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Non-Thermal Plasma Treatment on Seed
The use of non-thermal plasmas in agriculture or plant biology has also been widely reported in the last few years. The topics, related to the decontamination of seeds, modification of surface properties, metabolomic pathways, and enzymatic activity, enhancing seed germination and the initial growth, are summarized e.g., in Plants 2021, 10, 1616; https://doi.org/10.3390/plants10081616. Disease control and mycotoxin degradation were also reported.
Plasma, called also the fourth state of matter, is a partially or fully ionized gas. A distinction is made between thermal (high temperature, equilibrium) and non-thermal (cold, low temperature, non-equilibrium) plasma. The thermal plasma reaches the temperatures of thousands of Kelvins and occurs in the Sun, lighting, electric sparks, tokamaks, etc. and it is therefore not applicable in biological applications. On the other hand, the non-thermal plasma (NTP), also called low-temperature or cold plasma, occurs at nearly ambient temperature and the high kinetic energy is stored in electrons only. Its biological and also medical applications are very wide and include, among others, disinfection processes, acceleration of blood coagulation and improved wound and infection healing, dental applications or cancer therapy. These are summarized in numerous reviews, such as , or in the comprehensive books of Shintani and Sakudo  and Metelmann et al. .
The use of non-thermal plasmas in agriculture or plant biology has also been widely reported in the last few years. The topics, related to the decontamination of seeds, modification of surface properties, metabolomic pathways, and enzymatic activity, enhancing seed germination and the initial growth, are summarized e.g., in . Plant disease control  or mycotoxin degradation  were also reported. The nature of chemical reactions in NTP is rather complex, see e.g., .
NTP may be easily generated in various electric discharges, among which the most commonly used ones are corona discharges, plasma jets (called also plasma needle, plasma torch or plasma pen), dielectric barrier discharges, gliding arcs and microwave discharges. For a general description of plasma sources, see e.g., . In addition, the described effects are not constrained to the direct NTP treatment, but on a lower scale are also mediated by the effects of plasma-activated water (PAW) or air, i.e., the water exposed to NTP prior to the application to desired objects. The described effects can persist for many months after exposure, mainly due to the presence of stable reactive oxygen and nitrogen particles as described e.g., in . The use of NTP has already been addressed by the authors in relation to wheat (Triticum aestivum)  and seeds that can be used as raw seed .
2. Surface Seed and Sprout Decontamination
Both microbial and toxin decontamination of seed surface and plant sprout were included. Runtzel et al.  reported the effective fungal inactivation of Aspergillus parasiticus and Penicillium sp. on the surface of common bean after 10–30 min exposure of dielectric barrier discharge (DBD). Selcuk et al.  reported the inactivation of pathogenic fungi — Aspergillus sp. and Penicillium sp.— by NTP in a SF6 atmosphere on artificially contaminated seeds of common bean, chickpea, lentil and soybean without affecting the cooking time and other food qualities. A significant reduction of 3-log was achieved within 15 min.
A significant reduction of the seed-borne microbial contamination on pea was observed by Khatami and Ahmadinia , where the amount of microorganisms decreased by ca 3-log from initial 5.5 to 2.5 log CFU/mL/cm² (the units are not explained in the original paper) after 60 s of exposure. Peanut decontamination was reported by Basaran et al. , where 1-log and 5-log reductions of Aspergillus parasiticus after 5 min treatment in air or a SF 6 atmosphere were observed, respectively. The decontamination of several leguminous species (blue lupine, goat-rue, honey clover, soybean, and pea) has been studied , where the plasma treatment contributed to better fungicidal effect against of Fusarium sp., Alternaria sp., and Stemphilium sp. on seeds. On the other hand, a possible reduction of toxin production on peanut kernels was reported , where a significant reduction of aflatoxin levels without any negative sensory effect was reported.
The decontamination of soybean seeds contaminated with bacteria using PAW was reported by Lee et al. . PAW reduced the overall 4.3-log CFU/mL amount of aerobic microbes and 7.0-log CFU/mL of artificially inoculated Salmonella Typhimurium within 5 min and 2 min, respectively. Two following works reported the decontamination of sprouts, both by PAW only. Schnabel et al.  contaminated mung beans sprouts with the bacteria Escherichia coli, Pseudomonas fluorescens, Pseudomonas marginalis, Pectobacterium carotovorum, and Listeria innocua. The experimental results showed a reduction from 2.5-log to 3.5-log of bacteria and better growth of the mung bean sprouts, while untreated samples became strongly glassy and cell liquor was released, no influence of treated samples was observed. Similar results with mung bean were reported also by Xiang et al. , where reductions of 2.3 to 2.8-log were observed in aerobic bacteria, yeasts and moulds.
