ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter 8. RESISTANCE AND ADAPTATION OF PLANTS AND ANIMALS

8.2. Plant Resistance and Adaptation to Adverse Environmental Factors

Xerophytes include plants with reduced, scale-like leaves, such as saxaul, ephedra, and juzgun. Their root system is well developed, and Photosynthesis takes place in the Cells of green shoots.

In the temperate climate of middle latitudes, mesophytes grow—plants that are less drought-resistant than xerophytes. These include the majority of meadow grasses and legumes (creeping couch grass, meadow timothy, alfalfa, etc.), agricultural crops (wheat, barley, oats, peas, corn, beets, etc.), as well as most fruit and vegetable plants. They feature a well-developed root system with relatively high root pressure. The Tissues of these plants have a high water-holding capacity. The transpiration rate of mesophytes is regulated by stomatal activity, as well as the shedding of leaves and fruits.

Drought-resistant shrubs and woody plants share the same characteristics as herbaceous plants. It is worth noting that, alongside physiological and biochemical features, the drought resistance of shrub and tree species heavily depends on morphological and anatomical traits. Primarily, these include deep root penetration into the soil and groundwater, a smaller leaf surface area of the crown, leaf pubescence, and others.

Heat tolerance is the ability of plants to withstand the effects of high temperatures. Like drought resistance, it is determined by genotypic potential, the ontogenetic state of the plants, and the interaction between elevated temperatures and other environmental factors.

Based on their heat tolerance, plants are divided into three main groups: 1) heat-resistant plants, which mainly include lower plants, thermophilic Bacteria, and cyanobacteria capable of withstanding temperatures of 75 — 90 °С; 2) heat-tolerant plants, which are typical of arid climatic zones—primarily xerophytes capable of withstanding temperatures up to 54 °С, with some (succulents) enduring up to 60 °С; 3) heat-sensitive plants—mesophytes that withstand temperatures of 40 — 47 °С, and aquatic plants that tolerate 38 — 42 °С.

A distinction is made between inherent and induced heat tolerance. Inherent heat tolerance is driven by the high thermal stability of biological macromolecules—primarily Proteins, Nucleic Acids, CARBOHYDRATES, and Lipids—which can endure the Direct impact of high temperatures without coagulation. Induced heat tolerance refers to the ability of plants to protect themselves against overheating through adaptive protective mechanisms.

The heat tolerance of different plant Organs varies. For instance, flowers and ears of grain withstand higher temperatures compared to leaves. Plant tissues possess a significant water-holding capacity, which is ensured by the accumulation in cell vacuoles of substances that regulate osmotic pressure, such as carbohydrates, organic acids, soluble nitrogen, and mineral compounds. Among all tissues, cambial tissues are the most resistant to high temperatures. In comparison to the root system, SHOOT growth processes are much more sensitive to elevated temperatures.

One of the plant responses to high temperatures and drought is enhanced transpiration. During severe heat, intensive transpiration can lower the temperature of plant organs and tissues by 10 — 15 °С. However, this occurs only up to a certain temperature threshold, beyond which increased transpiration no longer prevents pathological, physiological, and biochemical alterations.

Apart from transpiration, plants possess other adaptive mechanisms, some of which have already been mentioned. The main ones include positioning leaves parallel to the sun's rays; leaf pubescence or The formation of scales that shade The surface of above-ground plant organs; the presence of a dense cuticle; The Development of cork tissue protecting the phloem and cambium; the presence of light-colored pigments in above-ground organs; and a low water content in the Cytoplasm.

Short-term drought in mesophytes triggers the intensification of most physiological processes relative to overall growth rates. This is likely due to adaptive metabolic reorganizations in cells that enhance plant viability.

