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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5217_Библиотеки_им_академика_М_И_Перельмана.pdf
Figure 7.3: Leaves of Mentha piperita and Urtica dioica.
Figure 7.4: Leaves and flowers of Mentha spicata.
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 273
and roots, where it can degrade plant quality and overall performance. Arsenic exposure can trigger oxidative stress and cause severe cellular damage, ultimately leading
to stunted growth and lower yields [41]. Studies from Poland indicate that arsenic levels in Mentha × piperita range between 0.05 and 0.13 mg/kg, while in Urtica dioica, concentrations vary from 0.09 to 0.24 mg/kg (Figure 7.3) [42].
In Austria, a study analyzing plant samples grown in chernozem soil – a fertile,
humus-rich black soil – found that arsenic concentrations in the leaves and branches
ranged from 1.2 to 2.0 mg/kg, while the soil itself contained 18 mg/kg of arsenic [43].
Similarly, for Mentha spicata (spearmint), the reported average arsenic content is approximately 0.2 mg/kg (Figure 7.4) [44].
Furthermore, analyses of fennel seed samples from India have detected arsenic concentrations ranging from 0.51 to 0.59 mg/kg [45]. Research conducted in Bulgaria has
revealed that commonly consumed tea bags contain arsenic levels between 0.02 and
0.25 mg/kg [46]. Additionally, significant arsenic accumulation has been reported in
macroalgae and sea moss, with concentrations varying from 20 to 100 mg/kg [47].
These results underscore the need for stringent monitoring of arsenic levels in food
and medicinal products, particularly in edible and therapeutic plants.
274 Negar Valizadeh and Gülen Özyazıcı
Although nickel is an essential micronutrient for certain plants, excessive amounts
can be harmful. At high concentrations, it interferes with protein and enzyme structures, disrupting vital metabolic activities such as respiration and photosynthesis. Moreover, nickel exposure can induce genetic and structural alterations in plants, potentially
leading to long-term damage. Such effects are particularly concerning for medicinal
and aromatic plants, as their bioactive compound composition must remain stable and
of high quality [18]. Consequently, identifying sources of contamination and mitigating
heavy metal levels in the environment is crucial.
7.3 Mechanisms of heavy metal uptake
and accumulation in plants
The soil solution, which serves as the medium for plant mineral absorption, consists
of a complex blend of elements. Consequently, interactions between these elements
frequently occur in plants. Due to the chemical resemblance between cadmium and
zinc, their interactions have been widely observed in various plant species. Research
on plants cultivated in different soil types has shown that higher zinc levels in flax
seeds (Linum usitatissimum) lead to a significant decrease in cadmium accumulation
[48]. A pot experiment conducted in North Dakota, USA, further confirmed that soil
enrichment with zinc can lower cadmium content in flax seeds. However, when cadmium and zinc were applied together, cadmium concentration in the seeds increased
compared to when cadmium was added alone [49]. The application of monoammonium phosphate in flax seeds has been found to elevate cadmium concentrations
while decreasing zinc levels [48]. Chizzola and Mitteregger [50] explored cadmiumzinc interactions in chamomile plants, revealing that soil enrichment with zinc significantly lowered cadmium accumulation in the aerial parts. However, this reduction
was insufficient for plants in contaminated areas to match the cadmium levels of
those in uncontaminated regions. In the case of Achillea millefolium (yarrow), the concurrent addition of cadmium and zinc did not significantly influence cadmium accumulation [51].
Cadmium-manganese interactions have also been documented in lettuce, where
an increase in cadmium concentration within the nutrient solution led to greater
manganese absorption and translocation to the aerial parts [52]. Similarly, nutrient
solution studies indicated that introducing a small amount of cadmium enhanced
iron uptake in sorghum [24]. In Picea abies (Norway spruce), elevated calcium levels
reduced cadmium and zinc accumulation, whereas the presence of cadmium or copper hindered calcium uptake [53]. Additionally, interactions between phosphorus and
zinc at both the plant and soil levels have been reported [54].
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 275
7.4 Processes of heavy metal uptake by roots
Roots serve as the primary interface between plants and soil, playing a crucial role in
the absorption of heavy metals. This process occurs through two main mechanisms:
active and passive uptake. Active uptake involves the transport of heavy metal ions
into root cells via specialized membrane proteins, such as H
energy-driven mechanism enables plants to absorb heavy metals efficiently from the
soil. In contrast, passive uptake depends on diffusion or nonspecific ion channels, allowing heavy metals to enter the plant without energy expenditure [55].
