mercury (Hg), lead (Pb), chromium (Cr), arsenic (As), silver (Ag), and antimony
(Sb) are toxic to plants even at low concentrations [55, 134].
Heavy metals are the main group of inorganic pollutants, which contaminate
large soil areas, and many of them cause serious risks for agricultural plants and,
respectively, human health [135]. The most important sources of heavy metals in
the environment are human activities such as mining, smelting procedures, metallurgical industry, chemical industry, traffic, intense using of pesticides and detergents, etc.
[134]. Heavy metals are non-biodegradable; therefore, they can be accumulated in the
soils [135, 136]. Plants, including legumes, are able to uptake heavy metals from soils
that commonly have a negative impact on their physiological and biochemical processes. Plant responses to heavy metal exposure are dose-dependent. For essential
metals, these responses have several dose-dependent phases – from deficiencysufficiency at low doses of the metal, tolerance at moderate doses, and to toxicity at
high doses. For nonessential metals, only tolerance and toxicity stages take place
[137]. The adverse effects of heavy metal include inhibition of seed germination and
seedling development, reduction in root and shoot biomass, mutagenic effects, accelerated senescence and death of plants, and decreased quality of flowers and crop yield
[134, 137–139]. Many of these effects are caused by the ability of heavy metals to
directly modify many proteins and DNA and to inhibit biosynthesis of chlorophylls
and proteins [140–142]. As a result, progressive chlorosis and necrosis and decreased
protein content are typical features of heavy metal toxicity in plants [143].
Heavy metals (Cd, Cr, Ni, Hg, etc.) are present in the soils as free metal ions,
soluble metal complexes (sequestered to ligands), exchangeable metal ions, organically bound metals, and precipitated or insoluble compounds such as oxides,
carbonates, and hydroxides, or they may constitute a part of the structure of silicate
materials [144]. The primary toxicity mechanisms of the metal ions may be different
due to their chemical properties, especially valence, ion radius, and capacity to form
organic complexes. Metal toxicity is also greatly influenced by the coexistence
of other metals in the soil, which could have both synergic and antagonistic effects
depending on the relative concentrations and other soil properties (i.e., presence
of nutrient elements). For example, Ca
2+ strongly inhibits the uptake of Ni
in Arabidopsis bertolonii, whereas the opposite effect was observed in Berkheya
coddii [55, 145]. Heavy metals can inactivate directly many metal-containing
proteins via substitution of the primary metal or causing protein denaturation.
In particular, chromium ions (VI) inhibit such enzymes as nitrate reductase [146,
147] and Fe
3+ reductase in plant roots [148]. In plant mitochondria, Cr
6+ can inhibit
electron transport by replacing Cu and Fe ions in prosthetic groups of many
mitochondrial redox carriers [145, 149].
4.2
Oxidative Stress as a Mechanism of Heavy Metal Toxicity
Similar to other biotic and abiotic stresses, heavy metal exposure also induces
oxidative stress development in plant cells [117, 119, 145, 147]. Depending on the
chemical properties and behavior of metals in biological systems, one of the
310
U. Y. Stambulska and M. M. Bayliak
(Sb) are toxic to plants even at low concentrations [55, 134].
Heavy metals are the main group of inorganic pollutants, which contaminate
large soil areas, and many of them cause serious risks for agricultural plants and,
respectively, human health [135]. The most important sources of heavy metals in
the environment are human activities such as mining, smelting procedures, metallurgical industry, chemical industry, traffic, intense using of pesticides and detergents, etc.
[134]. Heavy metals are non-biodegradable; therefore, they can be accumulated in the
soils [135, 136]. Plants, including legumes, are able to uptake heavy metals from soils
that commonly have a negative impact on their physiological and biochemical processes. Plant responses to heavy metal exposure are dose-dependent. For essential
metals, these responses have several dose-dependent phases – from deficiencysufficiency at low doses of the metal, tolerance at moderate doses, and to toxicity at
high doses. For nonessential metals, only tolerance and toxicity stages take place
[137]. The adverse effects of heavy metal include inhibition of seed germination and
seedling development, reduction in root and shoot biomass, mutagenic effects, accelerated senescence and death of plants, and decreased quality of flowers and crop yield
[134, 137–139]. Many of these effects are caused by the ability of heavy metals to
directly modify many proteins and DNA and to inhibit biosynthesis of chlorophylls
and proteins [140–142]. As a result, progressive chlorosis and necrosis and decreased
protein content are typical features of heavy metal toxicity in plants [143].
Heavy metals (Cd, Cr, Ni, Hg, etc.) are present in the soils as free metal ions,
soluble metal complexes (sequestered to ligands), exchangeable metal ions, organically bound metals, and precipitated or insoluble compounds such as oxides,
carbonates, and hydroxides, or they may constitute a part of the structure of silicate
materials [144]. The primary toxicity mechanisms of the metal ions may be different
due to their chemical properties, especially valence, ion radius, and capacity to form
organic complexes. Metal toxicity is also greatly influenced by the coexistence
of other metals in the soil, which could have both synergic and antagonistic effects
depending on the relative concentrations and other soil properties (i.e., presence
of nutrient elements). For example, Ca
2+ strongly inhibits the uptake of Ni
in Arabidopsis bertolonii, whereas the opposite effect was observed in Berkheya
coddii [55, 145]. Heavy metals can inactivate directly many metal-containing
proteins via substitution of the primary metal or causing protein denaturation.
In particular, chromium ions (VI) inhibit such enzymes as nitrate reductase [146,
147] and Fe
3+ reductase in plant roots [148]. In plant mitochondria, Cr
6+ can inhibit
electron transport by replacing Cu and Fe ions in prosthetic groups of many
mitochondrial redox carriers [145, 149].
4.2
Oxidative Stress as a Mechanism of Heavy Metal Toxicity
Similar to other biotic and abiotic stresses, heavy metal exposure also induces
oxidative stress development in plant cells [117, 119, 145, 147]. Depending on the
chemical properties and behavior of metals in biological systems, one of the
310
U. Y. Stambulska and M. M. Bayliak