Applications of Plasma Technology in Agriculture: background, devices and conditions, and pre-harvest results (Part 2)
In the part I technote we explored the basics of cold plasma technology and gave a few examples. In this part II technote, we will dig deeper into the applications.
Background
This technote focuses on the pre-harvest opportunities that can be exploited in the space of non-thermal plasma technology to improve food security. This is particularly important given the multitude of stresses that seeds and plants have to surmount to increase germination and standing crop. Notable stresses include – but are not limited to, water scarcity, waterlogging, high toxin levels in soils, high soil salinity and extreme temperature due to climate change. When plants are subjected to these stressors yields reduce [1].
Climate change and other stressors can lead to an outbreak of pathogens that infest seeds. A variety of environmental and natural factors might delay seed germination. Soils might also be subject to attack by pathogens that might go on to attack seeds when they’re planted. All these and many more have opened up agricultural applications of non-thermal plasma in the following domains among many others:
- Seeds sterilization
- Seed germination improvement
- Reduction of pathogen invasion in soils
Non-thermal plasma can change seed morphology, genetic expression and enhance protein levels thereby increasing yields [2].
To demonstrate the biological effect of low-temperature plasma on wheat germination, a plasma-treated wheat seedling supernatant was analyzed and it showed activity for certain enzymes. The activity of the following enzymes was tested: amylase (ATP), peroxidase (POD) and superoxide dismutase (SOD), all of which are very important in plant growth. Comparing with wheat grown in controlled environments, plasma-treated samples showed an increase in enzyme activity of 60-143 %, with ATP content increasing by 9.1-62.3 %. The ATP contents of plant leaves increased by 13.9-178.5 % with the root system increasing by about 7.5 %. The plasma-treated wheat withstood drought with protective enzyme activities due to a sharp increase in peroxidase and superoxide. [2].
Reactive species such as electrons, ions and oxygen and nitrogen, together with neutral ones are generated from plasma. Plasma also generates UV radiation and electric fields. It also changes the pH, electrical conductivity and oxidative-reductive potentials of solutions. In agricultural applications, the seeds must first be placed in the plasma and a seed-plasma contact should be effectively established. Once in contact with seed substrates, the generated plasma species penetrate the seed epithelium, change the level of enzyme activity, increase the rate of seed germination, and enhance plant growth over conventional treatment methods. Advantages include: short treatment time, easy accessibility and low temperature during treatment [1-3].
Innovative development of Low Temperature Plasma (LTP ) devices and optimum conditions
Conceptually, a Low-Temperature plasma system (or device) is assembled by first selecting an appropriately sealed container and evacuating it to create a vacuum condition at a pressure of about 100 Pa. In the sealed container two parallel plates are installed to serve as electrodes. These electrodes are connected to the outside by a wire. At a pressure of 100 Pa, the system is powered on under a certain voltage and the air between the two electrodes is ionized. The air plasma created typically is composed of reactive nitrogen and oxygen species (RONS) and electrons between the two electrodes. There is direct contact between these species and any seed sample placed between the electrodes. After a certain time and a certain intensity of plasma treatment the activity of the seed can be improved. Tests showed that these plasma-processed seeds when sown result in plants with greater primary productivity.
So far Russia has taken the lead in the production of low-temperature plasma devices, although Russian domestic departments and units have introduced some foreign-built plasma modifications. These foreign-made units are partly defective, lack reproducibility of performance and consume very high energy. This is because the radio frequency electric field created a DC discharge problem. After careful diagnosis, it has been found out that there is an asymmetry between the assembled electrodes which create an AC signal with a radio frequency (RF) 13.56 MHz. Because the metal container (cylinder) acts as one of the two electrodes (or ground) and discharges to the other electrodes (or target), a large amount of charge cannot be released and a DC potential is created. This reduces the power of the RF electric field which affects process reproducibility.
Low-temperature plasma seed processing technology is quite new and is gaining a foothold in China. The plasma seed-treatment units typically have four parts [2]:
a) a radio frequency matcher;
b) a vacuum system;
c) a plasma generating device; and
d) a plasma transmission system (which guides the energy or species of the plasma to the substrate).
An example of a seed-treatment plasma device is shown in the Figure below for radish sprouts cultivated with and without O2 plasma irradiation. In part a) the 13.56 MHz RF matcher is attached to plasma generator (fitted with electrode) and the gas is transmitted via a funneling device to the target [3,4]. A vacuum pump is attached to create a pressure less than atmospheric. Sprouting results are displaced pictorially in figure b’s top and bottom pics.
Different configurations of non-thermal technology are applicable in agriculture. They include, dielectric barrier discharge (DBD), corona discharge (CD), spark discharge (SD), and atmospheric pressure plasma jet (APPJ).
In DBD, a high voltage is applied to a gas-filled gap between two electric materials or is applied to one of the parallel electrodes separated by a dielectric barrier, acting as an insulating layer between them. DBD creates micro-discharges that generate a significant number of high-energy electrons and ions. DBD is mainly used in applications where thermal damage needs to be minimized.
In CD, partial ionization of the gas surrounding a conductor occurs due to the influence of a high electric field. It uses mainly air as plasma gas. CD is mainly used in the decontamination of gaseous medium and in the treatment of soil samples.
SD occurs when a high voltage is applied across the electrodes, creating a strong electric field in the gap. As the voltage increases, the electric field strength eventually exceeds the breakdown voltage of the gas leading to ionization and the formation of a conductive plasma channel. SD is applicable for the direct treatment of surfaces that are not subject to significant changes under heat. It is useful in plasma activated water (PAW) and plasma treated solutions (PTS) applications.
