Symptoms of Heavy Metal Toxicity on Cereal Plants

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This entry is adapted from the peer-reviewed paper 10.3390/agriculture13101983

The contamination of soils by heavy metals poses a substantial environmental quandary with far-reaching repercussions for the growth and development of cereal crops. These crops are indispensable for sustainable food systems as they absorb water and nutrients from the soil, potentially uptaking these toxic metals in the process. This phenomenon, known as bioaccumulation, can lead to elevated levels of heavy metals in edible plant parts, such as grains, thereby endangering consumers when these tainted crops are consumed.

cereal crops metal toxicity metal uptake metal tolerance mycorrhizal symbiosis soil

1. Introduction

The manifestations of heavy metal toxicity on cereals are multifaceted, ranging from visible symptoms, such as chlorosis and stunted growth, to intricate physiological changes at the cellular level ^[1]^[2]. These manifestations are influenced by various factors, including the specific heavy metal present, its concentration in the soil, the duration of exposure, and the inherent tolerance and susceptibility of cereal crop varieties. Some of these symptoms have been discussed in previous sections concerning the interaction between heavy metals and cereals, as well as the mechanisms behind heavy metal-induced growth impairment in plants. Researchers will now elaborate on these symptoms in this section.

2. Chlorosis

One of the prominent effects of heavy metal toxicity on cereals is chlorosis, a condition marked by the yellowing of leaves due to the disruption of chlorophyll synthesis. Chlorophyll, the green pigment located in chloroplasts, plays a decisive role in photosynthesis, allowing plants to capture light energy and convert it into chemical energy. ^[3]^[4]. As mentioned above, heavy metals like cadmium and lead can interfere with chlorophyll production, leading to reduced photosynthetic capacity and a characteristic yellowing of leaves. Cadmium can disrupt the biosynthesis of chlorophyll in several ways. It interferes with the activity of enzymes involved in chlorophyll synthesis, impairing the conversion of precursor molecules into functional chlorophyll molecules. As a result, chloroplasts within affected leaf cells become deficient in chlorophyll, leading to a noticeable loss of green colour ^[5]^[6]. Similarly, lead, another significant heavy metal pollutant, can inhibit various enzymes and metabolic pathways involved in chlorophyll biosynthesis. By disrupting the delicate balance of chlorophyll synthesis, lead-induced chlorosis manifests as a progressive yellowing of leaves in cereal crops. Some heavy metals can directly inhibit enzymes involved in chlorophyll biosynthesis. For example, cadmium can inhibit enzymes like δ-aminolevulinic acid dehydratase (ALAD) and protoporphyrinogen oxidase (PPO), both of which are essential for chlorophyll production. Inhibition of these enzymes disrupts the chlorophyll synthesis pathway, leading to chlorosis ^[7].

Chlorosis typically begins in the older leaves of cereal plants, as heavy metals are translocated from the soil to the roots and then upward to the leaves. As the heavy metal concentration in the leaves increases, the chlorophyll content decreases, leading to a gradual yellowing of the leaf tissue. In severe cases of heavy metal toxicity, chlorosis may progress to necrosis, where the yellowing leaves eventually wither and die. The consequences of chlorosis in cereals are significant ^[8]. Reduced chlorophyll content translates to reduced photosynthetic capacity, resulting in decreased carbohydrate production. As photosynthesis is the primary process through which plants synthesize their food, compromised photosynthetic efficiency can lead to stunted growth and reduced biomass accumulation in cereal crops. Moreover, chlorosis negatively impacts grain filling and yield in cereals. During the grain-filling stage, the plant translocates carbohydrates from leaves to developing grains. Heavy metal-induced chlorosis diminishes the availability of carbohydrates for grain filling, leading to smaller and poorly filled grains, ultimately reducing the overall yield of cereal crops ^[9]^[10]. Chlorotic plants are generally weakened and more susceptible to environmental stressors, diseases, and pests. Their overall health and vigor decline, and they may show signs of reduced growth.

Chlorosis is a noticeable manifestation of heavy metal toxicity in cereals, signifying disruptions in chlorophyll synthesis and photosynthetic capacity. The severity of chlorosis in cereals depends on several factors, including the type of heavy metal, its concentration in the soil, the duration of exposure, and the inherent tolerance of the cereal variety ^[10]^[11].

3. Stunted Growth

One of the most noticeable and common manifestations of heavy metal toxicity in cereals is stunted growth. Stunted growth refers to the inhibited development of cereal plants, where they fail to achieve their expected size and exhibit a reduced number of leaves and smaller stems ^[12]^[13].

