Cadmium for the Plant: Destroyer
Cadmium (Cd)
For the plant – a Destroyer
For the plant organism cadmium is also very toxic. It is easily absorbed from the soil through the root system, as well as from the atmosphere. It is mainly localized in the roots and to a lesser extent – in the stems, petioles, and main veins of the leaves.
Toxicity of cadmium for plants is explained by its similarity in chemical properties to zinc and its replacement in many biochemical processes, which leads to disruption of the activity of enzymes involved in protein, nucleic acid, and other metabolisms, inhibition of photosynthesis, disruption of transpiration and carbon dioxide fixation, inhibition of biological reduction of NO2 to NO, alteration of cell membrane permeability (up to rupture), complications in the uptake and metabolism of zinc, copper, manganese, nickel, selenium, calcium, magnesium, and phosphorus in plants (this antagonism is utilized to suppress excessive accumulation of cadmium in plants by improving their supply of these elements).
Up to 70% of cadmium that enters the soil binds to soil chemical complexes that are available for uptake by plants. The soil microflora also participates in the processes of cadmium-organic compound formation.
Usually, the cadmium content in plants is 0.001% (on a dry matter basis). In areas with elevated cadmium levels in the soil, a 20–30-fold increase in its concentration in the above-ground parts of plants is observed compared to plants from unpolluted areas. Contaminated plants can contain up to 400 mg/kg of cadmium or more. In contrast to other mineral elements (except for zinc), cadmium can accumulate in relatively large amounts in generative organs. On average, its content in seeds of plants growing in contaminated areas increases to 4 mg/kg (in seeds from relatively “clean” habitats – 0.2 mg/kg of cadmium).
With increased cadmium levels, plants exhibit leaf chlorosis, reddish-brown color of their edges and veins, growth retardation, and root system damage. The main source of cadmium contamination in soils is the application of fertilizers, especially superphosphate, where cadmium is included as a micro-additive.
In terms of sensitivity to cadmium, plants are arranged in the following ascending sequence: tomatoes → oats → lettuce → meadow grasses → carrots → radishes → beans → peas → spinach.
Cadmium hyperaccumulator is St. John’s wort Hypericum perforatum L., Hypericaceae (herb).
Medicinal plants containing cadmium:
• Siberian phlojdicarpus Phlojodicarpus sibiricus (Steph. ex Spreng.) K.–Pol., Apiaceae (rhizome with roots);
• marsh cudweed Gnaphalium uliginosum L., Asteraceae (herb);
• creeping tribulus Tribulus terrestris L., Zygophyllaceae (herb);
• Indian thorn apple Datura inoxia Mill., Solanaceae (leaves);
• purple foxglove Digitalis purpurea L., Scrophulariaceae (leaves);
• lily of the valley Convallaria majalis L., Convallariaceae (leaves, herb, flowers).
• Siberian phlojdicarpus Phlojodicarpus sibiricus (Steph. ex Spreng.) K.–Pol., Apiaceae (rhizome with roots);
• marsh cudweed Gnaphalium uliginosum L., Asteraceae (herb);
• creeping tribulus Tribulus terrestris L., Zygophyllaceae (herb);
• Indian thorn apple Datura inoxia Mill., Solanaceae (leaves);
• purple foxglove Digitalis purpurea L., Scrophulariaceae (leaves);
• lily of the valley Convallaria majalis L., Convallariaceae (leaves, herb, flowers).
Fungi have selective ability to accumulate cadmium, and, what is particularly dangerous, – edible ones. The process of cadmium accumulation occurs in them 7 times more intensively than in higher plants and other organisms. The birch bolete and parasol mushroom are characterized by a high degree of accumulation.