Prolonged disruption of the water balance leads to Changes in the intensity and direction of physiological and biochemical processes within plants. Thus, as noted earlier, one of the primary manifestations of dehydration caused by high temperatures and drought is a reduction in plant growth rate, which indicates an adaptive response. Plant growth is known to be regulated by phytohormones. For instance, intensifying drought leads to the accumulation of Ethylene and Abscisic acid in plant tissues, alongside a decrease in the levels of Auxins, Gibberellins, and Cytokinins.

It has been established that The production of ethylene, auxins, and nucleic METABOLISM Enzymes is stimulated by phytohormones known as non-Steroids. These are considered key Hormones that help increase Plant resistance to high temperatures and drought. Proline, betaines, and Polyamines exert a similar protective effect.

Prolonged exposure to temperatures of 35—40 °С in most plants (mesophytes) alters the ratio between Photorespiration and photosynthesis. Photorespiration drops sharply at temperatures above 30—35 °С, whereas photosynthesis declines at 40—50 °С. Consequently, at 35—40 °С, the consumption of organic substances through photorespiration exceeds their synthesis. This is accompanied by the accumulation of ammonia, which the plant fails to incorporate into Protein Biosynthesis in time. As a result, ammonia toxicity occurs, leading to plant exhaustion, susceptibility to various diseases, and other adverse effects.

High temperatures cause cell mitosis disturbances in various plants. For instance, at 38 °C, mitosis is observed in only every seventh cell, whereas raising the temperature to 42 °C reduces Cell Division by nearly 500-fold.

Cell lysis also depends on temperature. When the temperature rises to 38 °C, nuclear lysis occurs in 5.3% of cells, while at 52 °C it happens in almost all of them.

Prolonged exposure to high temperatures and severe drought trigger Metabolic Disorders that lead to intensified Breakdown and Oxidation processes. Furthermore, disruptions occur in the Structure of Cell Membrane Lipids, particularly thylakoids; the content of free Fatty acids increases, while The amount of Phospholipids decreases. The impairments caused by elevated temperatures and drought are by no means limited to those mentioned above. They manifest differently across various plant species, which calls for further research.

One of the ways to mitigate the NEGATIVE IMPACT OF high temperatures and drought involves agrotechnical and ameliorative measures, as well as plant breeding. It is essential to properly cultivate the soil, apply fertilizers, establish shelterbelts, create water reservoirs, and irrigate crops. Another approach is to reduce plant transpiration intensity using chemical substances known as antitranspirants. Substances that induce stomatal closure include abscisic acid, phenylmercuric acetate, and others. Another group of compounds forms a film on the leaf surface that does not interfere with Light absorption yet prevents water evaporation.

To enhance plant heat tolerance, foliar feeding with zinc sulfate can be applied, which shifts photorespiration processes toward the synthesis of organic acids. This compound neutralizes the Toxic effects of ammonia that accumulates under high-temperature stress and incorporates it into protein biosynthesis. A similar effect is observed when plants are sprayed with manate.

winter hardiness and cold hardiness of plants. Perennial grasses and winter crops are more productive than annual and spring ones. This is primarily due to better utilization of autumn and early spring moisture, autumn root system development and intensive tillering, less damage from pests when winter crops are sown at optimal times, and active Differentiation of the spike primordium in spring, among other factors.

Winter hardiness. The ability to withstand adverse environmental conditions during the winter and early spring growing periods is referred to as winter hardiness. The causes of plant damage or even death during the wintering period can include a significant drop in temperature (frosts), an excessively deep snow cover, the formation of an ice crust, and more. Plant resistance to adverse factors associated with low temperatures is determined primarily by their genetic adaptations and hardening (acclimatization for winter).

Plants capable of sprouting at relatively low temperatures are called psychrophiles (or frigophilic). The main causes of plant damage under low temperatures include metabolic shifts, disruption of Membrane Structure functioning due to alterations in their physical properties, and damage to pigment complexes. Injury and death of overwintering plants also occur as a result of water freezing in intercellular spaces and cells, leading to osmotic Shock, degradation, and mechanical trauma to cell membranes.