Heavy metal toxicity interferes with critical physiological processes, disrupting plant growth and development at various stages of the plant’s life cycle. The presence of heavy metals in the soil can directly affect root growth and function. As cereal plants take up water and essential nutrients through their root systems, heavy metals can enter the roots and cause damage. For example, cadmium exposure can lead to reduced root elongation and impaired root branching, limiting the plant’s ability to explore the soil for water and nutrients. This restricted root development hinders the plant’s capacity to take up essential elements and sustain normal growth. Arsenic toxicity leads to reduced plant height and tillering in cereals. It disrupts photosynthesis and interferes with nutrient uptake, particularly for phosphorus, resulting in stunted growth. Lead exposure can result in stunted growth by interfering with cell division and elongation in roots and shoots. It can also disrupt water and nutrient uptake processes, further exacerbating growth problems. ^[14]. Also, heavy metals can interfere with cell division and elongation in the shoot system of cereal plants. As cells in growing tissues are particularly sensitive to heavy metal stress, the elongation of stems and leaves can be hindered, leading to smaller plant size. The disrupted cell division can also result in a reduced number of leaves, further limiting the photosynthetic capacity of the plant. Furthermore, heavy metals can inhibit various enzymatic activities involved in critical metabolic processes ^[15]^[16]. For instance, lead exposure has been shown to disrupt the activity of enzymes essential for the synthesis of plant growth hormones, such as auxins and gibberellins. These hormones play key roles in regulating plant growth and development. The inhibition of growth hormones can disrupt cell elongation and division, contributing to stunted growth in cereals ^[17]^[18].

Stunted growth in cereals due to heavy metal toxicity has significant consequences for crop productivity. Smaller plants with reduced biomass have limited photosynthetic capacity, leading to reduced carbohydrate production. As a result, cereal crops exposed to heavy metal-contaminated soils may produce smaller and less-filled grains, ultimately leading to decreased yield ^[3]^[19]. Additionally, stunted growth can impact the overall health and resilience of cereal crops. Smaller plants may be more susceptible to environmental stressors, pests, and diseases, making them less resilient to adverse conditions.

4. Reduced Root Growth

Roots are a critical component of plant architecture and function, serving as the primary organs for nutrient and water uptake from the soil. They are particularly sensitive to heavy metal stress, as they are the first point of contact with contaminated soil. Heavy metals, such as cadmium, lead, and copper, can accumulate in the soil surrounding the roots, within the rhizosphere, leading to direct exposure and potential damage to the root system of cereal crops. High concentrations of heavy metals in the soil can directly impact root growth and function ^[20]^[21]. One of the primary effects is the inhibition of root elongation and branching. For instance, cadmium exposure can lead to the accumulation of ROS in root tissues, which in turn disrupt cell division and elongation, impairing root growth. The reduction in root elongation limits the plant’s ability to explore a larger volume of soil for water and nutrients, ultimately restricting its access to essential resources ^[22]^[23].

Furthermore, heavy metals can interfere with the proper development of root hairs. Root hairs are thin, hair-like extensions of root epidermal cells that significantly increase the surface area for nutrient absorption. However, heavy metal stress can disrupt the formation and elongation of root hairs, further compromising the cereal plant’s nutrient uptake capacity ^[24]. As the root system is responsible for water and nutrient absorption, reduced root growth has direct consequences on nutrient uptake by cereal crops. Heavy metals can inhibit the activity of root transporters responsible for taking up essential nutrients, such as potassium, calcium, and magnesium. The interference with nutrient uptake leads to deficiencies in these vital elements, adversely affecting plant health and development ^[25]. Additionally, heavy metals can displace essential nutrients in the soil, reducing their availability for uptake by plant roots. As mentioned above, cadmium can compete with zinc for uptake by roots, leading to reduced zinc uptake and consequent zinc deficiency in cereal crops. Zinc is essential for various enzymatic reactions and plays an important role in many physiological processes ^[19]^[26]^[27]^[28].

The reduced root growth and compromised nutrient uptake caused by heavy metal toxicity have far-reaching consequences on cereal crop productivity and health. Cereal crops with stunted root systems may exhibit poor nutrient efficiency, leading to reduced growth and overall yield.

5. Leaf Deformities

Leaf deformities represent another distinctive manifestation of heavy metal toxicity in cereals. Certain heavy metals, such as cadmium, mercury, and nickel, can cause physical deformities in leaves, altering their normal appearance and structure ^[29]^[30]. These deformities may include curling, twisting, irregular shapes, and abnormal growth patterns. One of the mechanisms through which heavy metals induce leaf deformities is the disruption of hormone regulation. Heavy metals can interfere with the synthesis, transport, and perception of plant growth regulators, such as auxins and gibberellins, which play critical roles in leaf development ^[3]. Distorted hormone signalling can lead to abnormal cell division and elongation, resulting in the misshapen and twisted leaves observed in heavy metal-exposed cereals. For example, mercury exposure has been shown to disrupt auxin signalling, leading to leaf curling and abnormal leaf development in cereal crops. Similarly, cadmium can interfere with gibberellin biosynthesis, contributing to leaf twisting and irregular leaf shapes ^[31]^[32].

