Mercury for Plants: Metabolism Inhibitor
Mercury (= Hydrargyrum) (Hg)
For the plant – a metabolic inhibitor
In plant organisms mercury causes inhibition of cellular respiration, photosynthesis, chlorophyll formation, gas exchange, and reduction of enzymatic activity. The key reaction explaining the disruption of metabolic processes is the interaction of mercury with the sulfhydryl groups of amino acids.
Although it is known that mercury strongly binds to sulfur atoms in amino acids, which are part of many proteins and enzymes, it is apparently easily transported in plants.
Toxic effects on young barley are described at a mercury content of 3 mg/kg dry weight, and at a concentration of 0.01 mg/kg in ash, mercury is highly toxic. The toxicity of vaporous free mercury and some methylated compounds (methylmercury) for plants is the most significant.
Plant damage is observed even at a mercury concentration of 1 µg/kg in the nutrient solution.
Symptoms of mercury poisoning in plants – delayed growth of seedlings and root development, inhibition of photosynthesis, and consequently – reduced yield.
Accumulation of mercury in root tissues inhibits the absorption of potassium ions by the plant, although at low mercury concentrations its stimulating effect on K+ uptake has been described.
In plants, mercury content is about 0.0001 mg%. The minimum amount (0.01–0.02 mg/kg) accumulates in plants growing in soils with low mercury concentrations (up to 0.10 mg/kg), but as the concentration in the soil increases, the mercury content in the aboveground and underground organs of plants increases to 0.018–3.0 mg/kg. In soils, most of the mercury is deposited by humic acids and humin. An increase in the content of humic acids in the soil reduces the amount of mercury that is absorbed by plants due to the formation of mercury-organic complexes. Under the influence of microorganisms, such complexes can be broken down, resulting in the formation of metallic mercury, which, upon evaporation, re-enters the atmosphere.
The rhizomes of algae can absorb mercury from contaminated bottom sediments and serve as a source of mercury for many epiphytic organisms. Thus, planktonic organisms accumulate mercury, which is consumed by crustaceans. Crustaceans are eaten by fish, and fish by birds. The final links in food chains are often seagulls, grebes, and ospreys.
However, regardless of how mercury enters the water, microorganisms methylate it, and methylmercury is always formed. This compound is fat-soluble, extremely toxic, and very persistent. In Sweden, the concentration of methylmercury in the bodies of birds, a significant portion of which feed on fish, has now approached levels at which grain-eating terrestrial birds are already dying from mercury obtained by consuming seed grain (in Sweden in the 1940s, grain was treated with methylmercury dicyanamide).
In the aquatic food chain, the concentration of methylmercury increases from link to link. Since methylmercury is soluble in fats, it easily transfers from water to aquatic organisms. When the smallest living organisms are captured by larger ones, which serve as their food, mercury is retained in the latter. Since mercury has an exceptionally long half-life (especially in organisms with low metabolic rates) (in humans – 70 days), the toxin is not excreted but, on the contrary, accumulates in the body. Marine mammals suffer particularly from this, as they live entirely on a fish-based diet.
However, regardless of how mercury enters the water, microorganisms methylate it, and methylmercury is always formed. This compound is fat-soluble, extremely toxic, and very persistent. In Sweden, the concentration of methylmercury in the bodies of birds, a significant portion of which feed on fish, has now approached levels at which grain-eating terrestrial birds are already dying from mercury obtained by consuming seed grain (in Sweden in the 1940s, grain was treated with methylmercury dicyanamide).
In the aquatic food chain, the concentration of methylmercury increases from link to link. Since methylmercury is soluble in fats, it easily transfers from water to aquatic organisms. When the smallest living organisms are captured by larger ones, which serve as their food, mercury is retained in the latter. Since mercury has an exceptionally long half-life (especially in organisms with low metabolic rates) (in humans – 70 days), the toxin is not excreted but, on the contrary, accumulates in the body. Marine mammals suffer particularly from this, as they live entirely on a fish-based diet.
In higher plants fine roots accumulate mercury more than coarse ones and act as a barrier.
Mercury entering from the atmosphere as vapor is adsorbed and firmly retained by higher spore plants (mosses) and conifers.
Organic mercury compounds are used in agriculture as fungicides and pesticides. For the reclamation of mercury-contaminated lands, plants that concentrate selenium are used. They facilitate the “pulling” of mercury from deeper to upper soil horizons and then convert it into poorly soluble mercury selenide, which does not participate in biochemical processes.
According to sanitary norms in Ukraine, the content of mercury compounds in soil is 2.1 mg/kg, in plants (potatoes, vegetables, grains) – 0.02–0.03 mg/kg.
The super-concentrating mercury plant is the paper birch Betula papyrifera Marsh., Betulaceae (leaves, content in ash – up to 1%).
Medicinal plants containing mercury:
• common dandelion Taraxacum officinale Webb, Asteraceae (roots);
• common oregano Origanum vulgare L., Lamiaceae (herb).
• common dandelion Taraxacum officinale Webb, Asteraceae (roots);
• common oregano Origanum vulgare L., Lamiaceae (herb).