedding of Vanda flowers was recorded both before and after storage at each time point throughout the experiment, and the data were represented as a percentage. The results were calculated by the following equation:

$$\mathrm{Percentage} \mathrm{of} \mathrm{flower} \mathrm{abscission} (\%) =100 -{\displaystyle \frac{ (\mathrm{number} \mathrm{of} \mathrm{flowers} \mathrm{in} \mathrm{each} \mathrm{day}\times 100)}{\mathrm{number} \mathrm{of} \mathrm{initial} \mathrm{flowers}}}$$

(5)

In addition, vase life concluded when over 30% of the florets within an inflorescence exhibited diminished quality, characterized by petal discoloration, necrosis, wilting, and/or abscission. The scoring criteria are shown as follows (

Table 3. The scoring criteria for the visualization of Vanda florets.

Score (Points)	The Visualization of Vanda Florets
5	Normal florets
4	Florets experience one type of deterioration, such as wilting or veination
3	Florets experience two types of deterioration, such as wilting and petal discoloration, or veination
2	Florets experience more than two types of deterioration, such as wilting, petal discoloration, veination, or necrosis
1	Floret abscission

):

2.8. Statistical Analysis

The data from each treatment were analyzed using the SPSS software, version 27.0.1 (IBM Corp., Armonk, NY, USA). The one-way analysis of variance (ANOVA) was performed, followed by a subsequent multiple range test using the least significant difference method (LSD) to assess the differences between the groups. Statistically significant differences at a probability of p < 0.05 were represented by different lowercase letters.

3. Results and Discussions

3.1. Weight Loss and Water Uptake

The change in weight loss of inflorescences ranges from 0 to 57% and tends to increase continuously throughout the experimental period. The results revealed that on days 6, 9, and 12 of vase life after treatment, vanda inflorescences treated with 1-MCP (T2) and cold plasma discharge (T3) exhibited the lowest weight loss and trended towards a slight increase throughout the vase life period. However, treatment of cold plasma discharge appears to maintain the weight loss of Vanda inflorescences (Figure 5A). This may be due to 1-MCP and cold plasma playing an important role in retarding the weight loss of vanda inflorescences. Jia et al. [55] also found that weight loss of tomatoes treated with atmospheric cold plasma at 60 kV was significantly lower than in the untreated group. Wustman and Struik [56] reported that the loss of fresh weight occurred during storage time through respiration, where internal starch is used for energy supply and is associated with an increase in transpiration or water loss.

The water uptake rate of vanda cut flowers tends to increase with the vase life period, except for the control treatment (T1). Although the daily water uptake rate did not differ significantly among the treatments, the experiment results found that cold plasma discharge tended to enhance the water uptake rate compared to other treatments (Figure 5B). One of the factors that contributed to decreasing water uptake in cut flowers, such as microbial growth, particularly bacterial proliferation, in vase water of cut flowers can lead to blockages in the xylem, resulting in impaired water uptake and accelerated wilting. RONS generated by cold plasma may kill microorganisms in the xylem vessels during the discharge process, leading to an increase in water uptake observed in vanda inflorescences. Atmospheric cold plasma not only generates H₂O₂ but also involves a combination of complex reactive molecules, charged particles, and UV light simultaneously. These reactive species are diffused from the atmospheric cold plasma device into cells, initially causing damage to the outermost cytoplasmic membrane. The shrinkage and destruction of cells resulting from this membrane damage are widely recognized as a primary cause of bacterial cell death induced by atmospheric cold plasma [57]. One possible explanation is that cold plasma treatment enhances the surface hydrophilicity and permeability of peduncle cells. Although previous studies have reported such effects, most findings are based on seed coats [58]. It is, therefore, plausible that similar mechanisms may occur in flower stems or peduncles exposed to DBD plasma during treatment, contributing to improved water uptake. However, further research is needed to confirm this mechanism in flower tissues and clarify its role in postharvest water balance.

3.2. Discoloration

The decline in floret color significantly reduces the market value of horticultural plants. Therefore, it is important to evaluate the preservative method affecting the flower color. Colorimetric measurements were conducted within 21 days after treatment. We observed non-significant differences in L*, a*, b*, and ∆E parameters among treatments until the 21 days after treatment. However, on the 3 days after treatment, the a* value in Vanda petals treated with 1-MCP was higher than the control group but showed nonsignificant differences compared to plasma discharge. Furthermore, the highest ∆E value was observed in the plasma discharge treatment on the 3 days after treatment (

Table 4. The color parameters of Vanda cut flower petals were measured at 0, 3, 6, 9, 12, 15, 18, and 21 days after treatment (DAT).

