Hemp or marijuana ( Cannabis sativa ) is undoubtedly one of the most controversial and controversial plants. Despite being a drug of abuse due to its psychoactive activity, its pharmacological properties have aroused scientific interest for some time. Apart from this, certain varieties are of great use in other sectors of society: as a source of fibers for textiles, livestock feed and even papermaking.
Recently, it has also shown its usefulness in the field of environmental protection through phytoremediation , which consists of the use of plants to recover a natural environment disturbed by pollutants. It is a cost-effective and reliable technology for removing a wide range of organic and inorganic chemicals, including heavy metals, as well as radioactive contaminants.
heavy metals
The contamination of soil and water by heavy metals during the last 20th century became one of the biggest environmental problems. They are substances that persist in the natural environment for a long time because they are not biologically or chemically degradable . Some can pass to living beings, accumulate in them and pass from one to another through feeding relationships, integrating into food webs.
The marijuana plant can be very useful to help decontaminate environments that accumulate heavy metals. It has been proven that this plant has the property of absorbing and accumulating nickel, lead, cadmium, zinc, copper and chrome in its tissues , especially in its leaves. It has been proven that a single kilogram of plant can accumulate more than 150 milligrams of cadmium, about 123 milligrams of nickel, up to 1.5 grams of copper, and up to 4.4 grams of lead.
Of course, the toxicity of these products causes a lower growth of the plant, which would affect its extraction capacity. Its effectiveness is therefore limited to moderately contaminated soils. There is a limit to the concentration of heavy metals in the soil, depending on the plant, its genetics, and the environmental conditions in which it grows, from which hemp is no longer viable .
One way to make it easier for the plant to extract these contaminants is the application of biodegradable chelating agents . These types of substances are so useful that it has been experimentally observed that, using hemp, it is possible to go from extracting just under 130 grams of lead per hectare to more than 26 kilograms .
The role of genetics
Recently, several plant genes related to the ability to grow under conditions of heavy metal contamination have been identified.
More specifically, it has been observed that in the presence of heavy metals there is a higher expression of two genes, GSR and PLD 𝛼. The expression of these genes induces the production of antioxidant enzymes that protect plant cells from oxidative stress. This work is especially important under stress conditions, since free radicals are released from oxygen that can cause serious damage to cells and tissues. Therefore, these genes are associated with plant defense mechanisms against the stress produced by these pollutants.
Genetic manipulation could facilitate the production of marijuana varieties —or other species— with a higher expression of the GSR and PLD 𝛼 genes, which can grow better in even more polluted conditions, which would be planted where current varieties do not grow, and help eliminate heavy metals accumulated in these soils.
What to do after?
Removing heavy metals from the soil is a difficult task, which can be greatly facilitated by the use of plants. However, we are left with a problem. What is done with the plants with all that contamination retained in their tissues? Can they themselves be an environmental problem?
Fortunately, hyperaccumulating plants —as plants capable of retaining high concentrations of heavy metals in their tissues are called— tend to have an unpleasant taste for herbivores, so it is difficult for these metals to end up in food webs . However, if they die and decompose, they release the metals back into the soil, and the problem is by no means solved.
However, this problem opens the door to a new solution. It is enough to carry out a good monitoring of the plants and remove them, once they have accumulated the maximum amount of contaminant tolerable for them. These plant remains, properly treated, can become a source of metals with a certain commercial value, and be reincorporated into the industrial system, thereby reducing the costs of soil treatment.
REFERENCES:
Ahmad, R. et al. 2016. Phytoremediation Potential of Hemp (Cannabis sativa L.): Identification and Characterization of Heavy Metals Responsive Genes.
CLEAN – Soil, Air, Water, 44(2), 195-201. DOI: 10.1002/clen.201500117
Alufasi, R. et al. 2018, marzo 9. Potential Applications of Cannabis sativa inEnvironmental Bioremediation. A Review. 6th Scientific Conference with International Participation all about People: Challenges For Science and Education, Slovenia.
Awa, S. H. et al. 2020. Removal of Heavy Metals in Contaminated Soil by Phytoremediation Mechanism: a Review. Water, Air, & Soil Pollution, 231(2), 47. DOI: 10.1007/s11270-020-4426-0
