Next Article in Journal
An Effective Motion-Tracking Scheme for Machine-Learning Applications in Noisy Videos
Previous Article in Journal
Assessment of Challenges to the Adoption of IOT for the Safety Management of Small Construction Projects in Malaysia: Structural Equation Modeling Approach
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 13
Issue 5
10.3390/app13053341
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Plasma-Activated Water Affects the Antioxidant Contents in Water Spinach

by
Shu-Chen Hsu
1,
Ting-Khai Kong
2,
Chung-Yu Chen
1 and
Hsiu-Ling Chen
2,*
1
Bachelor’s Degree Program in Food Safety/Hygiene and Laboratory Sciences, Chang Jung Christian University, Tainan 711, Taiwan
2
Department of Food Safety/Hygiene and Risk Management, College of Medicine, National Cheng Kung University, Tainan 701, Taiwan
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(5), 3341; https://doi.org/10.3390/app13053341
Submission received: 6 January 2023 / Revised: 13 February 2023 / Accepted: 1 March 2023 / Published: 6 March 2023
Browse Figures
Versions Notes

Abstract

:
Several studies suggested that plasma-activated water (PAW) increases crop yields and confronts drought. This study determined the total phenolic (TP) and total flavonoid (TF) contents of water spinach to elucidate whether PAW induced changes in the antioxidant potential of water spinach planted in soil with and without metal contaminants. PAW was selected as the irrigation water for pot experiments. Results showed that PAW increased the TP of water spinach planted in soil without any contaminants and, to some extent, in Cd-added soils but decreased the TP of those planted in Pb-added soil. PAW significantly enhanced the TF of water spinach planted in Cd-added cultivation soil, but not for Pb-added cultivation soil. Meanwhile, PAW can elevate the TF of water spinach by adding fertilizer and sunlight exposure. This study demonstrated that PAW application could increase TF, powerful antioxidants of water spinach planted with fertilizer and sunlight, in the cultivation soil with Cd, or without any contaminant.

1. Introduction

Plasma is generated by feeding different kinds of gases to an electric or electromagnetic field, thereby, providing adequate energy for ionizing or dissociating these gases by exciting collisions [1]. It can be categorized into high-temperature plasma, thermal plasma, and nonthermal or cold plasma [2]. The latter is an emerging nonchemical pretreatment technology that has been recently applied in agricultural applications and food safety to induce changes in plant characteristics [3,4,5], eliminate pathogens or inhibit microorganisms [6,7,8], promote seed germination [9], increase crop quantity [10,11], improve crop preservation, and reduce pesticides [12,13].
Cold atmospheric pressure plasma is applied to water to generate reactive oxygen species (ROS), reactive nitrogen species (RNS), ultraviolet (UV) radiation, and other chemical species in liquids. The newly formed solution is termed as plasma-activated water (PAW), which is acidic in nature resulting from the changes in oxidation-reduction potential (ORP), electrical conductivity (EC), and formation of ROS and RNS, such as the long-lived hydrogen peroxide (H2O2), nitrite (NO2−), and nitrate (NO3−) species in PAW [4,14]. Judée et al. [15] investigated the potential of utilizing cold atmospheric pressure dielectric barrier discharge plasma treatment of tap water (PATW) for agricultural purposes. The reaction between cold plasma’s gas-phase substances and tap water leads to the formation of ROS and RNS in water, such as H2O2, NO2−, NO3−, or ammonia (NH4+), which can induce and enhance crop growth as well as bicarbonate (HCO3−) ions, which can promote seedling growth, as a result of PAW-based nitrogen fixation [16,17,18]. The species generated depends on the plasma source used, treatment time and gases used, the distance between the liquids, and the nature of the electrode [4]. In addition, the species and reactions generated in the PAW might be different between PAW generated above the water surface and PAW generated directly in the liquids [4]. An advantage of generating PAW is that it does not require any additional chemicals to be added to the water, making it eco-friendly [14,19]. Overall, PAW can disinfect foods, promote the growth of seedlings, increase crop yield, and confront drought [4,14,19,20].
Oxidative stress plays a major role in aging and age-related diseases, such as cancer, cardiovascular disease, chronic inflammation, and neurodegenerative diseases [21]. Epidemiological studies have shown that a high intake of foods of plant origin is strongly associated with a reduced risk of the above-mentioned pathological conditions. These beneficial effects have been partly attributed to the compounds present in plants that possess antioxidant activity. Additionally, antioxidant compounds possess antibacterial, antimutagenic, or antiviral activities to a greater or lesser extent [22,23]. Antioxidants are substances that prevent damage to cells from free radicals, which are highly reactive, unstable molecules, that lead to cell damage linked to various chronic diseases [24,25]. Green leafy vegetables contribute to a balanced diet and are rich in bioactive compounds, such as polyphenols, carotenoids, flavonoids, flavones, catechins, vitamins, minerals, and so on [26].
Phytochemicals, which are natural bioactive components sufficient in plants, have been reported to possess antioxidant properties. They are small nonessential nutrients with putative health-promoting actions. Polyphenols and flavonoids represent the largest two categories of phytochemicals and serve as powerful antioxidants; they have received much attention because of their ability to prevent excessive free radicals in oxidative stress that cause several degenerative diseases in humans [27,28,29]. Phytochemicals usually increase in abundance during stressful events [28]. Heavy metal contamination of the environment is an important concern because plants can absorb chemical toxicants and further facilitate the entry of toxic metals into the food chain [30]. Plant cells growing in soil containing heavy metals are damaged by the imbalance between ROS and the protective systems of cells [31]. Heavy metals, such as lead (Pb) and cadmium (Cd), are of particular importance because of their dire consequences on the environment and human health [32,33]. Plants have automatically developed a range of protective systems, including protective systems to overcome invasion, such as increasing antioxidative enzymes viz. catalase, peroxidase, superoxide dismutase, and ascorbate peroxidase, involved in the metabolism of antioxidants [30,31,34]. Several studies have shown that plants’ secondary metabolites increase under heavy metal stress [34,35,36] and that heavy metals exhibit the ability to induce oxidative stress by generating ROS [37].
Choi et al. [38] found that plasma could significantly increase the total phenolic (TP) content and antioxidant activity of mandarin and freshly cut pitaya fruits [39]. By contrast, Ramazzina et al. [40] indicated that the effects of plasma on the antioxidant content and activity of kiwifruit were not significant. Meanwhile, studies investigating the effect of PAW on the antioxidant content and activity of leafy vegetables under heavy metal stress are lacking. Therefore, this study aimed to elucidate whether PAW treatment induced changes in the antioxidant potential of water spinach by investigating the TP and total flavonoid (TF) contents of water spinach planted in soil contaminated with Pb or Cd.