3. Effects on Seed Surface Properties
The applications of NTP or PAW also affect the properties of samples. These changes appeared to be beneficial and are listed below, and may be divided into seed surface properties and seed internal content properties. The primary effect on the surface is a decrease of the surface energy leading to better wettability or higher hydrophilicity, as measured by the contact angle of water droplets. This change in the wetting properties of seeds is at least partially due to oxidation of their surface by NTP.
Bormashenko et al.  reported a contact angle decrease in common beans and their markedly accelerated water absorption after tens of seconds of cold radiofrequency plasma treatment. The treatment leads to hydrophilization of the cotyledon and tissues constituting the seed coat when they are exposed to plasma separately. On the contrary, when the entire seed is exposed to plasma treatment, only the external surface of the common bean is hydrophilized by the cold plasma. All these statements are in agreement with the results of , where the authors observed pea seeds. SEM and Fourier transform infrared (FTIR) surface analyses showed small changes in the surface layer caused by the oxidation of lipids and polysaccharides (the consequences are mentioned in the original work). Moreover, the result of performed genotoxicological tests also confirmed that the level of DNA damage is minimal. A significant increase in water imbibition was also reported for soybean seeds .
Surface modification was also reported after treatment by PAW. Fan et al.  reported a water absorption rate increase from 65% for control samples to 75% for treated mung beans. Sajib et al.  reported a similar lipid or wax coat alteration of black gram due to the interactions of NO²− and H2O2 with wax. Zhou et al.  confirmed by SEM that the seed coat of mung bean is chapped and that it improves the water and nutrients absorption, which is a condition that could enhance the germination rate of mung bean and promote the growth of hypocotyls and radicles.
4. Field Production and Quality Crop Yield
The following three works enhance the studies up to the level of field experiments. In Tarrad et al. , seeds of Egyptian clover (T. alexandrium cv. Gemmiza 1 and cv. Fahl) were treated by pulsed atmospheric-pressure plasma jet, that increased the final yield. The total dry matter yield increased by about 15 % and 9 % over the non-treated control for Gemmiza 1 and Fahl cultivars, respectively.
In , the authors reported for blue lupine NTP treated seed that due to the decrease of seed infection and stimulation of field germination, to early seedling growth and to plant resistance to pathogens, the yield increased by 27 %. In , the authors exposed soybean seeds to NTP in various atmospheres of air, O2 or N2. Under greenhouse conditions, dry weight of roots, plant height, stem diameter and yield of plants grown from either healthy or infected seeds were improved. The plant height, stem diameter and root dry weight of plants from plasma-treated seeds showed increases of 3 %, 8 % and 12 %, respectively; the NTP treatment had positive effects on all the monitored parameters, as compared with either infected plants or fungicide control.
Non-thermal plasma (NTP) has become a widely used technique in various fields of biology, medicine, food processing and others.
The exposure to NTP or to plasma-activated water (PAW) can significantly affect the different properties of legume seeds. Namely, germination starts from water uptake, and the capability of water absorption could be significantly influenced by the action of plasma. Important surface properties and some physiological parameters of seeds could be also modified. Oxidation processes of plasmatic reactive species may increase water adsorption capability by increasing wettability of seed coats and could also be associated to gas exchanges and to electrolyte leakage by the seed. It is likely that NTP can effectively change dormancy of hard seeds by affecting seed permeability and triggering subsequent processes. NTP can positively influence the germination and growth of the seed, and subsequently also the properties of the seedlings. NTP treatment could reduce the hardness associated with mechanical dormancy of many Fabaceae species (alfalfa, blue lupine, grass pea, honey clover, Mimosa sp., Trifolium sp., etc.). NTP can be advantageously used in decontamination of plant seed surfaces or legume products. Legumes tolerate this physico-chemical treatment well, and the mild stress it causes appears to have a positive effect on them. Changes in physiological factors can then have a positive effect on the number of crops in the field and their yield.
Acknowledgement: This study was supported by Charles University research program Progress Q25.
The entry is from 10.3390/plants10081616
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