DAT (Day)	Treatment	Color Parameters
		L*	a*	b*	$\mathbf{\Delta }\mathbf{E}$
0	Control	30.28 ± 2.15	32.45 ± 3.44	−25.81 ± 4.22	-
	1-MCP	31.10 ± 4.91	39.15 ± 3.71	−29.56 ± 1.22	-
	Plasma discharge	26.09 ± 2.09	36.95 ± 0.96	−29.92 ± 0.47	-
Sig. (LSD < 0.05)		ns	ns	ns
3	Control	29.87 ± 0.46	31.93 ± 2.43 b	−25.54 ± 3.48	2.12 ± 0.95 b
	1-MCP	30.95 ± 4.82	38.69 ± 3.34 a	−29.10 ± 3.34	1.44 ± 0.31 b
	Plasma discharge	28.67 ± 0.53	35.61 ± 1.69 ab	−29.43 ± 1.69	4.38 ± 1.60 a
Sig. (LSD < 0.05)		ns	*	ns	*
6	Control	30.96 ± 2.40	30.39 ± 3.82	−30.39 ± 3.82	2.83 ± 0.87
	1-MCP	31.51 ± 4.01	37.87 ± 3.77	−37.87 ± 3.77	1.82 ± 1.06
	Plasma discharge	29.57 ± 1.67	34.50 ± 0.81	−34.50 ± 0.81	4.89 ± 1.64
Sig. (LSD < 0.05)		ns	ns	ns	ns
9	Control	32.89 ± 3.93	29.66 ± 3.38	−29.66 ± 3.38	4.23 ± 1.58
	1-MCP	34.60 ± 3.73	35.29 ± 3.39	−35.29 ± 3.39	5.93 ± 3.01
	Plasma discharge	29.46 ± 1.77	33.87 ± 1.31	−33.87 ± 1.31	4.34 ± 1.37
Sig. (LSD < 0.05)		ns	ns	ns	ns
12	Control	31.82 ± 2.92	25.74 ± 1.20	−25.74 ± 1.20	8.25 ± 3.96
	1-MCP	34.05 ± 3.76	31.94 ± 7.00	−31.94 ± 7.00	9.06 ± 7.13
	Plasma discharge	28.19 ± 1.42	31.77 ± 1.28	−31.77 ± 1.28	6.31 ± 2.09
Sig. (LSD < 0.05)		ns	ns	ns	ns
15	Control	31.15 ± 3.50	24.99 ± 0.98	−22.06 ± 3.08	8.74 ± 5.37
	1-MCP	33.60 ± 5.28	30.45 ± 7.80	−23.66 ± 3.09	11.14 ± 7.32
	Plasma discharge	27.69 ± 1.44	30.63 ± 1.64	−26.51 ± 0.00	7.49 ± 3.12
Sig. (LSD < 0.05)		ns	ns	ns	ns
18	Control	31.15 ± 3.50	24.99 ± 0.98	−20.69 ± 2.21	9.42 ± 5.94
	1-MCP	33.60 ± 5.28	30.45 ± 7.80	−23.51 ± 3.47	11.22 ± 7.32
	Plasma discharge	27.69 ± 1.44	30.63 ± 1.64	−25.93 ± 1.20	7.79 ± 2.95
Sig. (LSD < 0.05)		ns	ns	ns	ns
21	Control	31.15 ± 3.50	24.99 ± 0.98	−20.33 ± 2.11	9.61 ± 6.02
	1-MCP	33.60 ± 5.28	30.45 ± 7.80	−23.24 ± 3.87	11.38 ± 7.58
	Plasma discharge	27.69 ± 1.43	30.63 ± 1.64	−25.83 ± 2.60	7.82 ± 3.31
Sig. (LSD < 0.05)		ns	ns	ns	ns

* Denotes that the result is statistically significant (p < 0.05); a, b = means followed by the same letters in the same column are not statistically significantly different from each other (p < 0.05); ns = not statistically significant, according to the least significant difference (LSD) test (p < 0.05).

). The results are similar in tomatoes and lettuce treated with cold plasma, as they both exhibit significant color change [59]. In this study, the total color change in plasma treatment is high in the primary stage of the vase life period and is considered continuously stable. Despite the lack of direct studies on the effects of cold plasma treatment on orchid pigments that are currently available, related research on ginger has shown that ROS generated by cold plasma can lead to the oxidation and photo-oxidation of pigments such as carotenoids, resulting in measurable color changes. For instance, the ΔE in plasma-treated ginger increased significantly due to ROS-mediated degradation and isomerization of pigments [60,61]. These findings support the hypothesis that similar oxidative mechanisms may occur in orchid petals, particularly during the early stages of cold plasma exposure, explaining the initial increase in ΔE observed in this study. Over time, however, pigment stabilization and improved water balance might contribute to the reduction in ΔE during the later stages. Although the total color charge on the 3 days after treatment was significantly different in each treatment, there were no significant differences observed in Vanda petals across all treatments from the 6th to the 12th day after treatment. Nevertheless, on the 12 days after treatment, the total color change in Vanda treated with cold plasma appears to be lower than other treatments. Additionally, the color in certain plants experiencing the browning reaction may be stabilized by cold plasma, whereas in plants containing carotenoids, the color may experience a slight reduction due to the destruction of carotenoid pigments by reactive species derived from cold plasma [59,62]. Petals of orchid flowers are recognized as rich sources of anthocyanins [63]; therefore, plasma may not markedly affect the color of blue Vanda orchids.

