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Negar Valizadeh✶ and Gülen Özyazıcı
Chapter 7
Impact of heavy metal on the medicinal
and aromatic plants’ biochemistry
Abstract: Medicinal plants have long been vital to human health and well-being, serv-
ing as critical resources for food, medicine, and environmental sustenance. These
plants are characterized by their active biochemical compounds, including alkaloids,
flavonoids, and essential oils, which possess therapeutic and aromatic properties.
Their importance spans traditional and modern medicine, as well as the pharmaceutical, food, and cosmetic industries. However, environmental factors, particularly abiotic stresses such as heavy metal contamination, significantly influence the production of these bioactive compounds. Heavy metals such as lead, cadmium, mercury,
and arsenic are among the most harmful environmental pollutants, originating from
industrial activities, mining, and agricultural practices. These metals accumulate in
soil and water, disrupting plant growth, metabolic functions, and secondary metabolite production. While some metals, like copper and zinc, are essential micronutrients,
their excessive concentrations, alongside toxic metals, can lead to oxidative stress,
stunted growth, and bioaccumulation in plants and the food chain, posing serious
risks to human and ecological health. Medicinal plants are particularly susceptible to
heavy metal contamination due to their bioaccumulation tendencies. Studies have revealed significant heavy metal levels in commonly used species like mint, lavender,
and rosemary, potentially jeopardizing their medicinal quality. However, this absorption ability also positions medicinal plants as key agents in phytoremediation, a biotechnological approach to mitigating environmental pollution. By tolerating and sequestering heavy metals, these plants help remediate contaminated soils while
maintaining some level of secondary metabolite production. Despite their phytoremediation potential, heavy metals can alter the composition and yield of essential oils,
impacting their medicinal and aromatic properties. These changes depend on factors
such as metal concentration, plant species, and metabolic pathways. While low metal
concentrations may stimulate essential oil production, higher levels typically inhibit
it, underscoring the complex relationship between heavy metals and plant biochemistry. To address these challenges, advanced detection technologies, improved agricultural practices, and stringent regulations are essential for minimizing heavy metal
✶
Corresponding author: Negar Valizadeh, Research Division of Natural Resources, East Azerbaijan,
Agricultural and Natural Resources Research and Education Center, Agricultural Research, Education
and Extension Organization (AREEO), Tabriz, Iran, e-mail: n.valizadeh@areeo.ac.ir,
https://orcid.org/0000-0003-3066-2534
Gülen Özyazıcı, Department of Field Crops, Faculty of Agriculture, Siirt University, Siirt, Türkiye
266 Negar Valizadeh and Gülen Özyazıcı
contamination. Furthermore, future research must focus on understanding the genetic and biochemical mechanisms underlying these effects, broadening the study
scope to include diverse medicinal plant species. Such efforts will optimize the use of
medicinal plants in polluted environments and ensure their sustainability for human
and industrial applications.
Keywords: contamination, herbal medicine, heavy metals stress, plant secondary metabolites
7.1 Introduction
Plants have long been fundamental to human survival, providing food, oxygen, and
medicinal benefits even before human civilization emerged. Medicinal plants, in particular, serve as valuable genetic resources and play a crucial role in shaping plant
ecosystems. Their ability to adapt to diverse climates makes them a highly significant
natural asset for any country. These plants are indispensable in the pharmaceutical
industry, traditional medicine, and food production, as they contain bioactive compounds with therapeutic properties that contribute to health and wellbeing. The active components in medicinal plants can exert healing effects directly or indirectly
and have been used for centuries to treat ailments and support overall health [1].
In modern times, medicinal plants continue to be utilized in both traditional and
contemporary medicine. They serve as key ingredients in pharmaceuticals, herbal extracts, and essential oil production. These plants are particularly rich in bioactive
compounds such as alkaloids, flavonoids, terpenoids, and essential oils, all of which
contribute to disease treatment and support various bodily functions [2].