2. Materials and Method

2.1. Chemicals

Folin–Ciocalteu’s (FC) phenol reagent, gallic acid (C7H6O5, GA), quercetin, and sodium carbonate (Na2CO3) were obtained from Sigma-Aldrich Co. (St. Louis, MO, USA). Aluminum nitrate nonahydrate (Al (NO3)3·9H2O) and potassium acetate (CH3CO2K) were purchased from Amresco Inc. (Solon, OH, USA).

2.2. Generation of PAW

PAW was generated using a non-thermal atmospheric plasma generator (designed and constructed by Aerothermal and Plasma Physics Lab, Department of Mechanical Engineering, National Chiao Tung University, Hsinchu, Taiwan) as described in our previous study [41]. Briefly, a 1000 mL beaker was filled with 500 mL of reverse osmosis (RO) water. Two atmospheric pressure plasma jet (APPJ) electrodes (Patent US10,121,638B1) were immersed in the RO water, and then the beaker was covered using a plastic wrap. Power was set at 60 W for plasma generation and the RO water was activated for 20 min with air as the feeding gas (Figure 1A). The physicochemical properties of the PAW were measured as pH, NO2− concentration, NO3− concentration, ORP, and EC (Table 1).

2.3. PAW Treatments

2.3.1. Phase-1: Pot Experiment in Soil with Pb/Cd Contaminants

Water spinach (Ipomoea aquatica) seeds were procured from Known-You Seed Co., Ltd. (Kaohsiung City, Taiwan). First, 15 seeds were randomly selected and placed in a beaker containing 200 mL of RO water. A single APPJ electrode was immersed in the water, and the power was set at 60 W for plasma generation. The seeds were treated for 7 min (Figure 1B). For the untreated (control) group, 15 seeds were placed in the RO water for 7 min without applying a plasma jet. For seedling preparation, the non-treated and PAW-treated seeds were separately placed on a wet filter paper in a Petri dish for a 5-day germination. Six of the best-grown seeds were chosen and moved to pots for growing under PAW or RO water (control) irrigation. Planting pots with 13 cm height and 15 cm diameter were used for plantation. Cultivation (control) soil and man-made contaminated soil with added metals, such as Pb (Pb acetate trihydrate, 99–102.5%, Sigma-Aldrich Co., St. Louis, MO, USA) and Cd (Cd II acetate anhydrous, 99.995%, Sigma-Aldrich Co., St. Louis, MO, USA), were prepared in advance according to our previous study [41]. Each planting pot contained 550 g of either control or contaminated soil (Figure 2).
The PAW was applied for the seeds and irrigation, resulting in the following four treatment combinations: Group 1, non-treated seeds and irrigation (NTS + NTW); Group 2, non-treated seeds but PAW was used for irrigation (NTS + PAW); Group 3, plasma-treated seeds but none for irrigation (PTS + NTW), and Group 4, plasma-treated seeds and irrigation (PTS + PAW). Each group was planted indoors in triplicate, and each pot contained six seedlings. Planting was performed under neutral day conditions (photoperiod: 13 h light/11 h dark between 07:00 and 20:00 h) using a T9 fluorescent lamp (6500 K, 18 W). Each pot of water spinach was watered with 150 mL of either PAW (experimental group) or RO water (control group) three times weekly for 5 weeks.

2.3.2. Phase-2: Pot Experiment in Soil with Fertilizer

PAW can only offer nitrogen-based nutrients, such as NO2−, NO3−, and NH4+, for the growth of water spinach. However, the lack of other nutrients in the soil might restrict plant growth. Therefore, fertilizer was added to the soil in the Phase-2 experiment to fulfill the other essential nutrients (e.g., phosphorus, potassium, etc.) necessary during plant growth.
Cultivation soil with fertilizer was used for planting and the soil without fertilizer served as a control. Eight liters (3.64 kg) of cultivation soil mixed with 40 g of fertilizer were prepared at a ratio of 12% nitrogen, 18% phosphorus, and 12% potassium according to the manufacturer’s instructions (BENEFITCOTE Fertilizer, Green Orchids Company, New Taipei City, Taiwan). Untreated water spinach seeds were grown in planting pots containing 600 g of soil with or without fertilizer.
Plasma treatment was applied to the irrigation water; thus, four treatment combinations are listed as follows: Group 1, non-treated water and no fertilizer (RO); Group 2, non-treated water but with fertilizer (RO + Fert); Group 3, using PAW but no fertilizer (PAW), and Group 4, using PAW and fertilizer (PAW + Fert). Each pot of water spinach was watered with 150 mL of either PAW or RO water three times weekly for 4 weeks. A daylight T9 fluorescent lamp (6500 K, 18 W) was used as an aid for 13 h from 7 a.m. to 8 p.m. (Figure 2).

2.4. Water Spinach Sample Preparation

The edible parts of the grown water spinach were cut at a position approximately 1 cm above the soil surface. The harvested samples were weighed and then washed with RO water to remove dirt. The samples were dried using paper towels and then placed in zipper storage bags with corresponding labels. The samples collected from the same treatment combination (i.e., water spinach grown in 3 different planting pots of similar treatment) were kept in the same zipper bag for sample homogenization. The samples were stored in a −18 °C freezer for 24 h and then freeze-dried for 48–72 h. After freeze-frying, the samples were homogenized into smaller pieces using a blender. Then, the samples were filtered using a 20-mesh sieve and prepared for further tests.

2.5. Preparation of Water Spinach Extracts

First, 0.2 g of each freeze-dried water spinach powder was extracted with 10 mL of 75% ethanol in amber tubes at room temperature for 30 min using a vortex shaker. The extracted materials were then centrifuged for 10 min at 2500× g. The supernatants were collected in a new amber microcentrifuge tube for further analysis.