Plant frost resistance is driven by a complex of adaptive biochemical and physiological processes, primarily changes in protein, carbohydrate, and Lipid Metabolism, as well as cellular Functions, particularly their energy status. One of the consequences of The chemical composition restructuring in plants under low temperatures is the accumulation of so-called cryoprotectants within cells—substances capable of binding water (hydrophilic proteins, sugars, fats, polyphenols, lignins, pigments, Pectins, free Amino Acids, enzyme complexes, etc.). Cold-hardy plants are also able to synthesize substances with antifreeze properties that lower the freezing point of water (Glycoproteins, certain alcohols, etc.).

The ability of plants to survive long periods in winter is supported by the nutrient reserves accumulated within them, such as proteins, lipids, and carbohydrates. For instance, hypothermia induces the synthesis of stress proteins, which correlates with the development of frost resistance. A decrease in the content of Nucleic Acids and free amino acids halts Plant Growth and increases winter hardiness.

In late autumn, overwintering plants switch to an Anaerobic respiration pathway, and the coupling activity of oxidation and phosphorylation processes in Mitochondria changes. All of this leads to the economical use of ATP.

As previously noted, plant frost resistance largely depends on the physiological state in which they enter the winter period. During active growth, plants exhibit low tolerance to low temperatures, but in autumn, as growth ceases, their frost tolerance increases—the plants undergo acclimation. Frost resistance in plants develops as a result of prolonged hardening, the consequence of which is the realization of genotypic potential to enhance freezing tolerance.

Hardening of winter crops occurs in several stages. In The First stage, at daytime temperatures of 8–15 °C and nighttime temperatures around 0 °C, sugars (mainly glucose and sucrose) accumulate in the crown nodes and leaf sheaths. These are the sugars synthesized during the day through photosynthesis that are not entirely consumed by respiration. They not only perform a protective function but also serve as nutrients during the winter and as material for synthesizing Organic compounds at THE START OF spring vegetation.

With a further drop in temperature, plants enter a dormant state, which is primarily accompanied by changes in phytohormone levels—regulators of Plant GROWTH AND DEVELOPMENT processes. Specifically, the amounts of auxins and gibberellins decrease, while the content of abscisic acid increases.

The first stage of direct hardening in winter cereals, for example, takes place in light and darkness at temperatures of +5 to -5 °C over the course of several days, while in fruit and berry woody crops it takes several weeks. As growth and development processes gradually cease, metabolic changes occur within the cells, including partial dehydration of tissues, the transition of a portion of water into a bound state, the accumulation of lipids with a higher degree of fatty acid saturation, proteins, and sugars, alterations in enzyme properties, and the slowing of thermal molecular motion in the cytoplasm.

The next stage of hardening occurs when the air temperature drops to -10 to -15 °C and lower. This results in the stabilization of cytoplasmic structure, the outflow of water, and other processes. The complex of cellular adjustments taking place in plant cells at such low temperatures facilitates tolerance to the dehydration of polymers and membrane structures.

Agricultural crops vary significantly in their frost resistance. Under hardened conditions, winter rye withstands the lowest frosts—down to -25 to -30 °C in the crown node zone. Under a 20–35 cm snow cover, it can endure temperatures as low as -60 °C. Modern winter wheat varieties are capable of withstanding -18 to -20 °C in the crown node zone. Among cereals in Ukraine, barley is the least frost-resistant, with critical temperatures of -12 to -15 °C. Winter rapeseed in a snowless

winter with waterlogged soil can withstand temperatures of -7 to -10 °C, and under a 35–50 cm snow cover, down to -25 °C.

Crown nodes in cereals are more resistant to low temperatures than leaves. Their cells have a higher cell sap concentration due to both dehydration and the accumulation of proteins, carbohydrates, and lipids. Furthermore, they are protected by a layer of soil. Even if the leaves freeze off while the crown nodes remain uninjured, new shoots can develop from them.

The frost resistance of winter crops is influenced by agronomic practices, such as sowing at optimal times and balanced mineral Nutrition.