Furthermore, heavy metals can cause oxidative stress in leaf tissues, leading to cellular damage and deformities. ROS generated by heavy metal exposure can attack cellular components, including proteins and lipids, disrupting cellular structure and function. This oxidative damage can manifest as irregular leaf growth, distortion of leaf margins, and abnormal leaf shapes. Additionally, heavy metals can disrupt the balance of essential nutrients in leaf tissues, leading to nutritional imbalances that contribute to leaf deformities ^[15]^[20]^[33]. For instance, cadmium exposure can interfere with the uptake and translocation of essential elements like calcium and magnesium, which are vital for leaf development and structure. Deficiencies in these nutrients can lead to leaf malformations and altered leaf morphology ^[34]^[35].

The consequences of leaf deformities in cereals can be detrimental to plant health and productivity ^[36]. Deformed leaves may have reduced surface area for photosynthesis, impairing the plant’s ability to capture light and produce carbohydrates. This reduction in photosynthetic capacity can lead to reduced biomass accumulation and yield in cereal crops ^[37]. Likewise, leaf deformities can impact transpiration rates and water use efficiency. Altered leaf structures may affect stomatal conductance and transpiration, potentially leading to water loss and reduced drought tolerance in heavy metal-exposed cereals ^[38]^[39].

Leaf deformities are a distinct manifestation of heavy metal toxicity in cereals, reflecting disruptions in hormone regulation, oxidative stress, and nutrient imbalance. The abatement of leaf deformities caused by heavy metal toxicity requires understanding the specific mechanisms involved and implementing appropriate interventions ^[37]^[40].

6. Reduced Flowering and Fruit Development in Cereals

Heavy metal toxicity can have a significant impact on the reproductive processes of cereal crops, leading to reduced flowering and negatively affecting fruit development ^[41]^[42]. The reproductive stage is vital for the successful completion of the plant’s life cycle, as it determines seed production and the continuation of the species. However, heavy metals can disrupt various physiological and biochemical processes involved in flowering and fruit development, affecting the reproductive success of cereal crops. One of the ways heavy metals interfere with flowering is through the disruption of hormonal signalling. Plant hormones, such as cytokinins and gibberellins, play critical roles in regulating flowering and fruiting processes ^[43]^[44]. Heavy metal exposure can alter the synthesis, transport, and perception of these hormones, leading to delayed or suppressed flowering in cereal crops ^[45]. For example, high levels of cadmium in the soil have been shown to disrupt cytokinin signalling, resulting in delayed flowering in cereals such as wheat and rice ^[46]. Similarly, lead exposure can inhibit gibberellin biosynthesis, leading to reduced elongation of flower stalks and altered flower development. Additionally, heavy metal toxicity can impact the development of floral organs, such as sepals, petals, and stamens ^[47]. Deformities in these floral structures can impair pollination and fertilization processes, leading to reduced fruit sets in cereal crops. Moreover, heavy metals can affect the availability and transport of essential nutrients required for flowering and fruit development. For instance, zinc deficiency caused by cadmium competition can hinder flower and fruit development in cereals like maize and barley. Zinc is an essential micronutrient involved in various metabolic processes, including those related to flowering and fruit set ^[27]^[28]^[48].

The negative impact of heavy metal toxicity on fruit development is also significant. Fruits are essential structures in cereal crops that contain seeds and are essential for seed dispersal and propagation. Heavy metals can interfere with the growth and development of fruits, leading to smaller and malformed structures ^[49]. The reduced size and altered shape of fruits can affect seed production and dispersal, ultimately impacting crop yield and quality. Furthermore, heavy metals can interfere with the development of seed embryos within the fruits. Disturbances in seed development can lead to reduced seed viability and germination rates, affecting the overall productivity of cereal crops in subsequent generations. Decreased fruit sets can directly translate into reduced seed yield and, consequently, lower crop production ^[50]^[51].