2.6. Determination of TP Content

Determination of TP content of water spinach extracts was conducted using the FC method, which was modified by Quettier-Deleu et al. [42], with some modifications. The sample extract solution (50 µL) in an amber tube was mixed with 2% Na2CO3 and allowed to stand for 3 min. Then, 50 µL of 50% FC’s phenol reagent was added and thoroughly mixed. Incubation was performed in a water bath at 50 °C for 30 min for color development; then, the reaction mixtures were removed from the water bath and allowed to cool in an ice bath for 5 min. The reaction mixtures were centrifuged for 10 min at 2500× g. The absorbance of the supernatant at 750 nm was measured using GENESYS 10S UV-Vis Spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). A calibration curve was prepared using a standard solution of gallic acid. Each sample was determined in triplicate. The results are expressed in terms of milligram gallic acid equivalent per gram of dry matter (mg GAE/g DM). The calibration equation for gallic acid was y = 1.7898x − 0.0225 (R2 = 0.9996), where x is the gallic acid concentration in mg/mL and y is the absorbance reading at 750 nm.

2.7. Determination of TF Content

The TF content was determined according to Jia et al. [43] with slight modification. Briefly, 100 µL of the extracted samples were mixed with 300 µL of double-distilled water (ddH2O) and 20 µL of 10% Al (NO3)3·9H2O. After the mixture reaction for 5 min, 20 µL of 1 M CH3CO2K was added, and ddH2O was added to make the final solution up to 1 mL. The samples were mixed and reacted for 40 min at room temperature. The absorbance of the mixtures was determined using a spectrophotometer at 415 nm. A calibration curve was prepared, using a standard solution of quercetin. All determinations were conducted in triplicate. The results were expressed in terms of mg quercetin equivalent per gram of DM (mg QE/g DM). The calibration equation for quercetin was y = 5.1269x + 0.0587 (R2 = 0.9956), where x is the quercetin concentration in mg/L and y is the absorbance reading at 415 nm.

2.8. Statistical Analysis

SPSS software v.23.0 (SPSS Inc., Chicago, IL, USA) was used for statistical analysis. Average values from triplicate experiments were obtained, and a one-way analysis of variance was followed by Duncan’s test, which was performed to determine the differences in mean values of data from different sample types. The results are presented as mean ± standard deviation and were tested for significance level at 0.05. Statistical differences were considered significant at p < 0.05.

3. Results and Discussion

3.1. Plant Growth

The average height of harvested water spinach in the control soil grown under different plasma treatments in the Phase-1 pot experiment was measured weekly (Table 2 and Figure 3). There was a significant difference among the four treatment groups in the third week, but the average height of water spinach in the fifth week did not show a significant difference among the four treatment groups. The PTS + PAW group showed the shortest average height at the fifth week because of the varying height of plants within the treatment group.

3.2. TP Content

The TP contents in water spinach of different plasma treatments grown in different soils are shown in Table 3 and Figure 4. For water spinach cultivated in the control soil, significant improvement in TP was observed in each plasma treatment group. PTS + PAW treatment (18.83 ± 0.91 mg GAE/g) showed the best improvement at 30.9%, followed by NTS + PAW (17.73 ± 0.66 mg GAE/g) and PTS + NTW (16.23 ± 1.19 mg GAE/g) compared with NTS + NTW (14.39 ± 0.50 mg GAE/g). The improvement under plasma treatment was significant. This is expected as the PAW reactive species can increase the oxidative stress and elevate the production of antioxidants (e.g., phenolic compounds) in water spinach. The study by Kapoor [44] revealed that the TP content was increased by 1.4-fold at 0.6 mM Cd treatment than that of the control (from 7.14 to 10.01 mg/g FW). The overproduction of ROS under metal stress must be regulated by the free radical scavenging systems [44]. Here, the treatment of PTS + PAW showed the highest TP content in the control soil, which can be explained as the plant synthesizes various phytochemicals in response to environmental stress [41]. While ROS and RNS from plasma treatment were applied to plants, oxidative stress enhanced the enzymatic systems, and nonenzymatic systems comprise low molecular weight antioxidants and high molecular weight secondary metabolites [45].
Here, water spinach grown in two kinds of contaminated soils was explored to elucidate if plasma treatment affects TP and TF contents. In each pot, the initial Cd concentration of the Cd-added control soil was 18.18–22.28 mg/kg dry weight, whereas the initial Pb concentration of the Pb-added cultivation soil was 2064–2492 mg/kg dry weight. The designed concentrations were three times that of agricultural control levels in farmlands.
The TP content of water spinach grown in Cd-added soil decreased in the PTS + NTW treatment group (Table 3 and Figure 4). Plasma treatment of seeds would lower the TP content of water spinach grown in Cd-added soil, but irrigational PAW of the plasma-treated seed could improve the situation. The TP content of the NTS + PAW treatment group showed an improvement (18.04 ± 1.21 mg GAE/g); however, the usage of PAW seems to increase the TP content in the water spinach cultivated in Cd-added soil, with no observed significant difference.
The highest TP content value was obtained by NTS + NTW-treated water spinach (19.06 ± 0.29 mg GAE/g), which was planted in the Pb-added soil. The TP content in water spinach was decreased by 11.8% in the NTS + PAW treatment group compared to that in the NTS + NTW group by 20.3% in the PTS + NTW group and 21.6% in the PTS + PAW treatment group. Results showed that the plasma-treated seed or irrigation PAW did not increase the TP content of water spinach grown in Pb-added soil.
Several studies have shown that, under heavy metal stress, phenolic compounds can act as metal chelators and, alternatively, phenolic compounds could directly scavenge molecular species of active oxygen [34,35,36]. Zafari et al. [46] reported that Pb application significantly enhanced the phenolic acids and flavonoids content of Prosopis farcta. Rahimi et al. [33] demonstrated that Pb in soil significantly altered the content of polyphenolic compounds of basil (Ocimum basilicum L.); also, different Pb levels can have different effects on polyphenolic compounds. The number of polyphenolic compounds decreased with increasing Pb content from 0 to 50 ppm, and increased again at 75 ppm. Korkmaz et al. [31] found that the 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity and TP content values of basil (Ocimum basilicum L.,) increased with Cd application (from 2.5 to 40 mg/kg). Additionally, Kolahi et al. [30] observed oxidative stress induced by Cd and an increase in TP content in lettuce (Lactuca sativa Linn.).
This study’s results showed a significant difference in TP content between plasma-treated and control groups among the water spinach cultivated in the control, Pb-added, and Cd-added soils. Meanwhile, in the Cd- and Pb-added group, the application of PAW reduced the TP content of water spinach from those without PAW treatment, probably because of the excessive oxidative stress from both PAW and heavy metals. From a different perspective, there was no improvement in TP content in water spinach after plasma treatment, which might be due to Cd and Pb in the soil.