In trees and shrubs, exposure to low temperatures results in both external and internal damage. External damage includes frosting of branches, young shoots, and buds, as well as injury to roots and the root collar. Internal damage mainly involves tissue rupture and alterations in pith properties.

One of the indicators of frost resistance is the intensity of winter transpiration. The lower the water loss—which could otherwise lead to winter drought—the more resistant trees and shrubs are to low temperatures.

Factors that exacerbate the effects of low temperatures. There are several natural factors that intensify The impact of low temperatures during plant overwintering. One such factor is a transparent, crusted ice layer. It forms when snow thaws and, with a subsequent drop in temperature, the water freezes on the soil surface, encasing the plants in ice. The thermal conductivity of ice is nearly 5 times higher than that of snow. Consequently, an ice crust promotes winterkill in plants even during mild frosts.

A suspended, or bridged, ice crust forms in such a way that an air space exists between it and the soil. It is not as hazardous to plant survival as a crusted ice layer. Occasionally, a bridged ice crust can even protect plants against severe temperature drops. However, the formation of such a crust over weakly frozen soil can lead to plant suffocation. In such cases, the bridged ice crust is broken up to facilitate soil cooling. At the same time, it is inadvisable to mechanically break a crusted ice layer, as this damages the plants themselves. Instead, it is preferable to dust the ice crust with substances that accelerate melting, such as peat, soot, ash, or soil.

In the absence of snow cover during windy weather, soil blowing or even dust storms may occur. This can lead to both the deposition of dust on plants and the exposure and damage of crown nodes, as well as abrasion of leaves by sand particles.

At temperatures around 0 °C, when plant growth does not stop but slows down, plant winter injury (sprout rot) may occur. This process is most intense beneath a snow cover as photosynthesis slows down, causing nutrient depletion. Plants lose their hardening, and under a snow layer of 50 cm or more, available oxygen is depleted. The lack of light leads to leaf death and the destruction of Chloroplasts.

Winter injury leads to a decrease in sugars and an intense Breakdown of Proteins, which causes an increased content of free Amino Acids and the accumulation of toxic ammonia.

The consequence of plant exhaustion during winter injury is their increased susceptibility to infections, particularly fungal ones (such as Fungi Fusarium nivale, Fusarium culmorum, Sclerotinia graminearum, etc.), leading to the development of snow mold. It thrives at an air temperature of 0—1 °C and a humidity of about 100 %.

To increase plant resistance to winter injury and snow mold, it is necessary to carry out spring harrowing of winter crops, and in the case of snowfall on unfrozen soil, roll it with rollers. In areas where snow melts slowly, it should be dusted with peat, soot, ash, or soil. It is also advisable to select varieties resistant to snow mold.

Another unfavorable factor affecting winter crops is waterlogging during floods caused by thaws and the accumulation of meltwater in spring. This phenomenon is sometimes also observed in spring crops. The damaging effect of waterlogging is intensified when a temperature drop follows a thaw, forming an ice crust.

During flooding, anaerobiosis sets in and anaerobic respiration prevails; toxic alcohols, iron, manganese, and aluminum compounds accumulate. To reduce the negative impact of waterlogging, excess water must be drained from the fields in a timely manner and winter crops should be fertilized with nitrogen.

If plants fail to tiller and form nodal roots before winter, and are subjected to periodic thawing and freezing in spring, heaving may occur. This happens because plants are pulled out of the soil every day by the thickness of the ice crust formed during the periodic freezing of meltwater. When weak roots are located in a still-unfrozen soil layer, and the ice crust above the soaked soil layer is quite thick, root breakage and plant death are observed. This is primarily a consequence of late plowing and sowing. Therefore, It is important to improve agricultural practices. It is also necessary to select varieties characterized by a deeper placement of the tillering node.