Several factors influence the efficiency of heavy metal absorption by roots. Soil
pH is a major determinant, as it affects the availability of heavy metals for uptake. In
acidic conditions, heavy metals are more likely to exist in their ionic forms, making
them more accessible to plants. Additionally, soil characteristics such as organic matter content and cation exchange capacity also play a role in regulating heavy metal
absorption [7].
Dinu et al. [12] examined how peppermint (Mentha piperita) responds to soil contamination by heavy metals such as cadmium, lead, nickel, and arsenic. The study assessed the movement of these metals from the soil into different plant parts, including
roots, stems, and leaves, and compared the findings with a control group grown in
uncontaminated soil. Peppermint seedlings were exposed for 3 months to two metal
mixtures with similar concentrations. The first mixture contained arsenic and cadmium, while the second included arsenic, cadmium, nickel, and lead. The metal concentrations were 23.7 mg/kg for arsenic, 5 mg/kg for cadmium, 136 mg/kg for nickel,
and 95 mg/kg for lead.
The results showed that cadmium, nickel, and lead accumulated in different plant
tissues, with arsenic remaining undetectable. The accumulation pattern followed the
order: roots > stems > leaves. During the first month, no significant differences in
growth, development, or chlorophyll content were observed when compared to the
control plants. However, after 3 months, signs of phytotoxicity began to appear. Analysis of metal translocation and transfer factors yielded values below 1, indicating that
M. piperita primarily retained metals within its root system. These findings suggest
that peppermint can effectively stabilize heavy metals in its roots during short-term
exposure, demonstrating its tolerance to contaminated conditions when grown in nutrient-rich garden soil. This research highlights peppermint’s potential as a metalresistant plant, particularly for applications where root-based metal stabilization is
needed.
+
ions and H+-ATPase. This
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7.5 Transport and accumulation in various plant tissues
Once absorbed by the roots, heavy metals are transported to different plant tissues
via the vascular system. In flowering plants, this movement primarily occurs through
the xylem, which carries metals from roots to stems and leaves, and the phloem,
which facilitates transport between stems and leaves. During this process, heavy metals accumulate in various plant parts. While roots, stems, and leaves serve as the primary sites of accumulation, their specific distribution varies based on plant species,
metal type, and environmental conditions. For instance, cadmium predominantly accumulates in roots, whereas lead and zinc are more commonly found in stems and
leaves [6].
A study conducted in 2021, titled Hysteresis of Heavy Metal Uptake in Dandelions,
examined how heavy metals are absorbed and distributed within dandelion plants.
The results indicated that these metals could accumulate in different plant tissues, potentially affecting the plant’s medicinal properties [56].
7.6 The effect of soil type and environmental
conditions on heavy metal uptake
Soil composition and environmental conditions are key factors influencing the uptake
and accumulation of heavy metals in plants. Various physical and chemical characteristics of the soil, such as pH, moisture content, organic matter, mobile cation concentrations, and microbial activity, significantly affect the extent to which plants absorb
these metals. For example, in acidic soils, heavy metal ions are more readily available,
facilitating their uptake by plants. Conversely, in alkaline soils, heavy metals tend to
form mineral complexes or insoluble precipitates, reducing their bioavailability.
In addition to soil properties, environmental factors like temperature, light, and
moisture play a crucial role in heavy metal absorption and transport within plants.
Higher temperatures can stimulate enzymatic activity and metabolic processes,
thereby enhancing metal uptake. Similarly, moisture levels influence metal bioavailability, with heavy metals being more accessible to plants in wetter soil conditions
[13, 57].
A study investigated the effects of different exposure durations to thiram on the
uptake of manganese, cobalt, nickel, copper, zinc, cadmium, and lead in basil (Oci-
mum basilicum L.). In addition, the research assessed various physiological parameters of the plants. The experiment was conducted using two common agricultural soil
types found in rural areas of Poland. The methodology included soil analysis to determine bioavailable and total metal forms, measurements of chlorophyll content, and
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 277
evaluations of gas exchange in plants. Metal concentrations were quantified using
atomic absorption spectroscopy.