Aromatic medicinal plants are especially valued for their distinctive scents and
therapeutic benefits. These plants are known as primary sources of essential oils and
perfumes due to the presence of volatile compounds in their leaves, flowers, and other
parts. Well-known aromatic plants such as lavender, mint, rosemary, and thyme are
widely used as flavoring agents and fragrances in industries such as food, cosmetics,
and personal care. Beyond their aromatic qualities, the essential oils derived from these
plants are recognized for their antibacterial and antioxidant properties, making them
vital in both cosmetic formulations and medicinal applications [3].
Plants subjected to abiotic stresses display a range of physiological, biochemical, and
molecular responses. One of their primary defense mechanisms is the biosynthesis of secondary metabolites, including essential oils. Given the widespread application of these
bioactive compounds in the pharmaceutical, cosmetic, and food industries, it is crucial to
understand how environmental factors influence their production. While the effects of
specific abiotic stresses, such as salt stress, have been extensively researched, there is limited information on how toxic pollutants affect the synthesis of active compounds in medicinal plants. Meanwhile, pollution of soil and water by toxic and heavy metals has be-
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 267
come an escalating environmental concern. These metals accumulate in the food chain
and within living organisms, making them some of the most hazardous environmental
pollutants. Their presence in the environment is rising due to industrial activities, agricultural practices, and urban expansion, potentially posing serious health risks to humans
and other organisms [4].
Heavy metals are characterized as elements with an atomic number above 20
and a density exceeding five grams per cubic centimeter. Some of these, including
copper, zinc, nickel, molybdenum, manganese, and iron, are essential micronutrients
that contribute to plant growth, redox reactions, electron transfer, and various metabolic functions. However, when present in excessive amounts, these metals can disrupt metabolic activities and inhibit plant development. In contrast, nonessential metals such as lead, cadmium, chromium, and mercury can be highly toxic to plants, even
in small concentrations [5].
While heavy metals naturally exist in soil due to the weathering of parent rocks,
human activities have significantly increased their concentration in the environment.
Industrialization, metal smelting, mining, improper waste disposal, and the extensive
use of chemical fertilizers containing heavy metals have all contributed to their accumulation in ecosystems [6].
Lead from sources such as paints, batteries, and fossil fuels; cadmium, which is a
byproduct of zinc mining and phosphate fertilizer usage; and mercury, mainly emitted by chemical industries and power plants, are frequently introduced into the environment. Furthermore, mining operations and the irrigation of crops with contaminated water intensify arsenic pollution [7]. These persistent pollutants can disperse
over vast distances through air and water currents, leading to extensive ecological
and human health risks. Consequently, heavy metal emissions resulting from human
activities present a major environmental concern.
The toxicity of heavy metals varies depending on factors such as their chemical
state, concentration, and bioavailability. Once released into soil, water, and air, these
metals can easily enter the food chain and be absorbed by living organisms. Their interactions with cellular structures can severely disrupt essential biological processes,
including enzyme activity, oxidative balance, and DNA integrity. This disruption can
result in oxidative stress, impaired growth, and in severe cases, plant death. In humans and animals, exposure to these metals can cause neurological disorders and kidney damage.
Due to their low degradability, heavy metals persist in the environment for extended periods, increasing the likelihood of long-term exposure and bioaccumulation
[8]. High concentrations of these metals in soil pose a significant threat by degrading
soil structure, reducing biological activity and fertility, decreasing agricultural productivity, lowering crop quality, and raising metal concentrations in food products.
Ultimately, this contamination can have severe implications for human health by introducing toxic elements into the food supply [9].