Cold hardiness and frosts. In addition to winter hardiness, when exposed to low temperatures, plants are also characterized by cold hardiness—the ability to tolerate low positive temperatures. To assess it, THE CONCEPT OF the biological zero is used, which is the temperature value at which active plant growth stops. These values for various agricultural crops are given in Table 8.1.

Class="center">Table 8.1

BIOLOGICAL ZERO VALUES FOR VEGETATIVE AND GENERATIVE PLANT ORGANS

(according to Makrushyn et al., 1995)

When exposed to temperatures below the biological zero, biochemical and physiological changes occur that are similar to those determining plant winter hardiness. First of all, these include metabolic and energy conversion disorders, inhibition of The activity of A number of enzymes, changes in the permeability of Introduction/36.html">Biological Membranes, damage to The structure of Mitochondria and Plastids, a slowdown in aerobic respiration and photosynthesis, and the predominance of Catabolism over anabolism, etc.

To increase the cold hardiness of plants, thermal hardening of seeds or seedlings is used. Seeds are kept at 0—2 °C for a day or longer, and seedlings, before planting in the soil, are exposed to a gradual drop in temperature or alternating low and normal temperatures.

During the growing season, accidental temperature drops to negative values—known as frosts—may occur. Plant resistance to frost is determined primarily by genetic traits, physiological state, agricultural practices, etc. For example, root crops, most oilseed crops, and flax can withstand temperature drops down to -5...-8 °C; potatoes and sorghum down to -2...-3 °C; rice and cotton down to -1.5...-2 °C; and melons down to -0.5...-1.5 °C. Frosts are particularly dangerous during the flowering period and at the beginning of fruiting.

To reduce the damage sustained by plants during frosts, they are sown at optimal times when favorable, stable temperatures set in. Common practices include creating smoke screens, covering plants with straw mats, polyethylene film, paper caps, etc. Another method is sprinkling the plants with water before a frost. The high heat capacity of plants' cooling is reduced by water. A more rapid restoration of cell turgor also takes place.

PLANT RESISTANCE TO Chemical factors. In addition to high or low temperatures, excessive moisture, or drought, plants are also affected by A wide variety of chemical factors. Among them are elevated levels of salts, heavy metals, toxic gases, and others.

In a general sense, pollution is defined as the introduction or formation of agents uncharacteristic of the environment in excessive concentrations, which usually leads to negative consequences.

In this chapter, we will examine The Effect of the most important chemical pollutants on plants. The resistance of plants and other organisms to ionizing radiation is discussed in Chapter 9.

Salt Tolerance. Soils (solonetzes and solonetzic soils) contain elevated amounts of mineral salts—chlorides, sulfates, carbonates, etc. Soil salinity most often occurs under conditions of insufficient moisture. In this case, precipitation water does not leach mineral salts from the soil, and they accumulate. Another cause of soil salinity is the runoff of salt-saturated meltwater into lowlands and its accumulation after evaporation. A similar phenomenon is observed when salt-saturated groundwater rises to the upper soil layers and evaporates. Yet another cause of soil salinity is soil irrigation with poor water permeability in the underlying horizons, along with improper irrigation and sprinkling regimes.

According to salt tolerance, plants are divided into two groups: 1) halophytes, which are capable of growing on highly saline soils where the amount of soluble salts can be 1% or even more; 2) glycophytes, which are unable to grow on saline soils. In turn, halophytes are divided into 3 main subgroups. The most resistant to soil salinity are euhalophytes (salt-accumulating plants). These primarily include plants of semi-deserts and deserts, such as glasswort (salicornia), seepweed, etc. They are able to accumulate salts in the vacuoles of their tissue cells in amounts

up to 100% relative to all the water in the plants, which makes up 90% or more of the plant mass. These plants are characterized by high osmotic pressure and significant suction force, along with weak transpiration. This enables them to absorb water in saline soils. For the normal growth of euhalophytes, the presence of a certain amount of water-soluble salts is a prerequisite for their growth.