3.3. TF Content

Table 4 and Figure 5 compared TF content in water spinach with different treatments grown in different soil in the Phase-1 experiment. For water spinach cultivated in the control soil, the TF content of those without plasma treatment was 7.95 ± 1.61 mg QE/g DM and 9.77 ± 0.16 mg QE/g DM in the PTS + PAW group. However, there was no significant difference in the TF content of water spinach among the four groups. Meanwhile, the TF content of water spinach grown in Cd-added soil was increased by plasma treatment and those grown in the control soil. TF content improved in the treatment with PAW (NTS + PAW and PTS + PAW), whereas the TF content of water spinach grown in Pb-added soil was decreased after plasma treatment of seeds (PTS + NTW and PTS + PAW). When only PAW was applied to water spinach grown in Pb-added soil, the TF content was slightly elevated by 5.21% (9.69 ± 0.07 mg QE/g DM) compared with the control (NTS + NTW) of 9.21 ± 0.34 mg QE/g DM. From the view of water spinach grown in different soil types, the heavy metals in soil did not affect the TF content of the NTS + NTW group but not for the TF content, which changed while the seeds were treated by plasma. The increase in TF content by plasma treatment was observed in water spinach grown in Cd-added soil. In other words, plasma treatment applied to water spinach grown in Cd-added soil could help enhance TF content, especially under the combined treatment of seed and water (PTS + PAW). Results also showed that PAW applied to seedlings or for irrigation in water spinach soil without any metal contaminant or in the soil with Cd can initiate the content of TP and TF, except for soil with Pb.
According to other studies, the TP and TF content of some agricultural crops increased due to Pb and Cd metal stress compared with the control [46,47,48,49]. Wang et al. [48] revealed that the TF content significantly increased in Vallisneria natans leaves after exposure to 10–75 µM Pb for 4 days compared with that in the control. The findings of this study are similar to that of Kolahi et al. [30], who observed an increment in TP content of lettuce cultured in nutrient solutions containing Cd; however, TF content was not significantly affected, but the differences with other studies may be due to factors, such as heavy metal concentration, plasma treatment, plant species, or cultivation condition [50].
To understand the influence of PAW on the TP and TF content of plants in the field, the plasma treatment of seeds and pre-germination of seeds were replaced by directly sowing into the soil, and the cultivation rack was taken outside the laboratory to increase the ventilation and probability to receive sunlight. Although PAW can only act as eco-friendly nitrogen-based nutrients, the lack of other nutrients in the soil might also restrict plant growth, so fertilizer was added to the soil to fulfill the nutrients needed during plant growth in the Phase-2 experiment (Table 5).
The usage of PAW only showed a 15.5% increase in TF content in water spinach compared to that of RO water. With the addition of fertilizer, the TF content elevated to 33.6%. The use of PAW or fertilizer can significantly increase TF content in water spinach, and it has a synergistic effect when both are used simultaneously. Although the TP content among the four treatments did not show a significant difference (as shown in Table 5 and Figure 6), the results of the Phase-2 pot experiment were different from those of the Phase-1 pot experiment.
In summary, for the Phase-1 pot experiment, TP content can be elevated by plasma treatment of the water spinach grown in cultivation soil. However, plasma treatment of water spinach inhibited the TP content, especially in the Pb-added soil. Meanwhile, plasma treatment also showed a trend to inhibit the TF content of water spinach grown in the Pb-added soil. In the Phase-2 pot experiment, following fertilizer application, the TF of water spinach was not improved by the enrichment of the soil nutrient, which is RO + Fert, containing a greater amount of TF than RO only, and PAW + Fert showed a greater amount than the PAW only. Overall, the particular findings in certain cultivation conditions do not agree to what is already known, that the TP and TF contents of plants can increase under heavy metal stress. Therefore, we suggest further studies investigating the effect of PAW, as well as its synergistic action with fertilizer, on the TP and TF contents of water spinach under heavy metal stress to understand the role of PAW in this phenomenon.
Additionally, the acidic environment offered by PAW during irrigation may influence the TP and TF contents of the water spinach under heavy metal stress. According to Bhargava et al. [51], the concentration of heavy metals in soil tend to increase with decreasing pH due to the increased activity of hydrogen ions when there is a decrease in pH. This event displaces metals from exchangeable sites on solid surfaces which increases the availability of those metals for plant uptake. For instance, Lian et al. [52] found that the degree of Cd uptake by plants increased with decreasing pH. Therefore, we hypothesize that the lower soil pH introduced by the acidic nature of PAW increased the uptake of heavy metals by water spinach in this study, which in turn increased the amount of TP and TF due to heavy metal-induced stress. However, to our knowledge, there are no other reports on the use of PAW on water spinach leaves, therefore, the observed effect of low pH introduced by PAW on the TP and TF concentrations should be further clarified.
Nowadays, plants could grow in farms, greenhouses, and plant factories. Compared with traditional farming and greenhouses, a plant factory is a new way of farming. Plant factories are a closed growing environment where lighting sources such as fluorescent lamps and light-emitting diodes as well as air conditioning are used to replace sunlight and natural ventilation. Such indoor farming could ensure adequate and efficient production of fruits and vegetables [53,54]. This study’s results found that the use of PAW could increase the content of TP or TF in water spinach under different conditions. Polyphenols and flavonoids are powerful antioxidants; therefore, PAW can increase the antioxidant potential of water spinach.

4. Conclusions

The findings of this study indicated that PAW increased the TP and TF content of water spinach under different cultivation conditions. Particularly, PAW could increase the TP content of water spinach planted in control soil and, to some extent, in Cd-added soil but not in Pb-added soil, whereas PAW could increase the TF content of water spinach planted in control, Cd-added, and, to some extent, Pb-added soils. Moreover, this study showed the synergistic effect of PAW irrigation and fertilizer application as shown in the higher TF content of water spinach in the PAW irrigation combined with fertilizer application treatment group than those in the fertilizer or PAW treatment groups alone. However, combined PAW irrigation and fertilizer application inhibited the TP content of water spinach. These findings can provide the timing of PAW usage in plant factories or general farming, which may be of interest to the functional ingredient industry.

Author Contributions

S.-C.H.: methodology and processing, resources, writing—original draft preparation, writing—review and editing; T.-K.K.: methodology and processing, chemical analysis, writing—original draft preparation; C.-Y.C.: chemical analysis, resources; H.-L.C.: conceptualization, supervision, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy issues.