Analysis of variance indicated that thiram treatment significantly affected metal
transfer from the soil to basil plants, influencing metal concentrations in both the
roots and aerial parts. These effects were most pronounced on the 14th day after fungicide application. Thiram altered metal uptake and distribution within basil by modifying the microbial composition of the rhizosphere. The impact was more noticeable
in plants grown in mineral soils than in organic soils, which have a higher buffering
capacity [58].
7.7 The effects of heavy metals on physiology,
metabolism, and secondary metabolites
in medicinal and aromatic plants
Heavy metals have a significant impact on various plant physiological functions, including growth, photosynthesis, respiration, and metabolic activities. One of the primary effects is their disruption of the photosynthetic process. Metals such as cadmium, lead, and zinc can inhibit key photosynthetic enzymes, including RuBisCO
(ribulose-1,5-bisphosphate carboxylase/oxygenase), and interfere with chlorophyll
production. This disruption reduces photosynthetic efficiency, limiting the energy
available for plant growth [59].
Additionally, heavy metals influence plant respiration by altering mitochondrial
structure and function, thereby reducing energy efficiency during the respiratory process [55]. These changes lead to decreased ATP production and an accumulation of
free radicals, which can damage cellular membranes, proteins, and DNA. Heavy metal
exposure also negatively affects plant growth, particularly in the early stages of development, leading to lower germination rates, reduced root and shoot elongation, and
decreased dry weight [60, 61].
The adverse effects of heavy metals are particularly concerning for medicinal
and aromatic plants, as their optimal growth is essential for synthesizing bioactive
compounds used in pharmaceutical and aromatic industries. These plants are known
for producing secondary metabolites with therapeutic and aromatic properties.
Heavy metal exposure can interfere with the production of these metabolites by affecting enzyme activity and disrupting metabolic pathways. Key affected compounds
include alkaloids (e.g., morphine, atropine, and papaverine), flavonoids (e.g., quercetin, apigenin, and luteolin), terpenoids (e.g., menthol, thymol, and linalool), and anthocyanins (Figures 7.5–7.7).
For instance, in aromatic plants such as mint and lemon, heavy metals can impair
the biosynthesis of terpenoid compounds, which contribute to their characteristic aro-
Figure 7.5: Alkaloids’ chemical structures.
Figure 7.6: Flavonoids’ chemical structures.
Figure 7.7: Terpenoids’ chemical structures.
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mas. Metals like cadmium and lead inhibit the activity of enzymes involved in terpenoid production, leading to a reduction in these vital compounds [62, 63].
Heavy metals can significantly influence the biochemical composition of medicinal
and aromatic plants, leading to notable reductions in anthocyanins, flavonoids, and
terpenoids. Anthocyanins and flavonoids, which belong to the phenolic compound
group, play a crucial role in protecting plants from environmental stress while also
providing antioxidant benefits to counteract free radical damage. Research suggests
that heavy metal exposure leads to decreased anthocyanin and flavonoid concentrations in plants such as thyme, basil, and sage. This reduction is likely attributed to disruptions in their biosynthetic pathways, triggered by oxidative stress resulting from
heavy metal accumulation [64].
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 279
Similarly, terpenoid production in aromatic plants, including basil, mint, and pennyroyal, can be significantly impaired by heavy metals. These compounds are essential not only for the characteristic scent of these plants but also for their medicinal
properties, such as anti-inflammatory and anticancer activities. A decline in terpenoid
biosynthesis negatively affects both the therapeutic efficacy and aromatic qualities of
these plants, diminishing their overall value in medicinal and aromatic applications [65].
Antioxidant activity is a fundamental medicinal property of plants, playing a vital
role in shielding them from damage caused by free radicals. When exposed to heavy
metal contamination, plants experience elevated levels of free radicals, placing considerable stress on their antioxidant defense systems. While some plants may temporarily enhance their antioxidant responses to counteract the damage, excessive heavy
metal accumulation can overwhelm these systems, ultimately reducing their effectiveness and making plants more vulnerable to oxidative stress [55].