268 Negar Valizadeh and Gülen Özyazıcı
As a result, ongoing research aims to further explore the toxic effects of heavy
metals, improve detection techniques, and develop effective methods for remediating
contaminated environments [10]. Heavy metal pollution is a pressing environmental
issue with significant implications for medicinal plants and, consequently, human
health. Many medicinal plants have a natural capacity to absorb heavy metals from
the soil, leading to the accumulation of these elements in their tissues. This is particularly concerning in areas where soil and water contamination levels are high. Metals
such as lead, cadmium, mercury, and arsenic are among the most prevalent pollutants
that accumulate in plant structures, including roots, stems, and leaves. Research indicates that certain plants, such as mint, lavender, and rosemary, are especially prone
to absorbing these metals [11, 12].
Interestingly, this absorption ability has an advantage – medicinal plants can be
used in phytoremediation, a biotechnological strategy for reducing environmental
pollution. Certain plant species are capable of withstanding environmental stressors,
including heavy metals, making them valuable tools for mitigating contamination.
Their ability to synthesize secondary metabolites and antioxidant compounds helps
counteract the damaging effects of metal toxicity [4]. Research has shown that medicinal plants in polluted environments can absorb and store heavy metals, such as cadmium and lead, while still maintaining the production of bioactive compounds [13].
A study conducted by Luo et al. [14], titled “Heavy Metal Contaminations in Herbal
Medicines,” revealed that 30.51% of the analyzed herbal medicine samples contained
at least one heavy metal exceeding permissible concentration limits. While 70.93% of
these samples were within an acceptable risk threshold, arsenic was identified as the
most hazardous element. The research indicated that plants such as Tetradium ruti-
carpum, Plantago asiatica, and Desmodium styracifolium exhibited the highest contamination risks.
Similarly, another study by Abou-Arab et al. [15], titled “Accumulation of Heavy
Metals in Selected Medicinal Plants,” investigated heavy metal accumulation in 88
medicinal plant species. Findings from this study demonstrated that certain species
contained metal concentrations surpassing the permissible limits set by international regulatory bodies. These results emphasize the need for strict monitoring of
contamination levels in medicinal plants, as prolonged consumption could lead to
serious health hazards.
Given the essential role of medicinal plants in healthcare, industry, and medicine,
controlling heavy metal contamination is of paramount importance. Recommended
strategies to mitigate contamination include adopting improved agricultural techniques, using uncontaminated soils, and enforcing strict regulatory measures to monitor plant-based products. Additionally, advancements in rapid contamination detection technologies and the implementation of innovative phytoremediation techniques
can significantly help minimize the adverse effects of heavy metal exposure.
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 269
7.2 Heavy metals and their effects on the environment
Heavy metals are a class of elements characterized by their high density compared to
water. While some are essential in trace amounts, many become toxic at elevated concentrations, posing risks to both ecosystems and human health. Common heavy metals include lead, cadmium, mercury, arsenic, zinc, copper, nickel, chromium, iron, aluminum, beryllium, cobalt, and manganese. These metals are widely recognized as
environmental pollutants due to their persistence and potential for bioaccumulation.
Although heavy metals naturally occur in the environment, their levels have significantly risen due to human activities. Industrial processes such as mining, metal
refining, battery manufacturing, and paint production contribute heavily to their accumulation. Agricultural practices, including pesticide application, and the disposal of
municipal and industrial wastewater further exacerbate contamination. The combustion of fossil fuels also releases heavy metals into the air, contributing to widespread
pollution. In particular, gold mining operations and emissions from burning fossil
fuels are major contributors to mercury pollution, allowing it to enter and persist in
the environment [16].
Arsenic primarily contaminates the environment through groundwater pollution,
which can result from both industrial processes and natural geological activities. It is
widely regarded as a major environmental pollutant in numerous regions across the
globe [17]. Due to their distinct physical and chemical characteristics, heavy metals
have a tendency to persist in ecosystems, leading to long-term adverse effects on living organisms. These metals dissolve easily in water, allowing plants to absorb them
through their root systems, ultimately introducing them into the food chain. By interacting with plant proteins and enzymes, heavy metals can interfere with essential
metabolic and physiological functions. Furthermore, their accumulation in plant and
animal tissues can contribute to growth abnormalities, decreased agricultural yields,
and the formation of harmful biotoxins in the food supply [18].