Acknowledgments

The authors acknowledge Chun-Ping Hsiao (post-doctorate researcher) and Jong-Shinn Wu of Mechanical and Mechatronics Systems Research Labs, Industrial Technology Research Institute (ITRI), Taiwan, for the design and development of the plasma jet system.

Conflicts of Interest

All authors declare no conflict of interest.

References

  1. Pankaj, S.K.; Keener, K.M. Cold plasma: Background, applications and current trends. Curr. Opin. Food Sci. 2017, 16, 49–52. [Google Scholar] [CrossRef]
  2. Yan, D.; Sherman, J.H.; Keidar, M. Cold atmospheric plasma, a novel promising anti-cancer treatment modality. Oncotarget 2017, 8, 15977. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  3. Takaki, K.; Yoshida, K.; Saito, T.; Kusaka, T.; Yamaguchi, R.; Takahashi, K.; Sakamoto, Y. Effect of electrical stimulation on fruit body formation in cultivating mushrooms. Microorganisms 2014, 2, 58–72. [Google Scholar] [CrossRef] [Green Version]
  4. Thirumdas, R.; Kothakota, A.; Annapure, U.; Siliveru, K.; Blundell, R.; Gatt, R.; Valdramidis, V.P. Plasma activated wa-ter (PAW): Chemistry, physico-chemical properties, applications in food and agriculture. Trends Food Sci. Technol. 2018, 77, 21–31. [Google Scholar] [CrossRef]
  5. Lu, B.J.; Lin, T.C.; Chao, H.R.; Tsai, C.H.; Lu, J.H.; Tsai, M.H.; Chang, C.T.; Hsieh, H.; Lu, I.C.; Arcega, R.D.; et al. The impact of air or nitrogen non-thermal plasma on variations of natural bioactive compounds in Djulis (Chenopodium formosanum Koidz.) seed and the potential effects for human health. Atmosphere 2021, 12, 1375. [Google Scholar] [CrossRef]
  6. Brun, P.; Bernabè, G.; Marchiori, C.; Scarpa, M.; Zuin, M.; Cavazzana, R.; Zaniol, B.; Martines, E. Antibacterial efficacy and mechanisms of action of low power atmospheric pressure cold plasma: Membrane permeability, biofilm penetra-tion and antimicrobial sensitization. J. Appl. Microbiol. 2018, 125, 398–408. [Google Scholar] [CrossRef] [PubMed]
  7. Ma, R.; Wang, G.; Tian, Y.; Wang, K.; Zhang, J.; Fang, J. Non-thermal plasma-activated water inactivation of food-borne pathogen on fresh produce. J. Hazard. Mater. 2015, 300, 643–651. [Google Scholar] [CrossRef] [PubMed]
  8. Chen, H.L.; Arcega, R.D.; Herianto, S.; Hou, C.Y.; Lin, C.M. Mycotoxin decontamination of foods using nonthermal plasma and plasma-activated water. In Mycotoxins and Food Safety—Recent Advances; IntechOpen: London, UK, 2022. [Google Scholar] [CrossRef]
  9. Ahn, C.; Gill, J.; Ruzic, D.N. Growth of plasma-treated corn seeds under realistic conditions. Sci. Rep. 2019, 9, 4355. [Google Scholar] [CrossRef] [Green Version]
  10. Ma, Y.; Zhang, Q.; Zhang, Q.; He, H.; Chen, Z.; Zhao, Y.; Wei, D.; Kong, M.; Huang, Q. Improved production of poly-saccharides in Ganoderma lingzhi mycelia by plasma mutagenesis and rapid screening of mutated strains through infra-red spectroscopy. PLoS ONE 2018, 13, e0204266. [Google Scholar] [CrossRef]
  11. Xu, Y.; Tian, Y.; Ma, R.; Liu, Q.; Zhang, J. Effect of plasma activated water on the postharvest quality of button mushrooms, Agaricus bisporus. Food Chem. 2016, 197, 436–444. [Google Scholar] [CrossRef]
  12. Sarangapani, C.; O’Toole, G.; Cullen, P.; Bourke, P. Atmospheric cold plasma dissipation efficiency of agrochemicals on blueberries. Innov. Food Sci. Emerg. Technol. 2017, 44, 235–241. [Google Scholar] [CrossRef] [Green Version]
  13. Arcega, R.D.; Hou, C.Y.; Hsu, S.C.; Lin, C.M.; Chang, W.H.; Chen, H.L. Reduction of pesticide residues in Chrysanthemum morifolium by nonthermal plasma-activated water and impact on its quality. J. Hazard. Mater. 2022, 434, 128610. [Google Scholar] [CrossRef] [PubMed]
  14. Zhou, R.; Zhou, R.; Zhang, X.; Zhuang, J.; Yang, S.; Bazaka, K.; Ostrikov, K.K. Effects of atmospheric-pressure N2, He, air, and O2 microplasmas on mung bean seed germination and seedling growth. Sci. Rep. 2016, 6, 32603. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  15. Judée, F.; Simon, S.; Bailly, C.; Dufour, T. Plasma-activation of tap water using DBD for agronomy applications: Identification and quantification of long lifetime chemical species and production/consumption mechanisms. Water Res. 2018, 133, 47–59. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  16. Chen, H.; Yuan, D.; Wu, A.; Lin, X.; Li, X. Review of low-temperature plasma nitrogen fixation technology. Waste Dispos. Sustain. Energy 2021, 3, 201–217. [Google Scholar] [CrossRef] [PubMed]
  17. Huang, Z.; Xiao, A.; Liu, D.; Lu, X.; Ostrikov, K. Plasma-water-based nitrogen fixation: Status, mechanisms, and opportunities. Plasma Process. Polym. 2022, 19, 2100198. [Google Scholar] [CrossRef]
  18. Peng, P.; Schiappacasse, C.; Zhou, N.; Addy, M.; Cheng, Y.; Zhang, Y.; Anderson, E.; Chen, D.; Wang, Y.; Liu, Y.; et al. Plasma in situ gas–liquid nitrogen fixation using concentrated high-intensity electric field. J. Phys. D 2019, 52, 494001. [Google Scholar] [CrossRef]
  19. Zhou, R.; Li, J.; Zhou, R.; Zhang, X.; Yang, S. Atmospheric-pressure plasma treated water for seed germination and seedling growth of mung bean and its sterilization effect on mung bean sprouts. Innov. Food Sci. Emerg. Technol. 2019, 53, 36–44. [Google Scholar] [CrossRef]
  20. Zhou, R.; Zhou, R.; Prasad, K.; Fang, Z.; Speight, R.; Bazaka, K.; Ostrikov, K.K. Cold atmospheric plasma activated water as a prospective disinfectant: The crucial role of peroxynitrite. Green Chem. 2018, 20, 5276–5284. [Google Scholar] [CrossRef]