7. Necrosis

Necrosis is a severe manifestation of heavy metal toxicity in cereals, referring to the death of plant tissues. High levels of heavy metals in the soil can cause necrotic lesions on leaves and other plant parts, leading to tissue death and further impairing plant function. Necrosis can occur in various plant organs, including leaves, stems, and roots, and is often a result of oxidative stress and cellular damage caused by heavy metal exposure ^[3]^[52]. One of the primary mechanisms through which heavy metals induce necrosis is the generation of ROS. Heavy metals, such as cadmium and lead, can stimulate the production of ROS in plant cells (Figure 4). Excessive ROS accumulation can lead to oxidative stress, causing damage to cellular components, including proteins, lipids, and DNA. The breakdown of cellular structures and organelles results in necrotic lesions on affected tissues.

For instance, cadmium exposure has been shown to trigger ROS accumulation in the leaves of cereal crops, leading to oxidative damage and the formation of necrotic lesions. Similarly, lead-induced ROS production can result in cell death and necrosis in various plant parts ^[53]^[54]. Also, heavy metals can disrupt essential physiological processes in plants, further contributing to necrosis. For example, cadmium exposure can inhibit photosynthetic electron transport and reduce ATP synthesis in chloroplasts, leading to cellular energy deficits and impaired maintenance of cellular integrity. This disruption of essential metabolic processes weakens plant tissues and makes them more susceptible to necrosis. In some cases, heavy metals can directly interfere with the activity of enzymes involved in cellular repair and defence mechanisms. This interference hampers the plant’s ability to cope with oxidative stress and repair damaged cellular structures, leading to increased tissue necrosis ^[54]^[55].

Necrosis in cereals has significant consequences for plant health and productivity. Affected tissues lose their functionality, leading to reduced photosynthetic capacity, nutrient uptake, and water transport. The death of plant tissues can also compromise the structural integrity of the plant, making it more vulnerable to physical damage and environmental stressors. Additionally, necrosis in cereal crops can lead to reduced grain filling and impaired seed development. The loss of functional photosynthetic tissues reduces carbohydrate availability for grain filling, leading to smaller and poorly developed grains, ultimately impacting crop yield. Thus, necrosis is a severe consequence of heavy metal toxicity in cereals, signifying the death of plant tissues due to oxidative stress and cellular damage. The application of antioxidants or plant growth regulators can provide protection against oxidative stress and limit the extent of tissue necrosis.

8. Water Stress

Heavy metal toxicity can lead to water stress in cereal crops, affecting their ability to take up and transport water within the plant ^[24]^[56]. Water is essential for plant survival and plays a crucial role in various physiological processes, including nutrient uptake, photosynthesis, and transpiration. However, heavy metals can disrupt water uptake and transport mechanisms, leading to water stress and wilting in cereal crops. One of the primary ways heavy metals induce water stress is by affecting the structure and function of root tissues. Roots are responsible for water absorption from the soil, and heavy metals can inhibit root growth and function, reducing the plant’s ability to access water. For example, cadmium exposure has been shown to restrict root elongation and disrupt root cell membranes, hindering water uptake in cereal crops ^[57]^[58]. Additionally, heavy metals can interfere with the functioning of root transporters responsible for the uptake of water and essential nutrients ^[24]^[59]. The process of short-distance water transport within roots holds a pivotal position in regulating the flow of solutes and water, both entering and exiting the vascular system, through the encompassing tissues of the conducting cells. This mechanism could potentially impact the pace of long-range water transportation toward the aboveground segments of the plant ^[56]^[60].

The disruption of these transporters can lead to reduced water uptake and nutrient deficiencies in cereal plants. Furthermore, heavy metals can impact the transpiration process, where water is lost from the leaves as water vapour ^[56]. Transpiration is essential for nutrient transport and cooling of the plant, but heavy metals can affect stomatal conductance and transpiration rates. For instance, lead exposure has been shown to reduce stomatal opening, limiting water loss through transpiration and leading to water stress in cereal crops. Water stress can manifest as wilting, where the leaves of cereal plants lose their turgidity and appear droopy. Wilting is a visible symptom of water deficiency in plants and is a response to reduced water uptake and impaired water transport within the plant. The consequences of water stress in cereals are significant. Reduced water availability can impair nutrient transport and photosynthetic efficiency, leading to reduced growth and overall plant health. The lack of water can also lead to reduced cell expansion and division, further inhibiting plant growth and development ^[31]^[56].

Water stress can be particularly detrimental during critical growth stages, such as flowering and grain filling, as water deficiency can significantly impact seed development and ultimately reduce crop yield. Thus, water stress is a significant consequence of heavy metal toxicity in cereals, arising from disruptions in water uptake and transport mechanisms ^[23]^[61].

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Subjects: Environmental Sciences

Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :

Ionela Cătălina Vasilachi

Vasile Stoleru

Maria Gavrilescu

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Update Date: 24 Oct 2023

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