Among heavy metals, lead is considered particularly hazardous due to its widespread use in products such as batteries, paints, and fuel additives. This metal enters
the soil and water, where it can be absorbed by plants, disrupting their physiological
processes. Lead exposure hinders plant growth, alters metabolic activities, and increases oxidative stress by affecting antioxidant enzymes [19]. A study conducted by
Usman et al. [20] examined how different lead concentrations influence the antioxidant response of Tetraena qataranse. The findings revealed that while low lead levels
stimulated plant growth, higher concentrations (100 mg/L or 1,600 mg Pb per kg of
soil) significantly inhibited root and shoot development. Additionally, lead accumulation was found to be much greater in the roots (2,784 mg/kg) than in the shoots
(1,141.6 mg/kg).
270 Negar Valizadeh and Gülen Özyazıcı
The activity of antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), and glutathione reductase (GR), showed an upward trend with increasing lead concentrations. This indicates that Tetraena qataranse possesses the ability to absorb significant amounts of
lead while mitigating its toxic effects through enhanced antioxidant defense mechanisms, thereby improving its chances of survival.
Cadmium is another highly toxic heavy metal known for its detrimental impact
on plant growth and physiological functions. It primarily enters biological systems
through soil and water contamination. This metal disrupts essential plant processes
by interfering with nutrient absorption and damaging cellular structures. One of the
most significant consequences of cadmium exposure in plants is the decline in bioactive compounds, which directly influences the medicinal value of these plants [18].
Several studies have explored the impact of cadmium on secondary metabolite
production in medicinal plants. For example, research examining Gynura procumbens
under cadmium and copper stress reported inhibited growth and modifications in
secondary metabolite synthesis, potentially affecting the plant’s medicinal properties
[21]. Similarly, a study focusing on Salvia miltiorrhiza found that higher cadmium concentrations in the soil led to increased cadmium accumulation in the plant’s roots and
leaves, which in turn influenced the biosynthesis of key active compounds [22].
Cadmium is highly mobile in soil, with its movement influenced by several factors, including pH levels, organic matter content, cation exchange capacity, and the
mineral composition of the soil. In acidic soils with low organic matter, cadmium
availability tends to increase. For example, studies on Hypericum species collected
from acidic soils in eastern Austria revealed higher cadmium concentrations compared to those in calcareous soils [23]. Additionally, research suggests that while
organic matter can retain cadmium within the soil solution, it may also facilitate
the transfer of cadmium from the plant’s roots to its aerial parts [24].
Exposure to cadmium triggers intense oxidative stress in plants by suppressing key
enzymatic activities and promoting excessive production of reactive oxygen species
(ROS). This oxidative stress can result in DNA damage, protein degradation, and lipid peroxidation in cell membranes, ultimately compromising both the quality and productivity
of medicinal plants. Studies have further demonstrated that cadmium accumulation in
the flowering parts of Matricaria chamomilla is influenced by climatic variations [25].
Certain medicinal plants possess a natural tendency to accumulate cadmium and
are classified as hyperaccumulators. Research on Hypericum perforatum has shown
considerable variation in cadmium uptake, with concentrations among 56 examined
accessions ranging from 0.04 to 7.8 ppm [26] in Figure 7.1. Figure 7.1 shows a view of
the flowering period of the Hypericum perforatum plant under field conditions.
Certain plant species, including yarrow (Achillea millefolium), German chamomile
(Matricaria chamomilla), and tobacco (Nicotiana sp.), are recognized as hyperaccumulators of cadmium [27]. Among them, the cadmium content in tobacco leaves varies
between 0.3 and 2.2 ppm, depending on geographical location [28]. Mistletoe (Viscum
Figure 7.1: Hypericum perforatum L.