  21. Hajam, Y.A.; Rani, R.; Ganie, S.Y.; Sheikh, T.A.; Javaid, D.; Qadri, S.S.; Pramodh, S.; Alsulimani, A.; Alkhanani, M.F.; Harakeh, S.; et al. Oxidative stress in human pathology and aging: Molecular mechanisms and perspectives. Cells 2022, 11, 552. [Google Scholar] [CrossRef]
  22. Şengül, M.; Yildiz, H.; Kavaz, A. The effect of cooking on total polyphenolic content and antioxidant activity of selected vegetables. Int. J. Food Prop. 2014, 17, 481–490. [Google Scholar] [CrossRef]
  23. Xu, D.-P.; Li, Y.; Meng, X.; Zhou, T.; Zhou, Y.; Zheng, J.; Zhang, J.-J.; Li, H.-B. Natural antioxidants in foods and medicinal plants: Extraction, assessment and resources. Int. J. Mol. Sci. 2017, 18, 96. [Google Scholar] [CrossRef] [PubMed]
  24. Hossain, A.; Khatun, M.A.; Islam, M.; Huque, R. Enhancement of antioxidant quality of green leafy vegetables upon different cooking method. Prev. Nutr. Food Sci. 2017, 22, 216. [Google Scholar] [CrossRef] [PubMed]
  25. Suyan, H.; Aziz, H.; Efdi, M. Comparison of iron reduction methods on the determination of antioxidants content in vegetables sample. Orient. J. Chem. 2018, 34, 2418–2424. [Google Scholar] [CrossRef] [Green Version]
  26. Bhat, R.; Liong, M.-T.; Abdorreza, M.; Karim, A. Evaluation of free radical scavenging activity and antioxidant potential of a few popular green leafy vegetables of Malaysia. Int. J. Food Prop. 2013, 16, 1371–1379. [Google Scholar] [CrossRef]
  27. Stagos, D. Antioxidant activity of polyphenolic plant extracts. Antioxidants 2019, 9, 19. [Google Scholar] [CrossRef] [Green Version]
  28. Martinez, K.B.; Mackert, J.D.; McIntosh, M.K. Nutrition and functional foods for healthy aging. In Chapter 18—Polyphenols and Intestinal Health; Academic Press: Cambridge, MA, USA, 2017; pp. 191–210. [Google Scholar] [CrossRef]
  29. Panche, A.N.; Diwan, A.D.; Chandra, S.R. Flavonoids: An overview. J. Nutr. Sci. 2016, 5, e47. [Google Scholar] [CrossRef] [Green Version]
  30. Kolahi, M.; Mohajel Kazemi, E.; Yazdi, M.; Goldson-Barnaby, A. Oxidative stress induced by cadmium in lettuce (Lactuca sativa Linn.): Oxidative stress indicators and prediction of their genes. Plant Physiol. Biochem. 2020, 146, 71–89. [Google Scholar] [CrossRef]
  31. Korkmaz, K.; Ertürk, Ö.; Ayvaz, M.C.; Özcan, M.M. Effect of cadmium application on antimicrobial, antioxidant and total phenolic content of basil genotypes. Indian J. Pharm. Educ. Res. 2018, 52, S108–S114. [Google Scholar] [CrossRef] [Green Version]
  32. Baseer, M.; Jain, K. Review of botany, phytochemistry, pharmacology, contemporary applications and toxicology of Ocimum sanctum. Int. J. Pharm. Life Sci. 2016, 7, 4918–4929. [Google Scholar]
  33. Rahimi, A.; Ahmadi, F.; Ozyazici, G. Effect of heavy metals stress on polyphenolic compounds in hydroalcoholic extract of Basil (Ocimum basilicum L.). Int. J. Sci. Technol. Res. 2019, 5, 57–64. [Google Scholar] [CrossRef]
  34. Giannakoula, A.; Therios, I.; Chatzissavvidis, C. Effect of lead and copper on photosynthetic apparatus in citrus (Citrus aurantium L.) plants. The role of antioxidants in oxidative damage as a response to heavy metal stress. Plants 2021, 10, 155. [Google Scholar] [CrossRef] [PubMed]
  35. Fahimirad, S.; Hatami, M. Heavy metal-mediated changes in growth and phytochemicals of edible and medicinal plants. In Medicinal Plants and Environmental Challenges; Ghorbanpour, M., Varma, A., Eds.; Springer International Publishing: Cham, Switzerland, 2017; pp. 189–214. [Google Scholar]
  36. Kısa, D.; Elmastaş, M.; Öztürk, L.; Kayır, Ö. Responses of the phenolic compounds of Zea mays under heavy metal stress. Appl. Biol. Chem. 2016, 59, 813–820. [Google Scholar] [CrossRef]
  37. Štolfa, I.; Pfeiffer, T.Ž.; Špoljarić, D.; Teklić, T.; Lončarić, Z. Heavy metal-induced oxidative stress in plants: Response of the antioxidative system. In Reactive Oxygen Species and Oxidative Damage in Plants under Stress; Springer: Cham, Switzerland, 2015; pp. 127–163. [Google Scholar] [CrossRef]
  38. Choi, W.-J.; Kang, S.-K.; Ham, S.; Chung, W.; Kim, A.J.; Kang, M. Chronic cadmium intoxication and renal injury among workers of a small-scale silver soldering company. Saf. Health Work 2020, 11, 235–240. [Google Scholar] [CrossRef]
  39. Li, X.; Li, X.; Gao, K.; Liu, R.; Liu, R.; Yao, X.; Gong, D.; Su, Z.; Jia, P. Comparison of deionized and tap water activated with an atmospheric pressure glow discharge. Phys. Plasmas 2019, 26, 033507. [Google Scholar] [CrossRef]
  40. Ramazzina, I.; Berardinelli, A.; Rizzi, F.; Tappi, S.; Ragni, L.; Sacchetti, G.; Rocculi, P. Effect of cold plasma treatment on physico-chemical parameters and antioxidant activity of minimally processed kiwifruit. Postharvest Biol. Technol. 2015, 107, 55–65. [Google Scholar] [CrossRef]
  41. Hou, C.Y.; Kong, T.K.; Lin, C.M.; Chen, H.L. The effects of plasma-activated water on heavy metals accumulation in water spinach. Appl. Sci. 2021, 11, 5304. [Google Scholar] [CrossRef]
  42. Quettier-Deleu, C.; Gressier, B.; Vasseur, J.; Dine, T.; Brunet, C.; Luyckx, M.; Cazin, M.; Cazin, J.-C.; Bailleul, F.; Trotin, F. Phenolic compounds and antioxidant activities of buckwheat (Fagopyrum esculentum Moench) hulls and flour. J. Ethnopharmacol. 2000, 72, 35–42. [Google Scholar] [CrossRef]