Crinohalophytes (salt-excreting plants) are able to excrete absorbed salts through special secretory cells—bladder hairs on the leaves, where excess salts accumulate. When these hairs become completely filled with salts, they burst, and the salt ends up on the leaf surface and is shed from it. New hairs grow in place of the old ones. Typical representatives of crinohalophytes are tamarisk, statice (limonium), frankenia, etc. Like euhalophytes, they supply themselves with water from saline soil through high osmotic pressure, but these plants are characterized by a high transpiration rate.

Another group based on salt tolerance consists of glycohalophytes (salt-impermeable plants), which do not accumulate excessive amounts of salts in their cells even under conditions of severe soil salinity. High osmotic pressure for absorbing water from the soil in these plants is achieved through the production of a large amount of organic compounds—carbohydrates, organic acids, etc.

Major cultivated plants, unlike wild halophytes, have weak or no resistance to salinity. High salt concentrations in them cause changes in osmotic pressure, resulting in the destruction of Cell Membrane Structures, inhibition of the functional activity of membrane-bound enzymes, etc. This leads to disruptions in photosynthesis and Oxidative Phosphorylation, which causes a disturbance in the energy state of the cells. Intensified protein degradation also occurs, resulting in the accumulation of free amino acids, the formation of ammonia, and other substances.

However, among cultivated plants, there are species capable of growing on saline soils. Sugar beets are close relatives of euhalophytes. Glycophytes include rye, corn, cotton, and certain other cultivated crops. Their roots have low permeability to salts, preventing their accumulation.

To mitigate the impact of soil salinity on crop growth, agrotechnical and reclamation measures are implemented, such as soil leaching and drainage, rational fertilizer application, foliar feeding, etc. Sodium accumulation can be counteracted by applying gypsum to saline soils. The introduction of salt-tolerant

varieties is also essential. For certain industrial crops, vegetables, fruit trees, and ornamental plants, treating pre-soaked seeds with soluble salt solutions under a controlled hardening regime can be effective.

Gas resistance. Compounds capable of adversely affecting plants include toxic gases. This issue has become particularly acute due to industrial atmospheric pollution resulting from a neglect of environmental protection measures, accidental emissions of toxic gases into the atmosphere, and similar factors. The most widespread gaseous atmospheric pollutants are carbon monoxide (CO), sulfur oxides (sulfur dioxide — SO₂), nitrogen oxides (NO, NO₂), hydrogen sulfide (H₂S), ammonia (NH₃), fluorine compounds, and others. Excessive levels of atmospheric CO₂ also negatively impact plant development.

Plants exhibit high sensitivity to sulfur dioxide and acids formed when it combines with atmospheric moisture. This process leads to the formation of acid rain, which inhibits plant growth and leaches essential nutrients from the soil.

The air in industrial regions contains nearly 50% of all gases as CO, along with 20% SO₂, 6–8% nitrogen oxides, 2–5% hydrocarbon compounds, and 16–20% particulate matter.

The primary source of global atmospheric pollution is the combustion of fossil fuels, such as coal, oil, and fuel gas. Motor vehicles pose a severe threat to air quality. Another major source of harmful gases is industry in all its forms. Indoor air in residential buildings is heavily contaminated by the open burning of natural gas, with CO being the primary hazardous component of these pollutants.

One manifestation of the damaging effect of toxic gases is the disruption of photosynthetic reactions. For instance, sulfur dioxide induces the degradation of chlorophyll and chloroplasts. Atmospheric pollution by fluorine causes significant changes in peroxidase activity. Carbon monoxide can bind to Cytochromes, notably P₄₅₀ and cytochrome c oxidase. There are many other manifestations of the toxic action of harmful gases; they disrupt membrane transport, the ionic balance of the cytoplasm, and more.

A crucial factor in the MECHANISM OF ACTION of atmospheric pollutants is that other chemical substances can potentiate their effects. For example, individually non-toxic concentrations of nitrogen and carbon dioxide cause severe poisoning when acting synergistically in various combinations with CO.