In addition to affecting antioxidant properties, heavy metals also influence the
antibacterial activity of medicinal and aromatic plants. These plants naturally produce antibacterial compounds that help them resist harmful bacteria and microorganisms. However, exposure to heavy metals can alter the composition of these bioactive
substances, leading to a decline in antibacterial potency and a subsequent reduction
in the plants’ therapeutic value. Studies have demonstrated that heavy metals such as
cadmium and lead can impair the antibacterial properties of plants, thereby diminishing their medicinal efficacy [7].
Furthermore, bioactive compounds – including alkaloids, glycosides, and essential oils – are critical components of medicinal and aromatic plants, contributing to
their therapeutic benefits. These compounds not only offer health-promoting effects
but also define the unique characteristics of each plant. Heavy metal contamination
disrupts the biosynthetic pathways responsible for producing these bioactive substances. For instance, alkaloids present in plants like Datura and Peganum harmala decrease under heavy metal stress due to the inhibited activity of key enzymes, such as
alkaloid synthase, which are essential for their synthesis [8].
Likewise, glycosides, which are found in plants such as ginseng and thyme, are
adversely affected by heavy metal stress. Alterations in the plant’s internal environment due to metal exposure result in decreased glycoside production [7]. Essential
oils, primarily composed of terpenoid compounds, are particularly vulnerable to
heavy metal contamination. Plants like mint and pennyroyal experience a significant
reduction in essential oil synthesis when exposed to metals such as cadmium and
lead. This decline negatively affects both the aromatic properties and medicinal efficacy of these plants [65].
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7.8 Plant defense mechanisms against heavy metals
Plants utilize various defense strategies to withstand stress caused by heavy metal exposure. These adaptive mechanisms involve alterations in their antioxidant systems
and the production of signaling molecules to mitigate heavy metal toxicity. The plant’s
response to heavy metals is highly complex and depends on factors such as contamination levels and the specific type of metal involved. One of the most notable responses to heavy metal stress is the increased activity of antioxidant defense systems.
Heavy metal exposure leads to the generation of free radicals within plant tissues,
which can harm cellular membranes, proteins, and DNA. To counteract this damage,
plants enhance their antioxidant defenses. Key antioxidant enzymes, including SOD,
CAT, and peroxidase (POD), play a crucial role in neutralizing free radicals and minimizing oxidative harm. Research indicates that plants such as wheat and maize exhibit a significant rise in antioxidant enzyme activity, particularly SOD and CAT,
when subjected to heavy metal stress as a means of reducing oxidative damage
[66, 67].
Under extreme heavy metal contamination, the efficiency of antioxidant defense
systems may decline significantly, compromising the plant’s ability to neutralize free
radicals and mitigate oxidative stress. In such conditions, cellular membranes, proteins, and DNA become increasingly vulnerable to damage, leading to the breakdown
of cellular structures and disruption of essential physiological processes. The weakened defense mechanisms can manifest as cellular stress symptoms, inhibited growth,
decreased synthesis of vital biochemical compounds, and, in severe cases, cell death.
Beyond enzymatic antioxidants, nonenzymatic compounds also play a vital role
in plant defense. These include flavonoids, phenols, and vitamin C, which help counteract heavy metal-induced oxidative stress through their antioxidant properties. Research suggests that exposure to heavy metals such as cadmium and lead can trigger
an increase in the levels of these antioxidant compounds in certain plants, including
beans and sainfoin, as part of their adaptive response to stress [67].
Plants synthesize specific signaling compounds to adapt to heavy metal stress.
These molecules function as internal messengers, enabling plants to regulate their responses to adverse conditions. Among these compounds are plant hormones such as
auxins, cytokinins, salicylic acid (SA), and ethylene. SA is a key signaling molecule involved in plant responses to environmental stress, including heavy metal exposure.
This hormone plays a vital role in strengthening plant defense mechanisms against
metal toxicity. Research indicates that elevated levels of SA can enhance the activity
of antioxidant enzymes, thereby alleviating the harmful effects of heavy metals on
plant health [68, 69]. Ethylene is another essential hormone produced in response to
heavy metal stress. It contributes to stress adaptation by regulating critical physiological processes, such as root development and tissue growth, which help plants better
withstand unfavorable conditions [70].
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 281
7.9 Molecular and genetic responses to heavy metal contamination
Plants adapt to heavy metal stress through various molecular and genetic modifications. These adaptations involve changes in gene expression, activation of genetic resistance pathways, and, in some cases, genetic engineering techniques aimed at enhancing
tolerance. Heavy metals such as cadmium, lead, zinc, and copper influence multiple signaling pathways, leading to alterations in the expression of specific genes that regulate
plant defense responses.