  43. Zhishen, J.; Mengcheng, T.; Jianming, W. The determination of flavonoid contents in mulberry and their scavenging effects on superoxide radicals. Food Chem. 1999, 64, 555–559. [Google Scholar] [CrossRef]
  44. Kapoor, D. Antioxidative defense responses and activation of phenolic compounds in Brassica juncea plants exposed to cadmium stress. Int. J. Green Pharm. 2016, 10, 228–234. [Google Scholar] [CrossRef]
  45. Kasote, D.M.; Katyare, S.S.; Hegde, M.V.; Bae, H. Significance of antioxidant potential of plants and its relevance to therapeutic applications. Int. J. Biol. Sci. 2015, 11, 982. [Google Scholar] [CrossRef] [Green Version]
  46. Zafari, S.; Sharifi, M.; Chashmi, N.A.; Mur, L.A. Modulation of Pb-induced stress in Prosopis shoots through an inter-connected network of signaling molecules, phenolic compounds and amino acids. Plant Physiol. Biochem. 2016, 99, 11–20. [Google Scholar] [CrossRef] [Green Version]
  47. Chew, Y.L.; Goh, J.K.; Lim, Y.Y. Assessment of in vitro antioxidant capacity and polyphenolic composition of selected medicinal herbs from Leguminosae family in Peninsular Malaysia. Food Chem. 2009, 116, 13–18. [Google Scholar] [CrossRef]
  48. Wang, C.; Lu, J.; Zhang, S.; Wang, P.; Hou, J.; Qian, J. Effects of Pb stress on nutrient uptake and secondary metabolism in submerged macrophyte Vallisneria natans. Ecotoxicol. Environ. Saf. 2011, 74, 1297–1303. [Google Scholar] [CrossRef]
  49. Chen, S.; Wang, Q.; Lu, H.; Li, J.; Yang, D.; Liu, J.; Yan, C. Phenolic metabolism and related heavy metal tolerance mechanism in Kandelia Obovata under Cd and Zn stress. Ecotoxicol. Environ. Saf. 2019, 169, 134–143. [Google Scholar] [CrossRef]
  50. Maleki, M.; Ghorbanpour, M.; Kariman, K. Physiological and antioxidative responses of medicinal plants exposed to heavy metals stress. Plant Gene 2017, 11, 247–254. [Google Scholar] [CrossRef]
  51. Bhargava, A.; Carmona, F.F.; Bhargava, M.; Srivastava, S. Approaches for enhanced phytoextraction of heavy metals. J. Environ. Manag. 2012, 105, 103–120. [Google Scholar] [CrossRef] [PubMed]
  52. Lian, M.; Ma, Y.; Li, J.; Sun, J.; Zeng, X. Influence of pH on the particulate-bound Cd speciation and uptake by plants. Pol. J. Environ. Stud. 2022, 31, 5511–5517. [Google Scholar] [CrossRef] [PubMed]
  53. Graamans, L.; Baeza, E.; van den Dobbelsteen, A.; Tsafaras, I.; Stanghellini, C. Plant factories versus greenhouses: Comparison of resource use efficiency. Agric. Syst. 2018, 160, 31–43. [Google Scholar] [CrossRef]
  54. Graamans, L.; van den Dobbelsteen, A.; Meinen, E.; Stanghellini, C. Plant factories; crop transpiration and energy balance. Agric. Syst. 2017, 153, 138–147. [Google Scholar] [CrossRef]
Applsci 13 03341 g001 550
Figure 1. (A) The PAW generation system consisting of two plasma jet electrodes, RO water supply, power supply, and air supply. (B) The PAW treatment of seeds.
Figure 1. (A) The PAW generation system consisting of two plasma jet electrodes, RO water supply, power supply, and air supply. (B) The PAW treatment of seeds.
Applsci 13 03341 g001
Applsci 13 03341 g002 550
Figure 2. A schematic diagram of the Phase-1 and Phase-2 pot experiments.
Figure 2. A schematic diagram of the Phase-1 and Phase-2 pot experiments.
Applsci 13 03341 g002
Applsci 13 03341 g003 550
Figure 3. The average height of water spinach in control soil under different treatments. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed. Different uppercase letters indicate significant difference between groups or treatments, while same uppercase letters do not show significant difference.
Figure 3. The average height of water spinach in control soil under different treatments. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed. Different uppercase letters indicate significant difference between groups or treatments, while same uppercase letters do not show significant difference.
Applsci 13 03341 g003
Applsci 13 03341 g004 550
Figure 4. Comparison of TP content in water spinach with different plasma treatments grown in different soil. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed. Different uppercase letters indicate the significant difference between plasma treatment, while different lowercase letters indicate the significant difference between cultivation soil.
Figure 4. Comparison of TP content in water spinach with different plasma treatments grown in different soil. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed. Different uppercase letters indicate the significant difference between plasma treatment, while different lowercase letters indicate the significant difference between cultivation soil.
Applsci 13 03341 g004
Applsci 13 03341 g005 550
Figure 5. Comparison of TF content in water spinach with different plasma treatments grown in different soil. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed. Different uppercase letters indicate the significant difference between plasma treatment, while different lowercase letters indicate the significant difference between cultivation soil.
Figure 5. Comparison of TF content in water spinach with different plasma treatments grown in different soil. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed. Different uppercase letters indicate the significant difference between plasma treatment, while different lowercase letters indicate the significant difference between cultivation soil.
Applsci 13 03341 g005