The rate and degree of plant changes induced by gas exposure are referred to gas sensitivity. Conversely, the ability of plants to maintain vital activity under the negative influence of gases determines their gas resistance. Several forms of plant gas resistance are distinguished:

1) anatomical — associated with Structural Features of plants that impede gas penetration into tissues; 2) physiological — determined by the specific interactions between Plant Tissues and the environment; 3) biochemical — related to the resistance of metabolic processes and energy conversion pathways to harmful gases; 4) habitual — achieved by reducing the extent of contact between plant organs and gases; 5) phenorhythmic — avoiding the overlap between the timing of harmful gas exposure and Critical Periods of plant vegetation; 6) anabiotic — associated with the state of dormancy in plants during winter or summer drought; 7) regenerative — the ability of shoots to regrow leaves and produce new shoots; 8) population-based — dependent on the age polymorphism of the population.

All of these and Other forms of gas resistance primarily regulate the influx of harmful gases into plants, maintain cellular buffer capacity and cytoplasmic balance, and enhance the plants' capacity for xenobiotic detoxification (see Chapter 10).

Plants vary in their degree of gas resistance. The most gas-resistant species (such as poplar, black locust, box elder, elm, oleaster, mulberry, gleditsia, and Turkestan oak) are capable of accumulating elevated amounts of sulfur and chlorine, which reflects the increased activity of their enzymes. Salt-tolerant plants also exhibit high resistance to harmful gases. The species most resistant to SO₂ include onions, corn, citrus fruits, and rapeseed; to HF — wheat, tomatoes, sunflowers, and asparagus; and to NO and NO₂ — cabbage, pumpkins, onions, and strawberries, among others.

To enhance gas resistance, plants undergo selection and breeding, alongside receiving adequate moisture and mineral nutrients. Rain and irrigation are capable of washing up to 30% of toxic substances from plant leaves. Pre-soaking seeds in a 0.1% solution of H₂SO₄ or HCl, followed by subsequent watering of young plants with a 0.2% solution of salts of these acids, has a positive effect on increasing gas resistance.

Plant resistance to heavy metals. Industrial development, when accompanied by a neglect of environmental protection measures, has resulted in the heavy metal contamination of vast territories. Heavy metals are defined as those with a density exceeding 5 g·cm^-3 or an atomic number greater than 20. This category includes extremely toxic chemical elements. Their toxicity is determined primarily by their complex-forming capacity, ionic radius, valence, and other properties.

Heavy metals can disrupt membrane permeability, leading to alterations in ionic balance, water status, and enzymatic activity.

The toxic heavy metals most frequently accumulated by plants include cadmium, lead, copper, mercury, zinc, selenium, and nickel, among others. These metals exhibit a high affinity for SH-groups. Consequently, they are capable of inhibiting the activity of enzymes that contain sulfhydryl groups.

Based on their resistance to heavy metals, plants are divided into three groups: 1) accumulator plants, which concentrate heavy metals in their above-ground parts; 2) indicator plants, which accumulate metals in their shoot systems in quantities reflecting the concentration of their soluble forms in the soil; and 3) excluder plants, in which the heavy metal content in above-ground tissues remains low and stable regardless of soil concentrations.

The high tolerance of certain plants to heavy metals, which they can accumulate in substantial amounts (exceeding 1% of dry mass), is attributed to several mechanisms. One of these is the Synthesis of specific low-molecular-weight proteins (molecular mass 3.0–10 kDa) with a high Cysteine content. These proteins bind heavy Metal Ions and form stable complexes with them. Located in the cytoplasm, these proteins prevent heavy metal concentrations from reaching critical toxic levels within a certain range of external exposures.

Another mechanism of plant resistance to heavy metals may involve structural modifications in metal-sensitive enzymes and the emergence of metabolic pathways that enable plants to survive under high metal concentrations. The sequestration of heavy metals within cell walls and vacuoles also contributes to enhanced plant tolerance to heavy metal stress.