Chapter 7 Impact of heavy metal on the medicinal and aromatic plants’ biochemistry 271
album) also exhibits variable cadmium accumulation, influenced by the host tree it
grows on. This plant tends to store lower cadmium levels when attached to fruit trees
like apple and hawthorn but accumulates higher amounts on hosts such as pine and
willow [29].
To minimize the detrimental effects of cadmium toxicity, proper soil management
and the selection of plant accessions with lower cadmium uptake play a crucial role.
For instance, poppy species cultivated in contaminated soils demonstrated significant
cadmium accumulation; however, when grown in uncontaminated conditions, their
cadmium levels decreased considerably [30]. Additionally, applying cadmium in a soluble form at the initial stages of an experiment leads to increased plant uptake, as the
metal has not yet had sufficient time to stabilize in the soil matrix [31].
Given the detrimental impact of cadmium on the quality of medicinal plants, it is
crucial to monitor pollution sources and adopt effective soil and plant management
strategies. Selecting suitable cultivation sites and growing heavy metal-resistant plant
species can help minimize cadmium entry into the food chain and maintain the medicinal properties of plants. Research indicates that introducing copper into soil at elevated levels (up to 300 ppm) can lead to its accumulation in the aerial parts of chamomile, reaching concentrations as high as 271 ppm, without causing visible toxicity
symptoms in the plant [32].
Moreover, utilizing composts rich in copper – containing up to 760 ppm of this
metal – has been found to have no adverse effects on the growth of medicinal species
such as peppermint (Mentha × piperita) and dill (Anethum graveolens). These plants
only absorb minimal amounts of copper (around 12 ppm) into their tissues [33]. Studies conducted on milk thistle (Silybum marianum) cultivated in highly polluted soils
demonstrated a decline in seed yield (Figure 7.2). Despite the significant presence of
heavy metals, essential compounds such as fats and silymarin remained unaffected,
ensuring that the final product was free from contamination [34].
Figure 7.2: Silybum marianum seeds.
272 Negar Valizadeh and Gülen Özyazıcı
Likewise, clary sage (Salvia sclarea) cultivated in soils with elevated levels of cadmium, lead, and zinc did not exhibit any trace of contamination in its extracted essential oil [35]. Similarly, essential oils obtained from different peppermint species (Men-
tha × piperita and Mentha arvensis) grown in highly polluted environments were also
devoid of heavy metal contamination. This trait suggests their potential use in the
gradual rehabilitation of contaminated soils [36].
Mercury, a highly toxic heavy metal, primarily enters ecosystems through industrial emissions and pollution from fossil fuel combustion. It readily dissolves in water,
making it easily absorbable by plants and subsequently entering the food chain. Mercury exposure can disrupt enzymatic functions, impair photosynthetic efficiency, and
ultimately hinder plant growth. Additionally, its accumulation in plant tissues poses
significant risks to both human and animal health upon consumption [37].
Mercury, as a heavy and toxic metal, plays a crucial role in environmental pollution. Research indicates that plant absorption of mercury is minimal, leading to
generally negligible concentrations in medicinal plants. For instance, an analysis of
wild chamomile flowers collected across Slovakia between 1995 and 2003 found an
average mercury concentration of 0.04 ppm [38]. Likewise, plant samples from Vac-
cinium species, birch, and willow, gathered in Finland and northeastern Russia, contained mercury levels below 0.04 ppm [39]. A comprehensive study by Gasser et al.
[29] in Germany also reported that mercury concentrations in nearly 120 medicinal
herb samples remained under 0.1 mg/kg.
Mercury accumulation is more prevalent in algae. For example, research along
Portugal’s northern coast revealed that Fucus species exhibited higher mercury levels
than the surrounding sediments [40].
Arsenic, another hazardous heavy metal, is commonly found in groundwater and
surface water. It is easily absorbed by plants, primarily accumulating in their leaves