Plasma is an ionized gas consisting of faction of free electrons, excited atoms, molecules and radicals and so also called as the fourth state of matter. It integrates about 99 per cent of the visible universe. There are significant charge carriers in plasma that make it electrically conductive and strongly influenced by electric and magnetic fields
(Ahn et al., 2019, Ding et al., 2011). It forms structures
viz., beams, filaments and double layers under the influence of a magnetic field,
(Nalwa et al., 2018). Plasma is also known as St. Elmo’s fire. For the first time in 1879 Sir William Crookes (1832-1919), a British chemist discovered a radiant matter created in glass container having two electrodes that ionize the gas inside. In 1928 this matter was later called plasma by American chemist Irving Langmuir (1881-1957) who thought it looks like blood plasma.
(Jiayun et al., 2014). This phenomenon is also visible in the Universe wherein at the high temperature of the Sun, ionization of a large portion of the atoms takes place by collisions between fast-moving atoms and this gas acts as plasma
(Schnabel et al., 2012). Examples of plasma are; lightening, Universe, Sun, Stars, neon and fluorescent light and plasma T.V.
What are different types of plasma used nowadays?
Glow discharge plasma is build up by applying low frequency Radio Frequency (less than 100 kHz) electric field in between two metal electrodes. It is a non-thermal plasma generated (Figure 1) e.g. fluorescent light tubes
(Kumar et al., 2019). Dielectric barrier discharge plasma is also a non-thermal discharge that is created when high voltages is passed across small gaps
(Nishioka et al., 2014) (Fig 1).
Other plasma used commercially
1. Capacitively coupled plasma
2. Cascaded Arc Plasma Source
3. Inductively coupled plasma
4. Wave heated plasma
5. Arc discharge
6. Corona discharge
7. Dielectric barrier discharge
8. Capacitive discharge
9. Piezoelectric direct discharge plasma
1. Different processes occurring inside plasma
A plasma constitute equal amount of positive and negative ions. It is constructed by applying a potential difference of a few 100 V to a few kV between two electrodes in a reactor
(Bormashenko et al., 2012). Inside the reactor (that is having filled with inert gas) due to the potential difference, electrons emitted by cathode hit the gas atoms or molecules to give rise to excitation, ionization and dissociation reaction.
(Filatova et al., 2013) The
excitation collisions generating excited species, contributes to glow nature of plasma (and is therefore also called ”glow” discharge). The ion-electron pairs initiated by
ionisation collisions forms a self sustained plasma whereas
dissociation collisions forms very highly reactive radicals (Fig 1).
Plasma in Seed Science and Research
Nowadays many seed quality enhancement techniques are employed to improve the quality of seed. These techniques as mentioned (Table 1) offers a great advantage to the seed producers in terms of better seed germination, vigour, storability, protection from seed pathogens and insect pest, higher crop productivity. But most of these treatments comprise the exposure of seeds to either high moisture or chemicals that can alter the physiological property of seeds. But this new technique of treating the seeds with plasma (Fig 2,3) do not exposes seed to moisture but handle seed with ionized gas that improves the seed quality by different processes
viz, excitation, ionization and dissociation. While treating seeds with plasma factors are to be considered
viz. seed type:- size, shape, surface, wrinkle or cervices of seed, exposure time, sensitivity of microorganism type, density of fungal loading and type of plasma.
Properties of plasma
• Plasma Etching (
Etching: German
ätzen, from a base meaning ‘cause to eat): plasma is used as an etchant instead of strong acid to cut metallic surfaces. High speed plasma of appropriate gas mixture is shot at a sample. It modifies the physical properties of the surface by reaction of radicals.
• Surface sterilization by reactive species produced in plasma.
• Plasma gasification: process which converts organic matter into synthetic gas and electricity using strong electric current.
• Plasma pyrolysis generation of a torch’s with temperature ranging from 2,200 to 13,900°C.
• Plasma gasification means plasma torch is powered by an electric arc that is used to ionize gas and catalyze organic matter into synthetic gas and solid waste (slag).
Where plasma treatment can be done?
Other Application of Plasma Technology in science and technology
1. Lights and Microelectronic technology.
2. Medicine (dental cavities, sterlization of various surfaces, treatment of skin diseases, delicate surgeries).
3. Food processing- decontamination.
4. Ion implantation to material modification in metal industry.
5. Textile industry.
6. Decontamination of grains
(Meral et al., 2008).
7. Surface sterilization clinical devices off microorganism.
8. Rubber industry.
9. Waste treatment by gasification of biomass and solid hydrocarbons, such as coal, oil.
Cold radiofrequency plasma treatment
can mutate the wettability of surface in seeds
viz.