3.3. Ethylene Production and Respiration Rate

The ethylene production in Vanda cut flowers was at low concentrations, reaching its peak on day 15 of vase life [33]. In this study, it was found that the use of cold plasma resulted in Vanda cut flower inflorescences producing lower ethylene concentrations and respiration rates compared to the untreated group (T1), which is considered to have similar efficacy to using 1-MCP (

Table 5. Ethylene production, respiration rate (on day 15 after treatment), and vase life of Vanda cut flowers in each treatment.

Treatment	Ethylene Production (µL C2H4/kg/hr)	Respiration Rate (mg CO2/kg/hr)	Vase Life (Days)
25 °C	40.93 ± 5.34 a	118.21 ± 15.41 a	10.33 ± 1.70 b
1-MCP	26.15 ± 4.51 b	63.92 ± 11.02 b	19.33 ± 1.25 a
Plasma discharge	25.20 ± 1.20 b	57.44 ± 2.72 b	19.33 ± 0.94 a
Sig. (LSD < 0.05)	*	*	*

* Denotes that the result is statistically significant (p < 0.05); a, b = means followed by the same letters in the same column are not statistically significantly different from each other (p < 0.05).

). Zhu et al. [64] reported that RONS generated from cold plasma discharge, especially NO, play an important role in blocking the ethylene synthesis pathway, which is useful to delay the plant senescence process. Interestingly, when the enzyme responsible for ethylene synthesis, such as ACC oxidase, is combined with NO, it forms a binary ACC oxidase–NO complex, and a stable ternary ACC-ACC oxidase–NO complex is formed through the chelation of ACC. Consequently, this reaction leads to the inhibition of ethylene production [65]. One factor that has an impact on plant senescence is respiration, as it involves the oxidation of organic molecules (starch, sugar, and organic acids) into simpler elements (CO₂ and H₂O), thereby generating energy used for synthetic reactions that precipitate their biochemical processes [66,67]. Previous studies have found that cold plasma at 60 kV can reduce the respiration rate of postharvest tomatoes by maintaining lower metabolic levels [55]. These results suggest that atmospheric cold plasma may suppress the respiration rate by reducing the ethylene synthesis of Vanda cut flowers. Nevertheless, a strong positive correlation between respiration rate and ethylene production in harvested plants has been identified in previous studies, although the underlying mechanism remains unclear [68,69]. In each cycle from methionine to ethylene, one molecule of ATP is consumed to generate S-adenosylmethionine (SAM), and an aminobutyrate group is added to regenerate methionine, allowing the methyl group of the original methionine to be preserved. This methionine salvage pathway enables sustained ethylene production without depleting methionine reserves. However, when respiration rates decline, ATP synthesis is also reduced, which may limit the availability of ATP required for ethylene biosynthesis [70,71,72,73]. This reduction in ethylene production can, in turn, contribute to delayed senescence in harvested flowers.

3.4. Percentage of Flower Abscission and Vanda Cut Flower Vase Life

Flower abscission is a common postharvest senescence symptom in many floral species. The attributes of flower senescence, such as floret abscission, were measured, and the days after treatment were determined. It was found that the percentage of flower abscission in vanda cut flowers treated with 1-MCP (T2) and cold plasma discharge (T3) was lower than that in the untreated group (T1). This difference was evident on the 21 days after treatment (Figure 6). In this study, the vase life of cut flowers of vanda in each treatment ranged between 10 and 19 days. Vanda cut flowers treated with 1-MCP (T2) and cold plasma discharge (T3) have a similar effect on extending the vase life of vanda, which is longer than the control group by around 9 days (Table 5, Figure 7). The longevity of cut flowers is influenced by phenomena such as water and nutrient loss due to increased respiration, ethylene biosynthesis, and pathogen infection [74,75]. These results suggest that cold plasma affects internal factors to preserve the flower quality, such as enhancing water uptake and suppressing ethylene production and respiration rate of Vanda cut flowers, like 1-MCP. Thus, the vase life of Vanda cut flowers was increased by both treatments.

4. Conclusions

Atmospheric cold plasma was applied to treat Vanda cut flowers using a dielectric barrier discharge with a discharge time of 90 min combined with 30 min of standing time in a closed-room system. For comparison, 1-MCP was employed as a representative traditional preservation method. The results showed no significant differences between cold plasma and 1-MCP treatments in terms of weight loss, ethylene production, respiration rate, flower abscission, and vase life of Vanda cut flower inflorescences. Although water uptake and petal discoloration did not differ significantly among treatments, cold plasma treatment tended to improve these parameters slightly.

Therefore, cold plasma discharge can be considered an alternative preservation method for extending the vase life of Vanda cut flowers, offering several advantages: it is a green and environmentally friendly technology, has a shorter treatment duration (approximately 3 h) compared to 1-MCP, and does not require low-temperature conditions during treatment. However, further investigation is needed to clarify the mechanisms by which RONS generated during cold plasma discharge contribute to maintaining fresh weight, reducing ethylene synthesis and respiration rate, and ultimately extending vase life. Additionally, the potential application of atmospheric cold plasma discharge in other flower species should be explored.