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 How the fourth state of matter can make a greenhouse greener — and boost plant growth

Stephan Reuter of Polytechnique Montréal uses his expertise in energy and matter to develop medical devices on a daily basis. He stood in a sea of green recently, though, to explore how a rain of charged particles would affect lettuce.

He'd been invited to one of Quebec's largest commercial greenhouses to assist growers in rethinking agriculture's energy use. Thousands upon thousands of lettuce plants floated on polystyrene mats in a hydroponic, or no-soil, growing system within the facility, which was encased in glass walls and covered more than four soccer fields. It was almost time to harvest, package, and export the crop. Reuter's mission was to apply physics to assist Mirabel-based Hydroserre Inc. is reducing its carbon footprint.

To that aim, the company is looking for novel ways to combat diseases and deliver fertilizer to plants in growth. Many fertilizers contain ammonia, which is created by a chemical reaction called the Haber-Bosch process from nitrogen (essential for plant growth) and hydrogen. This process transformed agriculture in the early twentieth century by allowing for bulk fertilizer manufacturing. The process, however, produces hundreds of millions of metric tons of CO2 each year.

"Ideally, we'd like a renewable fertilizer," says Reuter. And, to be genuinely green, it should be produced on the farm, eliminating the need for transportation, which is another carbon emitter. Reuter and an increasing number of chemists, physicists, and engineers believe they can figure out how to do it. These experts are aiming toward completely sustainable farms in the future, where energy from renewable sources such as wind or sun is harnessed to produce a high-quality fertilizer on-site. They intend to make this vision a reality by utilizing plasma.

Plasma everywhere

Reuter may appear to be an unusual consultant for an agricultural problem. After all, the physics of plasma, one of the four fundamental states of matter, along with solids, liquids, and gases, is his area of specialty.

Plasma is a very common substance. In fact, astrophysicists estimate that more than 99.9% of the matter seen in the known universe is in a plasma form. Plasma is created when lightning strikes. Likewise, those low-cost novelty lamps are found in museum gift stores. When you turn on the power, a high voltage is produced by an electrode in the sphere's core, which interacts with the gas contained inside the glass to form tendrils of colored plasma that radiate outward. When you touch the glass, plasma tendrils appear to reach out and touch your fingers.

The sun is a plasma and gaseous ball. The solar wind (SN: 12/21/19 & 1/4/20, p. 6) is a jet of plasma that peels away from the sun. When that wind collides with Earth's protective, plasma-rich magnetic cushion, it creates the aurora borealis and aurora australis, which are viewed as rivers of light.

Plasma is a current technology workhorse as well. Engineers utilize it to etch the millions of tiny transistors found on today's computer chips, automobiles, and musical birthday cards. Plasma television pixels contain gas that generates plasma, which is enclosed inside tiny cells sandwiched between two glass plates, and plasma causes neon signs and fluorescent lights to shine. Plasma engines, according to some former astronauts, will one day drive mankind to Mars.

But what is plasma, exactly? It's a soup of negative-charged electrons, positive ions, and neutral atoms that produce electromagnetic fields as well as ultraviolet and infrared light. Plasma is formed when a gas is super-energized — for example, by heat or an electric current — and electrons are liberated from atoms.

Plasmas can be found in nature or created by humans. It's called "hot plasma" when it's made at high temperatures, like in the sun, and "cold plasma" when it's created at room temperature and low pressure, like in a plasma ball. It's easy to detect because plasma balls are filled with a gaseous mixture including one of the very stable noble gases, such as argon, xenon, neon, or krypton. The luminous tentacles that extend out from the core are made of plasma. The high-frequency current causes electrons to split from the gas atoms. A mixture of noble gases and the air is used in much agricultural research to produce nitrogen and oxygen ions.

Remember those mesmerizing plasma balls? The tiny lightning inside is plasma, which forms when the high voltage at the center of the ball causes electrons to separate from atoms in the surrounding gas, yielding a mix of charged and neutral particles.