In APPJ applications, a stream of plasma is expelled from a nozzle or an electrode by subjecting a gas to an intense electric field. High energy systems can be implemented using a microwave plasma torch. The microwave excites the gas within a resonant cavity. Other fields of application include environmental remediation and PAW. Due to higher concentrations of RONS PAW applications a gaining grounds in agriculture. APPJ are becoming more attractive versus low pressure plasma because of the reduced cost linked to the eliminated need to maintain a vacuum source [3].
Configurations of the various NTP types as useful in agriculture are shown below [3,5].
To have a reproducible and stable plasma for optimum performance some key factors ought to be optimized such as the applied power (and hence the voltage), the type of plasma gas, the application time, the selected RF value etc. When optimum conditions are determined and tested, improvements in seed germination and enhanced plant growth have been observed in some studies.
In another study, the wheat seeds of triticum aestivum was treated by a glow discharge using a mixture of air/O2 gases at 1333 Pa pressure and 3-5 kHz radio frequency for 3-9 minutes. It was observed that a 6-minute glow treatment resulted in 95 -100 % seed germination and a 20 % increase in wheat yield [8]. Other researchers using the same wheat seeds on a RF plasma operated at 13.56 MHz frequency with air as the plasma gas observed an enhanced yield of 75 % relative to control sample [9]
Iqbal et al. treated the seeds of the wheat (Galaxy-2013) with Ar low pressure plasma at variable voltages (600-850 V). They observed that germination rate was 57-60% higher when the voltage was changed from 600-850 V compared to samples not processed with plasma [6]. A different group of researchers used the same wheat seeds (Triticum spp.) and treated it with He plasma for 15 seconds at 150 Pa pressure and 3 x 109 MHz frequency with 60-100 W variable power. The plasma treatment at 80 W showed 6 and 6.7 % improvement in seed germination potential and germination rate respectively compared to the control group. Additionally, the plant height, root length and fresh weight increased to 20.9 and 21.8 % respectively at the seedling stage. The wheat yield increased to 5.8 % relative to the control group [7].
In another study, Bormashenko et al. used inductive air plasma discharge to treat beans seeds for 2 mins at 10 MHz frequency, 6 x 10-2 Pa pressure and 20 W power. No significant change in germination percentage was observed but the speed of germination was faster for plasma-treated samples [10]. In a different study, a commercial computer-controlled plasma device (HD-2N) treated the soybean (Glucine max) seeds. The HD-2N plasma device worked at a frequency of 13.56 MHz and pressure of 150 Pa with variable power from 60-120 W. There was an improvement in seed germination and seedling growth at 80 W power. Shoot length, shoot dry weight, root length and root dry weight increased by 13.7, 21.95, 21.42, and 27.51 % respectively [11].
In conclusion the technote summarized the application of plasma technology in agriculture.
References
[1] Pankaj A, Kenji I, Takamasa O, Kazunori K and Masahuru S. Plasma Agriculture from Laboratory to Farm: A Review. Processes, 2020; 8:1001
[2] Bo Zhang. Application of low temperature plasma technology in crop seeds. Current Investigation in Agriculture and Current Research, 2020; doi:10.32474/CIACR.2020.08.000295
[3] Evgeny M K, Namik G, Dmitry E B, Leonid V K, Alexey S D, Andrey Y I, Babak S and Sergey V G. Advancements in Plasma Agriculture: a review of recent studies. Journal of Molecular Sciences, 2023; 24:15093
[4] Kitazaki S, Koga K, Shiratani M, Hayashi N. Growth enhancement of Radish Sprouts induced by pressure O2 radio frequency discharge plasma irradiation. Jpn. J. Appl. Phys., 2012;51:01 AE01
[5] Bychkov V L, Chernikov V A, Deshko K I, Zaitsev E S, Esakov I I, Vysikaylo P I. Corona Discharge Over Alcohol against germs in air. IEEE Trans. Plasma Sci., 2021; 49:1028-1033
[6] Iqbal T, Farooq M, Afsheen S, Abrar M, Yousaf M, Ijaz M. Cold plasma treatment and laser irradiation of Triticum spp. seeds for sterilization and germination. J. Laser Appl. 2019, 31, 042013.
[7] Jiang J, He X, Li L Li J, Shao H, Xu Q, Ye R, Dong Y E Effect of Cold Plasma Treatment on Seed Germination and Growth of Wheat. Plasma Sci. Technol. 2014, 16:54–58.
[8] Roy N C, Hasan M M, Talukder M R, Hossain M D, Chowdhury A N Prospective Applications of Low Frequency Glow Discharge Plasmas on Enhanced Germination, Growth and Yield of Wheat. Plasma Chem. Plasma Process. 2018, 38:13–28.
[9] Saberi M, Modarres-Sanavy S A M, Zare R, Ghomi H. Improvement of photosynthesis and photosynthetic productivity of winter wheat by cold plasma treatment under haze condition. J. Agric. Sci. Technol. 2020;21:1889–1904.
[10] Bormashenko E, hapira Y, Grynyov R, Whyman G, Bormashenko Y, Drori E. Interaction of cold radiofrequency plasma with seeds of beans (Phaseolus vulgaris). J. Exp. Bot. 2015, 66, 4013–4021
[11] Li L, Jiang J, Li J, Shen M, He X, Shao H, Dong Y. Effects of cold plasma treatment on seed germination and seedling growth of soybean. Sci. Rep. 2014, 4:5859.
Author: Dr. Eliasu A. Teiseh PhD