Triticum spp.,
Lens culinaris and
Phaseolus vulgaris (Bormashenko et al., 2012) by decreasing the apparent contact angle of seed surface with water. Lower contact angle encourage more water imbibition and speed of germination. Similar results are obtained by
(Basaran et al., 2008, Jiang et al., 2014 and
Kumar et al., 2019). Seed surface gets oxidized by plasma treatment as during this treatment seeds get in contact with oxygen containing functional groups that boost radical emergence
(Sera et al., 2008). Seeds treated with plasma impose anti-microbial and antifungal activity and also increased hydrophilicity of the treated seeds. It can be proposed to save a significant amount of water which is wasted in flood irrigating the sown fields. Similar results are observed by
(Stone et al., 1973, Krapivina et al., 1994 and
Dubinoy et al., 2000). Atmospheric air plasma excited at some volts, can improve the permeability of the seeds by seed etching
i.
e. formation of many tiny holes on the surface of the seeds. The roughing of surface may accelerate seed germination and seedling emergence, longer root length and higher the strong seedling percentage
(Jiayun et al., 2014). Plasma is also known to increase antioxidants
viz., the catalase (CAT) activity and catalase isoenzyme expression. Also malondialdehyde (MDA) that is generated when seedling is in environmental stress may also decrease with plasma treatment of such seeds. Plasma treatment modified the physiological and biochemical characteristics of
Andrographis paniculata seeds by eroding the seed coat off the hydrophobic wax layer that induced deeper penetration of plasma-induced UV-B radiation
(Jiayun et al., 2014). It can further favour the biochemical reactions for modifying the seed coat.
Glow discharge plasma treatment of seeds upto 20 minutes is found to curtail the fungal loads upto 25 per cent thus improving seeds of cereal crops
(Brasoveanu et al., 2015). The fungal decontamination of seeds can be firmly related to the type, size, shape,
(Schnabel et al., 2012, Preechayan et al., 2010) surface, wrinkle or crevices of the seeds, minimum contact area of the samples with the holder and density of the fungal loading, exposure time and spatial location. Similar results were obtained by
(Ding et al., 2011, Dubinov et al., 2000). Also, atomic oxygen and hydroxyl radicals generated in plasma treatment are the apparent inactivation agents of microbial load while the nitrogen plays an imperative role in the intensification of the biological processes in seeds
(Filatova et al., 2013). If seed is under stress conditions, then most of its biological processes are withheld
e.
g. germination and seedling emergence. Cold plasma treatment proves to be remedial measure that can improve germination of seed, seedling growth, increase concentration of antioxidant enzymes, reduced lipid peroxidation levels (by lowering malondialdehyde content) of seeds
e.
g. oilseed rape under drought stress conditions
(Li et al., 2015). Not only this cold plasma treatment augment shoot length and dry weights, root length and dry weight and more lateral root number by improving absorptive ability of seeds. Similar results were produced by
(Shao et al., 2013,
Guimaraes et al., 2015). Higher is the amount of antioxidant enzymes more is the capacity of seeds to tolerate stress. So cold plasma treatment is also meant to elevate the SOD, CAT activities under drought stress conditions that is central for seed growth and germination.
Plasma treatment can be a restorative way to give protection to young seedlings against drought stress damage. It requires short duration of exposure that is commercially attractive. Another advantage is that chemicals are excluded in the plasma process hence there is no chance of pollutants remain after the treatment, which proves it as environment friendly. Seeds after plasma treatment can be stored again if they are not used for germination since it is not destroyed. Further this treatment recovers absorptive ability and imbibitions in seed and therefore better germination under drought stress. Higher accumulation of reactive oxygen species is cause of drought stress that results in oxidative damage to plants. But if plant is self sufficient in antioxidant system then its resistance to drought stress can be elevated. Plasma treatment is an important tool to increase the levels of antioxidant enzymes, such as SOD, POD and CAT and non-enzymatic compounds. Plasma treatment can also regulating water balance by modulating antioxidant enzymes. Plasma treatment stimulated SOD and POD activity in tomato seedlings
(Li et al., 2015). These result are in agreement with findings of with
(Meiqiang et al., 2005) who reported that plasma treatment increased peroxidase (POD) isoenzyme content in tomato seedlings. Also accumulation of soluble sugars and soluble proteins retain plant turgidity, compliment its absorptive ability and defends membranes and macromolecules during drought stress. Other advantage of using plasma treatment is that it improves wettability of seed and reduced lipid peroxidation-linked membrane deterioration. Further plasma treatment is conducted in several seed crops (Table 2) in relation to seed physiological, health and biochemical characters.
It can be concluded that plasma is a fast, inexpensive and ecologically sustainable, pollution free treatment that may emerge as a promising alternative technique to be used in agriculture as pre-germination seed treatment to improve seed quality, breaking hard seed coat, protecting seed against drought stress, enhancement of plant growth and performance in different crop species both under stress and normal conditions. This technology helps in improving seed germination rate, enhancing speed of germination in both normal and stress conditions and rejuvenates old seeds. It’s a constructive technique to enrich seed surface with micronutrients and inactivation of seed pathogen and decontamination of microbiologically contaminated seeds. It’s a quick treatment with no side effect and can prove to be an alternative to traditional seed processing technologies.