Plasma's biological implications have long piqued scientists' interest. The width of growth rings in fir trees near the Arctic Circle matched the cycle of the aurora borealis, widening when the northern lights were greatest, according to Finnish physicist Karl Selim Lemström in the late 1800s. He theorized that the light show aided plant growth in some way. He erected a metal wire net over growing plants and sent a current through it to simulate the northern lights artificially. He claimed that under the correct conditions, the therapy resulted in higher vegetable yields.

For decades, scientists have known that exposing dangerous bacteria, fungi, and viruses to plasma can safely destroy them. Plasma has also been shown in animal studies to stimulate the formation of blood vessels in the skin. Reuter's study focuses on how to use these features to prevent new infections in wounds, speed up healing, and cure other skin diseases. However, he and other physicists have recently begun working on ways to exploit plasma's power to boost food production.

Experiments over the last decade have looked at a variety of methods for applying plasma to seeds, seedlings, crops, and fields. The plasma made from noble gases and plasma made from air are examples of this. Plasma is sometimes given directly to seeds or plants by plasma "jets" that flow over them. Another method employs plasma-treated water, which can be used for both irrigation and fertilizing. Some studies have found a variety of benefits, ranging from assisting plants in growing quicker and larger to helping them withstand pests.

"Even at this very early level of study using plasma, which has really only come into its own in the last 10 to 15 years," says plant pathologist Brendan Niemira of the United States Department of Agriculture's Eastern Regional Research Center in Wyndmoor, Pa. He approves of the strategy: Niemira's avatar on Zoom is an almond bathed in a strange purple plasma glow.

The current challenge, he argues, is determining whether plasma can deliver at the hectare level. "Can we make it operate in a field environment to provide a benefit that can be implemented into future grow systems?"

Many other challenges are nested inside that one, such as discovering a large-scale means to distribute plasma to plants, validating benefits claimed in lab research, and demonstrating that plasma is superior to present approaches. Finally, determine what the charged plasma soup is actually doing to plants.

According to Niemira, recent advancements were made feasible because scientists found efficient and cost-effective means to make cold plasmas by streaming high-energy electrons into a gas in the 1990s and early 2000s. When the electrons collide with gas molecules, they knock electrons off and produce charged particles. Since then, he claims, there's been a rush to test plasma on plants at all phases of development and using various tactics.

Two paths

Plasma is being studied for a variety of potential agricultural benefits. Plasma has been shown in several research to increase plant growth and yield. Others have found that plasma can help preserve food by removing dangerous germs and fungi from plant surfaces.

How plasma agriculture may help plants 

Preharvest

• Growth enhancement 
• Seed sterilization 
• Soil remediation

Post-harvest

• Food preservation 
• Food processing

SOURCE: P. ATTRI ET AL/PROCESSES 2020

Surface changes

According to Reuter, one of the most intriguing uses of plasma is as a fertilizer alternative to ammonia. His vision for the Mirabel greenhouse project, which he helped begin alongside scientists from the Quebec-based organization IRDA, or Research and Development Institute for the Agri-Environment, is as follows: Plasma is created by passing an electric current through a gas, which in this case is ideally just air. This process produces a mixture of charged and neutral particles, including electrons and ions, which can yield nitrogen and oxygen reactive species. Reuter and his colleagues will enrich the water with plasma in tabletop trials and then in the greenhouse to see if it may minimize infections and affect developing plants.

As their name implies, reactive species are ready to react with atoms and molecules, including those found in living things, and are biologically available to plants. Those reactive species dissolve when plasma is added to water. The plasma-infused water will next be utilized to irrigate the plants with physiologically accessible nitrogen. It will perform the same function as ammonia: nitrogen is given to plants as ions, excited molecules, and compounds in the water. While high amounts of reactive species might harm plant cells or DNA, Reuter claims that the amount in plasma-treated water is safe for plants.

For small-scale tests, physicist Stephan Reuter and colleagues use a setup like the one below. An electric discharge creates plasma, which adds reactive species of nitrogen and oxygen to the water in the dish. This plasma-treated water might be able to fertilize growing plants.

