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.
|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.
How plasma agriculture may help plants
- Growth enhancement
- Seed sterilization
- Soil remediation
- Food preservation
- Food processing
SOURCE: P. ATTRI ET AL/PROCESSES 2020
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.
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.
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.
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.
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.