Natural resistance to stress
It is not possible for plants to change their location, they cannot migrate in case their living conditions get worse, therefore it is no surprise that they have developed a mechanism that allows them to survive adverse environmental conditions. It appears that plants may be subjected to constant stress – biotic stress – caused by the living elements of the ecosystems: pests or pathogens, and abiotic stress – atmospheric factors, shortage or surplus of minerals or environmental pollution. Depending on the plant species and varieties, different plants can manage these hard conditions better or worse, but there are some that can handle them perfectly. Because of these properties, people use them to clean the soil.
Research
The research is conducted in two ways. The scientists first examine the plants and their properties to clean the soil, then the edible plants’ properties to extract harmful substances from the soil and whether they may be harmful to people.
The soil may be polluted in different ways (organic pollutants, heavy metals, pesticides, etc.), scientists from Lodz focus on the organic pollution.
This includes, for example, pollution form perfluoro alkoxy and pollution from using wastewater as fertilizer, which may contain dioxins – very poisonous chemical compounds that belong to the polychlorinated hydrocarbons. These are: PCDD (polychlorinated dibenzo-p-dioxin), PCDF (polychlorinated dibenzofuran), and PCB (polychlorinated biphenyl).
- explains Dr. Wyrwicka-Drewniak.
How do plants do this?
Plants can clean the soil in various ways. They can extract toxins from the soil through their root zones. They can also cooperate with other microorganisms that may be found in the rhizosphere – the part of soil with roots.
Microorganisms (for example: bacteria) have mechanisms that allow for the distribution of some organic compounds. They often develop better when they are in the rhizosphere, which means that they need plants, namely their roots.
- explains Dr. Wyrwicka-Drewniak.
At the same time, plants may extract toxins more efficiently, if the soil is inhabited by the right type of microorganisms.
It sometimes happens that the soil is intentionally filled with microorganisms that have proven qualities of improving plant growth. Because of that, the plant is able to develop, have a bigger biomass, and consequently extract more pollution from the soil.
- explains Dr. Wyrwicka-Drewniak.
Which plants can do this?
It appears that various plants have the ability to clean the soil. There are some, however, that ‘specialise’ in particular types of pollution remediation.
The research shows that there are some plant species that are hyperaccumulators, which mean that they can absorb even hundreds units more of harmful substances than other plants. It mainly refers to cleaning the soil from heavy metals. Such pants include: thlaspi (zinc, cadmium), thlaspi rotundifolium (lead), alyssum (nickel), berkheya coddii - a really efficient hyperaccumulator of nickel, cadmium, lead and zinc.
- says Dr. Wyrwicka-Drewniak.
When it comes to organic pollutants, the most efficient plants are willows and poplars. Why? First of all, these plants are not very demanding and can grow even on poor and polluted soil.
They also have an extremely fast biomass growth, which proves to be a desired characteristic from the remediation perspective. For instance, salix viminalis is used as an energetic plant. Its growth is so fast, that in every two years it is mowed with a special harvester.
- explains the scientist.
It is an amazing candidate from the remediation perspective. What is more, both species mentioned above are not edible, therefore there is no risk of poisoning by including them in your diet.
There are also plants used in water remediation. Typha, acorus calamus, iris, schoenoplectus lacustris, glyceria maxima have very extended root ecosystems, which clean the water perfectly.
The amount of nitrogen and phosphorus in the water becomes lower this way, which also protects water reservoirs from cyanosis.
- says Dr. Wyrwicka-Drewniak.
Scientists from different parts of the world are testing the remediation properties of cannabis sativa with regard to metal and metalloid remediation. Other plants, which have been tested with regard to soil remediation from dioxins and dioxin-like compounds, include cucurbitaceaes, such as pumpkins and courgettis.
- adds Dr. Wyrwicka-Drewniak.
What has to be tested?
The scientists are mainly testing whether a given plant (species/variety) can absorb pollutants. At the same time, they are testing whether the plant is not too sensitive to a given pollutant and whether it can grow in a polluted environment.
It can be done in two ways, for instance, by testing the physiological state of plants and evaluating photosynthesis parameters. We very often evaluate the plants living in similar conditions on similar soil, clear and polluted, and with that we are able to compare how the parameters, we are interested in, will change in both cases. The important thing is the fact that this test may be often repeated because it does not influence the state of the plants.
- explains Dr. Wyrwicka-Drewniak.
It often happens that the photosynthesis testing is not enough. In order to assess the physiological and metabolic state of the plant, it is necessary to perform various biochemical testing. In this case it is necessary to collect the plant cells, from both leaves and roots.
Form the remediation perspective, the most important aspect is to test how much of the substance, that we want to remove from the soil, can be found in the particular parts of the plant. Some pollutants will accumulate only in the roots, others will be efficiently transported to the stem and leaves. This information is essential because there is always a problem of what to do with the plants after the phytoremediation, and how they can be reprocessed.
- explains Dr. Wyrwicka-Drewniak.
How can it be tested?
Remediation testing has many stages. In order to know what kind of plant can be planted on a polluted soil, it is necessary to perform various laboratory tests.
At this stage we do not use plants on the polluted area but prepare the soil where the pollutants are intentionally added, and then put the plants there.
- says Dr. Wyrwicka-Drewniak.
Another stage involves moving the plants to the field conditions outside the laboratory.
There are some species that are efficient in the laboratory but after being placed outside to more demanding conditions, their efficiency is lower.
- the scientist adds.
Only after we are able to isolate the plants, that have the best ‘cleaning’ properties, they may be transported to post-industrial areas or other polluted regions.
What happens with the research results?
Such knowledge may be used in two ways. First of all, edible plants are tested, especially those which are consumed without any heat processing (various types of lettuce, endive, oregano, basil, thyme).
Such knowledge is crucial for us in order to avoid the risk of food contamination.
- adds Dr. Wyrwicka-Drewniak.
Then, on the basis of the research results it can be established which types of plants show the best remediation properties and are the most resistant to pollutants. Thanks to that, it is known which species and varieties of plants can be used in the polluted areas to clean the soil in the most efficient manner.
Content: Faculty of Biology and Environmental Protection, University of Lodz
Edit: Promotion Centre, University of Lodz