Heavy metals are a group of elements distinguished by their additional power, high atomic mass, large number of atomic elements, or toxic properties. Density criteria range from above 3.5 g/cm3 to above 7 g/cm3, with an atomic weight above 20. They are a natural component of the earth’s crust, but because of the activities of industry, agriculture, urbanisation and mining, their concentration increases. This excessive concentration poses a threat to the biosphere.
The content of heavy metals in the soil is related to their natural sources, such as the parent rock, precipitation of atmospheric dust and rain, and decomposed biological material, as well as to anthropogenic sources, such as emissions of metallic dust from non-ferrous metal smelters, energy and communication dust. (Kabata-Pendias, 2015), (Gupta, 2019)
The uptake of heavy metals from the soil by plants is influenced by many factors that overlap and differentiate the content of individual metals in different plant species.
The most important factors are the content of metals in the soil and the amount of their active forms, interactions between elements, soil condition, its mechanical and chemical composition, differences in uptake at the level of species and respective individuals. (Gupta, 2019)
The uptake of heavy metals by plants is most often correlated with the content of their active forms in the soil. The amount of metals in the soil solution is usually only 0.1% of their total content in the soil.
The type of speciation influences the amount and rate of heavy metal uptake by plants.
The main speciation of heavy metals in the soil solution is single ions, complexes with inorganic ligands, and complexes with soluble organic substances. (Qin, 2021)
The uptake of metals by a plant can be reduced or increased by some factors. Metals can interact synergistically with each other, increasing the absorption effect, or antagonistically – reducing it.
There is a clear antagonism between iron and other metals and between copper and many metals. The high amount of copper causes the plant’s iron content to act, possibly precipitating it as ferric phosphate in the roots. This affects the iron content in the aerial parts, mainly in the chloroplasts. The favourable copper to iron ratio varies depending on the species. Excess copper causes manganese deficiency.
There is a strong antagonism between copper and zinc because both elements can occupy the same positions on organic carriers and undergo mutual substitution.
One of the most important and recognised as one of the first antagonisms, is the one between phosphorus-zinc , which is the cause of many physiological diseases of plants. (Kabata-Pendias, 2015)
Soil factors may influence the availability of metals in the soil and their availability to plants, increasing the solubility of mineral and organic compounds.
The percentage of carbon effluent determines the amount of matter supplied to the soil, which is stored in humus. The material fractions bind heavy metals, reducing their availability to plants. The durability of these bonds increases with the degree of decomposition of the obtained solution and with the increase in pH.
Organic compounds with the ability to complex heavy metals include humic acids, and fulvic acids, as well as a group of low-molecular compounds, highly soluble in water. The following functional groups of organic compounds directly participate in metal complexation: carboxyl (-COOH), hydroxyl (-OH), carbonyl (=CO), amine (-NH2), sulfhydryl (-SH) and methoxy (-OCH3). If the metal is bonded by two or more functional groups, complexes with very high durability – chelates – are formed. At lower metal concentrations and at the same time, a large amount of organic ligands in the soil solution, compounds with high solubility are formed, which are characterised by a high anion to metal molar ratio. However, when the proportion is less than 1:1, insoluble complex compounds are formed.
The decrease in the solubility of metals is due to their non-specific absorption. Such absorption is understood as the typical exchange sorption of cations on permanent negative charges, located on the surface of clay minerals, resulting from the isomorphic substitution of higher valence cations with lower valence cations. The bond strength depends on the valence and diameter of the ion and is determined by the direct contact of the ion with the functional group or groups of the adsorbent, which results in forming the covalent or coordination bonds.
Another guardian that influences the behaviour of metals in the soil is the percentage of floatable parts.
Most metals are correlated with the content of floatable parts. These include Mn, Cu, Ni, and Fe. However, in the case of Pb and Cd, there is often no correlation with the floatable parts, but a significant correlation with the organic carbon content.
The important factor which influences heavy metal absorption is the pH of soil.
It has been shown that the content of heavy metals in meadow plants depends more on the soil pH than on the amount of these metals in the soil. The influence of organic compounds occurring in the soil and its pH on the stability of compounds containing Pb, Ni and Cd was demonstrated.
Plant reactions to too high metal concentrations vary; it may be the sum of the effects of individual metals or the resulting antagonistic or synergistic interactions. The ability to absorb and tolerate excess chemical elements is related to the nature of the metabolism or the adaptation of the species to existing ecological conditions. The influence of organic compounds occurring in the soil and its pH on the stability of compounds containing Pb, Ni and Cd was demonstrated.
The strategy of resistance to toxic metals involves avoiding the harmful factor and inactivating them, for example in the cell wall or inside the protoplasm. (Zi Y., 2022)
Avoiding or reducing metal absorption can be achieved through rhizosphere interaction (a pH change, root secretions), and biochemical and physical changes inside a cell wall and within a cell membrane.
Rhizosphere characteristics themselves can modify metal absorption.
Metabolic root secretions can change the pH in the rhizosphere. Root mucus can protect the root from the entry of toxic ions.
The reaction of plants to metals usually depends on their individual and species sensitivity, exposure time, metal concentration and the form in which the metals occur. At the organismal level, heavy metals inhibit or hinder plant germination, inhibit root elongation, reduce its thickness, lead to thickening and greater compactness of the root system, weaken or completely stop the development of root hairs and lead to such morphological changes as thickening of the apical part of the root and its dieback.
At the biochemical level, the concentration of heavy metals in plants causes enzymatic modification of the root surface, binding to the root cell wall, accumulation and detoxification in the form of complexes with organic acids. (Zi Y., 2022)
Due to the uptake of heavy metals, plants can be divided into:
– plants absorbing relatively large amounts of heavy metals in aboveground parts, regardless of their amount in the ground, (Rascio N., 2011)
– plants absorbing heavy metals and transport them to the above-ground parts, in proportion to their amount in the ground.
– plants absorbing heavy metals at different levels but accumulate their relatively low amount, characterised by a well-developed mechanism of avoiding and excluding heavy metals.
REFERENCES
Gupta, N. Y. K. K. V. K. K. S. C. R. P. a. K. A., 2019. Trace elements in soil-vegetables interface: Translocation, bioaccumulation, toxicityand ameliorat ion – a review. Sci. Total Environ. 651, p. 2927–2942.
Hu, B., 2020. Modellingbioaccumulation of heavy metals in soil-crop ecosystems and identifying itscontrolling factors using machine learning.. Environ. pollut., p. 262.
Kabata-Pendias, A. a. S. B., 2015. Trace elements in abiotic and biotic.. s.l.:CRC press.
Qin, G. N. Z. Y. J. L. Z. M. J. a. X. P., 2021. Soil heavy metal pollutionand food safety in China: Effects, sources and removing technology.. Chemosphere.
Rascio N., N.-I. F., 2011. Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting?. Plant Science, pp. 169-181.
Zi Y., F. Y. J.-L. L. H.-T. W. H. Y. Y. S. J. L. Y.-F. Z. Y.-R. L. K.-M. C., 2022. Heavy metal transporters: Functional mechanisms, regulation, and application in phytoremediation. Science of The Total Environment, 809.