One of the primary responses to heavy metal exposure is the increased expression of genes involved in the production of antioxidants and protective enzymes. For
example, exposure to heavy metals can stimulate the upregulation of genes responsible for synthesizing SOD, CAT, and POD. These enzymes are critical in neutralizing
free radicals, reducing oxidative stress, and safeguarding plant cells from heavy
metal toxicity [71, 72].
Additionally, heavy metals can activate genes associated with hormonal signaling
pathways, including those governing ethylene, SA, and auxins. These hormones play an
essential role in regulating plant defense responses to heavy metal stress [68]. To survive in polluted environments, plants employ complex genetic mechanisms that regulate the uptake, transport, storage, and detoxification of heavy metals within their cells
and tissues.
A crucial component of the plant defense system against heavy metal stress is the
role of transporter proteins, which regulate the movement of heavy metals within the
plant. These proteins help transfer metals from the roots to other tissues or confine
them within specific compartments for safe storage. For instance, metallothionein
(MT) proteins and heavy metal ATPases (HMA) play a significant role in absorbing
heavy metals and isolating them within designated plant structures.
Additionally, plants can minimize heavy metal uptake by stabilizing these contaminants in the soil. This process involves the secretion of chemical compounds,
such as organic acids and amino acids, which bind with heavy metals to reduce their
mobility. One such mechanism, known as phytostabilization, allows plants to release
these substances into the surrounding soil, thereby preventing further absorption of
heavy metals [73].
Another way plants resist heavy metal stress is by making structural and functional adjustments to their cell membranes. Some plants reinforce their membranes
to block heavy metal entry, while others modify their structure to reduce metal absorption. Given the significant role of medicinal and aromatic plants, genetic engineering has emerged as a promising strategy to enhance their ability to tolerate heavy
metals. This approach involves introducing genetic modifications that strengthen
plant defense mechanisms and improve their capacity for metal uptake and detoxification.
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One genetic engineering technique involves incorporating genes that encode MT
proteins and heavy metal transporters into medicinal plants. These genes help plants
effectively sequester heavy metals in specific tissues or expel them altogether. For example, transferring HMA or MT genes into medicinal plants has been found to enhance
their resistance to metals such as cadmium and lead. Additionally, genetic modifications
can target hormonal signaling pathways to improve stress resilience. Enhancing the activity of SA and ethylene signaling pathways, for instance, enables plants to better manage the adverse effects of heavy metals. Research has shown that increasing the production of these hormones through genetic engineering significantly improves the tolerance
of medicinal and aromatic plants to heavy metal contamination.
Furthermore, cutting-edge technologies like CRISPR/Cas9 offer precise gene-editing
tools to enhance plant resistance. This method allows for the targeted modification of
genes responsible for heavy metal detoxification, thereby strengthening the plant’s ability to tolerate and manage these contaminants more effectively [74].
7.10 Management and control of heavy metal
contamination in medicinal and aromatic
plants
To mitigate the effects of heavy metals on plants, various agricultural and management strategies can be employed. One crucial approach is reducing soil contamination
by addressing pollution sources. Effective management of industrial and agricultural
waste plays a vital role in preventing heavy metals from entering the soil, thereby
minimizing long-term environmental damage.
Additionally, techniques such as cover cropping and crop rotation help limit
heavy metal exposure. Cover crops assist in immobilizing heavy metals within the
soil, reducing their uptake by medicinal and aromatic plants. Likewise, crop rotation
and diversifying plant cultivation in a given area can prevent excessive accumulation
of heavy metals in the soil. For instance, planting crops that absorb and retain heavy
metals in their root systems can act as a protective barrier, shielding medicinal plants
from contamination [75, 76]. Effective soil and water remediation techniques play a
vital role in managing heavy metal contamination in affected regions. One commonly
applied approach is phytoremediation, which utilizes plants to cleanse environments
polluted with heavy metals. This method encompasses various processes, including
phytostabilization, hyperaccumulation, and phytodetoxification. Among these, phytostabilization is particularly significant, as it involves the use of plants to restrict heavy
metals within the soil, thereby preventing their dispersion and minimizing environmental hazards (Figure 7.8).