Experiments led by biochemist Alexander Volkov of Oakwood University in Huntsville, Ala., provide another example of plasma agricultural research. Volkov investigates the interactions between plants and electromagnetic. He's demonstrated, for example, how an electromagnetic input can activate the Venus flytrap's closing mechanism.

Volkov recently set out to investigate the effects of plasma on 20 seeds of dragon's-tongue, a cultivar of the bush bean Phaseolus vulgaris. It was a low-tech experiment. He and his colleagues balanced the seeds for one minute apiece on a plasma ball, then incubated them in water for seven hours. Two days later, the scientists discovered that the radicle — the little protrusion of root that turns a seed into a seedling — measured 2.7 centimeters in plasma-treated seeds, compared to 1.8 cm in untreated seeds, a 50 percent increase. In February 2021, the researchers published their findings in Functional Plant Biology.

Roots emerge

Alexander Volkov, a biochemist, and his colleagues exposed bush bean seeds to plasma jets before immersing them in water for a day. Plasma-treated seeds had larger radicles, or beginning roots, two days later. Seeds that had not been treated grew the slowest. Plasma was given to the others for 30 seconds, one minute, five minutes, and fifteen minutes (shown below). He had less dramatic outcomes when he used a plasma globe.

Volkov felt encouraged despite the extra growth being less than a centimeter. Because reactive nitrogen and oxygen species can't leave the glass sphere, the advantage couldn't have originated from them, but the treated seeds looked to take up more water and develop quicker.

To test that hypothesis, he and his colleagues used an atomic force microscope and magnetic resonance imaging, which shows how tissues absorb water. Volkov noticed that exposure had roughed up the surface of the seeds using the atomic force microscope's micrometer-level vision. The pictures resembled carved mountain ranges. He theorized that the ridges allowed the water greater surface area to cling to and more apertures through which to saturate the seeds' interiors. In comparison to untreated beans, MRI images of treated beans exhibited bigger swathes of white, indicating more water inside

"When we employ plasma balls or lamps, the water can readily pass through the pores and speed up germination," he explains.

Rocky road

An anatomic force microscope image of an untreated bush bean seed reveals a comparatively smooth surface (left). The surface becomes rough and corrugated after a one-minute plasma treatment (right), which may allow water to infiltrate the seed's shell more easily.

Growing evidence

a rocky path

Novena Pua, a physicist at Serbia's Institute of Physics, has conducted scores of research testing plasma on plants and has been working in the field for decades. She claims that the majority of studies, whether successful or not, have focused on two concepts: plasma as a disinfectant and plasma as a growth stimulator.

On the disinfection front, plasma jet treatments on foods like apples, cherry tomatoes, and lettuce for less than a minute can eliminate disease-causing bacteria including E. coli, Salmonella, and Listeria. Higher exposure times have also been investigated in several studies: Five minutes of plasma treatment inactivated 90% of harmful Aspergillus parasiticus fungi on hazelnuts, peanuts, and pistachios, according to a 2008 study.

Niemira is also involved in this line of study. He and colleagues reported in May 2019 in LWT–Food Science and Technology that plasma treatment combined with an existing sanitizer destroyed 99.9% of Listeria on apples in under four minutes. After an hour of working alone, the sanitizer produced comparable effects. He claims that the combo is far more effective than either one working alone.

Seed germination and plant growth studies are also looking encouraging. Soybean seeds were exposed to plasma by researchers at the Chinese Academy of Sciences in Nanjing. The roots were up to 27% thicker seven days after exposure than roots from untreated seeds, according to a study published in 2014. Researchers in Romania observed similar increases for radish roots and sprouts in the same year.

Researchers from Japan presented findings from a study of young seedlings treated directly with plasma and plasma-treated water in a rice paddy in the Aichi prefecture at last year's Gaseous Electronics Conference, sponsored online by the American Physical Society. Plants that were immediately treated with plasma early in the growing process yielded up to 15% more than untreated plants. However, treating plants late in the growth cycle reduced production. Pua believes that timing is crucial. So does the application method: plasma-treated water actually reduced yield in certain studies in Japan.

Engineer Katharina Stapelmann of North Carolina State University in Raleigh, who coordinated the session, says, "To my knowledge, this was the first study where plants were treated directly," rather than as seeds or after harvest for disinfection.

According to Pua, studies have linked plasma treatment to a variety of benefits, ranging from growth rate to yield. Other research suggests, however, that plasma will never be a one-size-fits-all method.

For example, while a six-minute plasma exposure increased barley sprout germination rates, an 18-minute exposure over three days had no effect on growth and reduced overall plant weight, according to a study published in the Journal of Physics D: Applied Physics in 2020. The effects of direct plasma jets on peas, maize, and radishes were studied in experiments published in 2000, and they found negative impacts that varied depending on the gas utilized in the plasma. The seeds were exposed for two to twenty minutes, and ones exposed for longer periods of time germinated slower than untreated seeds.

Researchers in South Korea discovered that untreated sprouts (far left) did not grow as well as sprouts that had one six-minute plasma treatment nine days after seeding barley (second from left). Sprouts treated for six minutes on two consecutive days (second from right) and three days in a row (far right) did not grow well, demonstrating that too much plasma may limit growth.

 J.-S. SONG ET AL/JOURNAL OF PHYSICS D: APPLIED PHYSICS 2020

According to Reuter, the findings demonstrate that scientists need to learn more about the many ways plasma might affect plants before it becomes a common practice on farms around the world.

For example, the UV radiation produced by plasma may play a role in plant success; UV radiation has long been utilized as a disinfectant. Reactive nitrogen and oxygen species, which can be beneficial or damaging to live cells depending on their utilization, are likely to act as nutrients and disinfectants. Electric and magnetic fields, as well as infrared and visible light, are all produced by the plasma. Their effect on plants has also not been completely investigated. Researchers know what's in the plasma and can observe how the plants react, but they don't have all of the specifics laid out, according to Volkov.

Gardens big and small

Plasma is being tested on big scales and in a variety of contexts all around the world. Portable "reactors" that employ plasma to create fertilizers from the air have been developed by Dutch scientists working in Uganda. They anticipate that this development will be able to meet the need for fertilizers in areas where farmers are unable to obtain ammonia. Reuter expects to publish his first results from desktop experiments in early 2022. At Hydroserre, he'll be able to fine-tune his process thanks to hydroponic growth technology.

He hopes that the research would demonstrate to future farmers how to substitute ammonia and cut carbon emissions.

While researchers and farmers wait for the results, citizen scientists, amateur physicists, and experimental gardeners have been known to put a plasma ball next to their rakes and shovels in the shed to conduct their own experiments.

Volkov has jumped into the fray. Last year, when the pandemic forced him to close his lab, he brought his work — and his plasma balls — home with him. He soaked the vegetable seeds for his garden in the lamp's rich, purplish glow for a minute before planting them.

"Cucumbers, tomatoes, eggplants, and cabbage," he explains. Volkov openly admits that a backyard trial run isn't proof of anything, and any gardener can attest that a fussy combination of variables may make or ruin a garden.

Last October, though, he witnessed a spectacular crop. By late October, he was still plucking large, ripe tomatoes off vines produced from plasma-treated seeds, even though most untreated seed plants had shriveled. Cucumbers were larger and juicier. He claims that the cabbages he planted in a friend's nursery were heavier and more flavorful. "I received a great deal on everything."

CITATIONS

H. Hashizume et al. Improvement of yield and grain quality by periodic cold plasma treatment with rice plants in a paddy field. Plasma Processes and Polymers. Vol. 18, January 2021, p. e2000181. DOI: 10.1002/ppap.202000181.

N. Puač et al. Plasma agriculture: A rapidly emerging field. Plasma Processes and Polymers. Published online November 17, 2017. DOI: 10.1002/ppap.201700174.

P. Attri et al. Plasma agriculture from laboratory to farm: a review. Processes. Published online August 17, 2020. DOI: 10.3390/pr8081002.

P. Ranieri et al. Plasma agriculture: a review from the perspective of the plant and its ecosystem. Plasma Processes and Polymers. Vol. 18, January 2021, p. e2000162. DOI: 10.1002/ppap.202000162.

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