Applsci 13 03341 g006 550
Figure 6. TP content and TF content of water spinach grown under different treatments. PAW: plasma-activated water; RO: reverse osmosis water; Fert: fertilizer. Different uppercase letters indicate the significant difference between treatment.
Figure 6. TP content and TF content of water spinach grown under different treatments. PAW: plasma-activated water; RO: reverse osmosis water; Fert: fertilizer. Different uppercase letters indicate the significant difference between treatment.
Applsci 13 03341 g006
Table
Table 1. The physiochemical properties of PAW.
Table 1. The physiochemical properties of PAW.
Nitrate (mg/L)Nitrite (mg/L)pHConductivity (µS/cm)ORP (mV)
42.7 ± 0.7014.7 ± 0.583.17 ± 0.06311.7 ± 12.01554 ± 2.65
Table
Table 2. The average height of water spinach grown under different treatments in control soil.
Table 2. The average height of water spinach grown under different treatments in control soil.
TreatmentPlant Height (cm)
W1W2W3W4W5
NTS + NTW6.4 ± 0.37 A8.3 ± 0.80 B9.9 ± 0.11 CD12.0 ± 0.26 E15.6 ± 1.53 F
NTS + PAW5.8 ± 0.21 A7.3 ± 0.57 B8.8 ± 0.19 C11.1 ± 0.75 E15.3 ± 1.71 F
PTS + NTW6.8 ± 0.42 A8.6 ± 0.52 B10.7 ± 1.16 D12.9 ± 2.09 E15.8 ± 4.09 F
PTS + PAW6.6 ± 0.87 A8.0 ± 0.53 B9.6 ± 0.40 CD11.3 ± 0.51 E13.4 ± 1.16 F
p-value0.1930.1200.036 *0.2770.622
Note: Different uppercase letters indicate significant difference (* p < 0.05) between groups or treatments, while same uppercase letters do not show significant difference. The data shown are the average values of three repeats. W1–W5 indicates treatment from week 1 to week 5. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed.
Table
Table 3. Comparison of TP content in water spinach with different plasma treatments grown in different soil of the Phase-1 pot experiment.
Table 3. Comparison of TP content in water spinach with different plasma treatments grown in different soil of the Phase-1 pot experiment.
TreatmentTP Content (mg GAE/g DM) of the Water Spinach Planted inp-Value
Control SoilCd-Added SoilPb-Added Soil
NTS + NTW14.39 ± 0.50 Aa17.09 ± 0.72 Eb19.06 ± 0.29 Hc<0.001 ***
NTS + PAW17.73 ± 0.66 BCd18.04 ± 1.20 Ed16.82 ± 0.40 Gd0.246
PTS + NTW16.23 ± 1.19 Be14.85 ± 1.11 De15.20 ± 0.18 Fe0.256
PTS + PAW18.83 ± 0.91 Ch16.98 ± 1.25 Eg14.94 ± 0.37 Ff<0.01 **
p-value<0.01 **<0.05 *<0.0001 ***
Note: Different uppercase letters indicate the significant difference between plasma treatment, while different lowercase letters indicate the significant difference between cultivation soil (* p < 0.05, ** p < 0.01, *** p < 0.0001). Samples used in the analysis are the mixtures of three pots of plants and the data shown are the average analysis values of three repeats. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed.
Table
Table 4. Comparison of TF content in water spinach with different plasma treatments grown in different soil of Phase-1 pot experiment.
Table 4. Comparison of TF content in water spinach with different plasma treatments grown in different soil of Phase-1 pot experiment.
TreatmentTF Content (mg QE/g DM) of the Water Spinach Planted inp-Value
Control SoilCd-Added SoilPb-Added Soil
NTS + NTW7.95 ± 1.61 Aa9.20 ± 0.13 Ba9.21 ± 0.34 Fa0.257
NTS + PAW9.00 ± 0.86 Ab9.99 ± 0.32 CDb9.69 ± 0.07 Fb0.145
PTS + NTW9.24 ± 0.37 Ad9.54 ± 0.27 BCd7.25 ± 0.04 Ec<0.001 ***
PTS + PAW9.77 ± 0.16 Af10.53 ± 0.44 Df7.60 ± 0.82 Ee<0.05 *
p-value0.194<0.01 **<0.001 ***
Note: Different uppercase letters indicate the significant difference between plasma treatment, while different lowercase letters indicate the significant difference between cultivation soil (* p < 0.05, ** p < 0.01, *** p < 0.0001). Samples used in the analysis are the mixtures of three pots of plants and the data shown are the average analysis values of three repeats. NTS: non-treated seed; NTW: non-treated water; PAW: plasma-activated water; PTS: PAW-treated seed.
Table
Table 5. TP content and TF content of water spinach grown under different treatments in Phase-2 pot experiment.
Table 5. TP content and TF content of water spinach grown under different treatments in Phase-2 pot experiment.
TreatmentTP Content (mg GAE/g DM)TF Content (mg QE/g DM)
RO14.05 ± 0.54 A9.99 ± 2.71 B
RO + Fert14.61 ± 0.75 A13.65 ± 1.79 CD
PAW14.99 ± 0.59 A11.54 ± 0.27 BC
PAW + Fert13.99 ± 0.14 A15.50 ± 1.19 D
p-value0.1620.021 *
Note: Different uppercase letters indicate the significant difference between treatment (* p < 0.05). Samples used in the analysis are the mixtures of three pots of plants and the data shown are the average values of three repeats. RO: reverse osmosis water; Fert: fertilizer; PAW: plasma-activated water.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Hsu, S.-C.; Kong, T.-K.; Chen, C.-Y.; Chen, H.-L. Plasma-Activated Water Affects the Antioxidant Contents in Water Spinach. Appl. Sci. 2023, 13, 3341. https://doi.org/10.3390/app13053341

AMA Style

Hsu S-C, Kong T-K, Chen C-Y, Chen H-L. Plasma-Activated Water Affects the Antioxidant Contents in Water Spinach. Applied Sciences. 2023; 13(5):3341. https://doi.org/10.3390/app13053341

Chicago/Turabian Style

Hsu, Shu-Chen, Ting-Khai Kong, Chung-Yu Chen, and Hsiu-Ling Chen. 2023. "Plasma-Activated Water Affects the Antioxidant Contents in Water Spinach" Applied Sciences 13, no. 5: 3341. https://doi.org/10.3390/app13053341

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop