PLANT PHYSIOLOGY WITH ELEMENTS OF BIOCHEMISTRY - Prytuliak R. M. - 2016

Lecture Notes

TOPIC No. 8. PLANT ADAPTATION AND RESISTANCE TO ADVERSE ENVIRONMENTAL FACTORS

Outline

1. Plant ADAPTATION TO ENVIRONMENTAL conditions As a result of evolution.

2. PHYSIOLOGICAL AND BIOCHEMICAL changes in thermophilic plants exposed to low positive temperatures. Chilling tolerance and Methods of its enhancement.

3. Effects of sub-zero temperatures on plants. Frost resistance and methods of its enhancement.

4. Plant hardening to minimum temperatures.

5. Winter hardiness of plants.

6. Plant lodging: causes and Prevention methods.

7. Effects of high temperatures on plants. Heat resistance.

8. Drought resistance of plants. Classification of plants based on Water requirements.

9. Salt Tolerance of plants and potential methods of its enhancement.

10. Plant resistance to atmospheric pollution, pesticides, and agrochemicals.

1. Plant adaptation to environmental conditions as a result of evolution.

Plant adaptation to adverse factors is evolutionary in nature. Adaptation occurs through the alteration of physiological and biochemical processes, changes in Anatomical Structure, and The Development of specific morphological adaptations.

Throughout evolution, plants have developed and genetically fixed various adaptive responses. In accordance with environmental fluctuations, a sequence of active life stages has evolved, aligning critical phases of individual development with the most favorable periods of the year. Common adaptive responses during ontogeny include photomorphogenetic reactions, as well as thermoperiodism and Photoperiodism. One manifestation of thermoperiodic response is vernalization, or the accelerated development of winter forms of annual and biennial plants induced by exposure to a period of low positive temperatures. Unlike vernalization, which requires a single prolonged cooling period, thermoperiodism relies primarily on the alternation of high and low temperatures. Both diurnal and seasonal thermoperiodism are recognized.

Photoperiodism, as an adaptive response to seasonal environmental rhythms, extends beyond photoperiodic control of flowering. It also governs processes such as tuber formation, bulb formation, and the transition of plants into a dormant state.

PLANT RESPONSE TO changing environmental factors depends on the intensity of these changes and the functional state of the Organism. Significant shifts can disrupt vital processes, potentially leading to organismal death. In all cases, the functional state of the organism transitions sequentially from activation to inhibition and, ultimately, to lethality.

For instance, as daylight duration decreases and temperatures drop, metabolic changes in plant Cells lead to reduced Hydration of Cells and Tissues, the accumulation of storage nutrients, and The formation of specific protective structures. Plants tolerate adverse factors better while in a dormant state. Examples of this include autumn leaf abscission in perennials, as well as the formation of seeds and storage Organs.

2. Physiological and Biochemical changes in thermophilic plants exposed to low positive temperatures. Chilling tolerance and methods of its enhancement.

When thermophilic plants (such as cucumber, soybean, corn, and buckwheat) are exposed to low positive temperatures, the following physiological and biochemical disturbances occur:

· Increased cytoplasmic viscosity.

· Reduced water uptake.

· Disruption of Oxidative Phosphorylation.

· ATP synthesis is inhibited.

· Chlorophyll degradation occurs.

· Photosynthetic processes are disrupted.

Cold tolerance. Cold tolerance refers to a plant's ability to withstand low positive temperatures, typically ranging from 0 to 10 °C. This category includes plants native to temperate and southern latitudes, which encompasses most agricultural crops. Conversely, tropical and subtropical plants are often damaged or perish under such conditions.

To assess cold tolerance, researchers often use METABOLISM/2.html">THE CONCEPT OF the "biological zero"—the Temperature at which active growth processes cease. For most agricultural crops, this point is approximately +4 °C. The higher this value, the lower the cold tolerance of the specific plant group.

The sum of biological temperatures provides an indication of cold tolerance. A lower value generally correlates with higher cold resistance and a faster completion of the plant's life cycle.

The sum of biological temperatures serves as an indicator of the earliness of plant varieties. For very early-maturing varieties, it is 1200 °C; for mid-season varieties, it ranges from 2200–2800 °C; and for late-maturing varieties, it reaches 3400–4000 °C.

Cold-tolerant crops include spring wheat, barley, oats, and peas; mid-season crops include annual lupine, fava beans, flax, sunflower, and buckwheat; low-resistance crops include millet, corn, soy, sorghum, and Kidney beans; while non-resistant crops include rice, cotton, and cucurbits.

The NEGATIVE IMPACT OF low temperatures primarily affects the state of the Cytoplasm (increasing its density), membrane permeability, and the disruption of nucleic acid and Protein metabolism. Damage to The structure of Cell/35.html">Mitochondria and Chloroplasts leads to impaired aerobic Respiration and the suppression of Photosynthesis.

Methods for Enhancing plant cold tolerance. Strategies to ensure plant resistance to low temperatures involve creating growing conditions that maintain high energy levels within tissue cells. This includes adequate lighting, increased air humidity, and the application of phosphorus and potassium fertilizers. Alternating exposure to optimal and reduced temperatures for germinating seeds and young plants is also quite effective. Additionally, soaking seeds in dilute solutions (0.25%) of micronutrients, ammonium nitrate, and other salts has a positive effect. The Role of selective breeding in developing cold-tolerant agricultural varieties is crucial, alongside the following measures:

· Hardening of young plants (seedlings).

· Seed hardening.

· Pre-sowing seed Treatment with micronutrients.

3. The Effect of sub-zero temperatures on plants. Frost resistance and methods for its enhancement.

Frost, which often occurs during the growing season, causes significant agricultural losses. These frosts can be classified as spring, autumn, or, most dangerously, summer frosts.

A plant's resistance to frost is determined by its species, physiological state, and growing conditions.

Early-sown crops, such as spring cereals and legumes, are the most resistant to spring frosts, capable of withstanding brief temperature drops to -7–10 °C. Late-sown plants often fail to prepare for low temperatures. ROOT crops, most oilseeds, flax, and hemp can tolerate short-term drops to -5–8 °C; soy, potatoes, and sorghum to -2–3 °C; and rice and cotton to -1.5–2 °C. Cucurbits exhibit the lowest frost resistance, with temperatures as mild as -0.5–1.5 °C causing significant damage.

Plant frost resistance is largely determined by the developmental stage.

Various protective measures are employed to mitigate frost damage. These primarily include optimizing sowing and planting dates and using seedlings for vegetable and flower crops, which allows for transplanting into open ground once stable, favorable temperatures are reached. Sprinkling plants before a frost or spring irrigation can be highly effective. Common protective methods also include smoke screens and covering plants with paper or plastic caps, straw mats, etc.

Processes occurring in cells during freezing. Frost resistance, defined as The ability to withstand low negative temperatures, is determined by specific plant traits. Annuals survive the frost period in the form of mature seeds. Most perennials lose their aerial parts partially or completely before the onset of freezing temperatures, overwintering deep in the soil as bulbs, tubers, or rhizomes. However, winter crops and woody plants are exposed to sub-zero temperatures and often suffer as a result. The cause of death from frost is the coagulation of protoplast Proteins and the physical impact of ice crystals on cellular structures. Damaged plants lose turgor, and their leaves appear scalded, turning brown and drying out. Water is released from fleshy tissues upon thawing.

If tissue freezing occurs gradually, ice forms in the intercellular spaces and cell walls. These ice crystals draw water out of the cells, causing an increase in the pH of The Cell sap, dehydrating the cytoplasm, and altering the cell's osmotic properties. Furthermore, ice crystals exert physical pressure on the cytoplasm. Upon thawing, frozen cells can remain viable if the ice has not mechanically damaged their cytoplasm.

Frost-resistant plants possess a range of protective Functions based on specific biochemical changes.

At low temperatures, plant cells exhibit increased levels of sugars and protective proteins, Changes in the composition of polar Lipids, and reduced cellular hydration.

The protective role of sugars involves forming hydrophilic bonds with cytoplasmic proteins to prevent Denaturation. Additionally, sugars lower the freezing point of the cytoplasm and increase its water-holding capacity. High concentrations of protective proteins and modified lipid molecules enhance cellular structural integrity.

Thus, frost resistance is a temporary adaptation rather than a permanent plant trait. Its development depends heavily on external factors. The determining factor is primarily The ratio of sunny days with cool nights to cloudy, rainy days with relatively low or high temperatures. The higher this ratio, the more effective the hardening conditions. Soil Nutrition also significantly influences frost resistance. Resistance in acid soils increases with regular liming and the application of sufficient potassium and phosphorus fertilizers for winter crops. The most effective and reliable method for preventing winter crop loss is the development of frost-resistant varieties and the application of scientifically sound cultivation technologies.

Therefore, to enhance frost resistance, the following points should be observed:

· Optimal sowing dates.

· Balanced mineral nutrition.

· Adequate supply of phosphorus and potassium.

4. Hardening of plants against sub-zero temperatures.

Increasing plant frost resistance is closely linked to hardening processes, which involve the continuous preparation of plants for exposure to low temperatures.

The THEORETICAL FOUNDATIONS OF frost resistance were established by Academician N.A. Maximov. Building on his research, I.I. Tumanov identified three stages of preparation for wintering in plants: the transition to a dormant state and two stages of hardening.

The first phase occurs under sufficient light and low positive temperatures during the night hours. In this phase, light is essential not only for photosynthesis but also for maintaining cellular ultrastructure. Under these conditions, photosynthesis still occurs, but growth and respiration processes are significantly inhibited. Consequently, sugars produced during illumination accumulate within the cell. The duration of the first hardening phase is approximately two weeks. After the first phase, plants are capable of withstanding temperatures down to -10 °С without damage.

The second phase does not require light and occurs as temperatures gradually decrease. It is accompanied by the release of excess water from cells and plant tissues into extracellular ice. This leads to the convergence of molecules within the cytoplasm, an increase in its density, and a slowing of the thermal motion of molecules in space. As a result, plants are able to withstand the dehydration of macromolecules and membrane structures. The second phase also lasts about two weeks. After the second phase of hardening, plants can withstand temperatures down to -20-25⁰С.

However, the hardening process is reversible. As temperatures rise, the process proceeds in the opposite direction.

5. Winter hardiness of plants.

During winter, weather conditions—such as temperature, precipitation, and humidity—change significantly. Severe frosts alternate with short-term or prolonged thaws. Blizzards are frequent, and in snowless winters, dry winds occur. This constant flux of weather factors significantly exhausts the plant organism and can even lead to its death. In unfavorable years, the loss of winter wheat crops can sometimes reach 70-80%. Not only winter cereals perish, but also perennial grasses, especially clover and alfalfa. Woody species are no less affected by frosty winters than winter crops. The complex of unfavorable winter factors can cause phenomena such as smothering, soaking, heaving, and winter drought.

Smothering of plants occurs during warm winters with deep snow cover, especially when snow falls on unfrozen, moist soil. Under these conditions, plants exhibit intense respiration, leading to a high expenditure of stored nutrients, primarily sugars. Exhausted by such conditions, plants perish in the spring from starvation and late frosts.

Soaking is observed during prolonged thaws when large amounts of water accumulate on the soil surface, flooding the crop, impairing Gas Exchange in the plant, and leading to increased Anaerobic respiration, the toxic products of which cause poisoning and death.

If frosts occur after a thaw, an ice crust forms, which can be suspended above the crop or in direct contact. Suspended crusts are easily broken by rollers, allowing plants to be released. If the crust is in contact, the plants freeze into the ice; aeration ceases, and the plant loses its resilience. If only the tillering nodes are frozen in while the leaves remain in the air, air can penetrate from the leaves to The Root System through intercellular spaces, and the plants survive.

Plants emerging from under the snow in spring at low air and soil temperatures are resistant to flooding. As temperatures rise, their resistance drops sharply.

Heaving is observed in the absence of snow cover or when the topsoil is dry. In such cases, soil freezing begins from deeper, moist layers. The ice layer formed at a certain depth gradually thickens due to moisture from even deeper horizons and pushes the topsoil upward along with the plants, causing their root systems to rupture. The plants are left on the soil surface. To prevent their death, timely rolling in the spring is necessary to re-establish contact between the root system and moist soil.

Winter drought occurs during periods of constant strong winds in winter, especially when precipitation is scarce. This leads to significant dehydration of the soil. Plants suffer from both a lack of moisture and soil overcooling.

Thus, winter hardiness is the ability of plants to withstand a complex of unfavorable environmental factors during the winter period. To determine winter hardiness, both field and laboratory methods are used. Among field methods, the most common is the Diagnosis of the state of overwintering plants in extracted soil monoliths. Laboratory methods include determining winter hardiness based on the staining reaction of the cytoplasm in the Cells of the growing cone (staining intensity increases when the organism is damaged).

Plant resistance can be improved by applying various agrotechnical measures, including the liming of acidic soils. Timely liming and Fertilization reduce the pH of the soil solution, improve the ionic balance in tissues, and thereby contribute to increased winter hardiness. During winter, snow retention measures should be implemented.

Soaking is the death of winter crops, perennial grasses, and orchards due to the stagnation of meltwater, rainwater, or floodwaters in fields. Death is the result of poisoning by toxic substances formed under anaerobic conditions, the disruption of nutrient and water uptake, and the death of the root system.

6. Lodging of plants, its causes, and methods of prevention.

The problem of crop lodging has become particularly acute due to the intensification of agriculture and crop production, and the sharp increase in The Use of organic and mineral fertilizers. 30-60% of sown areas are subject to lodging. Winter cereals are the most frequently affected.

In lodged crops, In addition to yield loss, grain quality (test weight, vitreousness, flour yield, etc.) significantly deteriorates, harvesting and grain drying become more difficult, and labor productivity decreases.

Plant lodging is observed when the ratio between the mass of the above-ground part of the plant and the strength of the lower part of the stem is disrupted. This phenomenon is caused by insufficient thickening of the straw and weak development of mechanical elements within it. When lodging occurs, the plant's geotropic response is disrupted, and stem etiolation takes place.

The primary factors inducing plant lodging include excessive soil and air moisture, high crop density, over-fertilization, strong winds accompanied by rain, and the morphological traits of the plant variety (long-stemmed forms are more prone to lodging than short-stemmed ones).

Many modern high-yield wheat varieties are capable of producing 70-80 centners/ha or more under optimal agricultural conditions; however, in practical field settings, yields are typically 40-50 centners/ha. One significant reason for this is lodging. This phenomenon occurs not only in regions with sufficient moisture but also in semi-arid climates. In the Forest-Steppe and Polissia regions of Ukraine, lodging is observed almost every year. The severity of this issue depends on both meteorological conditions and specific varietal characteristics.

Plants that maintain a consistent accumulation of dry matter and reserve CARBOHYDRATES in the stem throughout the growing season can exhibit resistance to lodging.

Combating lodging in any specific case requires appropriate agronomic measures: proper soil cultivation; correct seed sowing depth; optimal seeding rates and plant stand density; balanced mineral nutrition; and the Selection of lodging-resistant varieties.

Alongside measures to prevent lodging, plant growth retardants are widely used to shorten and thicken stems, enlarge leaf blades, and enhance root growth. The most common retardants include chlormequat chloride (CCC) and its analogs, such as Dehydrel and Kompazan. The effectiveness of these retardants depends on the application of chemical crop protection agents, nitrogen fertilizers, and their specific management systems.

The development of short-stemmed varieties is considered a radical approach to controlling lodging. However, this trait in winter wheat is often associated with poor winter hardiness, underdeveloped root systems, lower grain quality, and reduced resistance to fungal diseases.

7. The effect of maximum temperatures on plants. Heat resistance.

Heat resistance is the ability of plants to withstand high temperatures. Based on this trait, plants can be broadly divided into 3 groups: 1) primarily lower

plants (thermophilic Bacteria, blue-green Algae) that can endure temperatures of 75-90 °С without damage; 2) heat-tolerant plants of arid zones (succulents that withstand temperatures up to 60 °С and xerophytes up to 54 °С); 3) heat-sensitive plants, including mesophytes and aquatic plants, which can withstand temperatures up to 40 °С.

Plants growing in dry, sunny, and well-warmed locations are generally more resistant to high temperatures than shade-tolerant ones.

Heat resistance depends significantly on both the absolute temperature values and the duration of exposure. Short-term exposure to extreme temperatures (43-45°) can be just as lethal as prolonged exposure to slightly lower temperatures that exceed the optimum range.

High air temperatures reduce leaf area and photosynthetic activity. Virtually all generative cells undergo structural changes, losing their activity and ability to divide. At high temperatures, pollen becomes sterile, and the germination of fertile pollen grains on the stigma is inhibited. This is one of the reasons for reduced wheat yields.

During heatwaves with high air humidity, The regulation of leaf temperature via Transpiration is limited. Under such conditions, exceeding the optimal temperature threshold leads to partial or complete Protein Denaturation, causing damage to the protein-lipid complexes of membranes. This results in the disorganization of many physiological processes.

Rising temperatures are particularly dangerous under intense solar radiation. Plants possess a range of adaptive mechanisms to protect against heat damage, such as transpiration, vertical leaf orientation, chloroplast phototaxis, lighter leaf coloration, protective bark layers, a cuticle layer, high cytoplasmic carbohydrate concentration, and others.

In field conditions, the degree of heat damage is influenced by a complex of environmental factors, including soil moisture deficiency. The most sensitive links are the Hill reaction and phosphorylation. The inhibition of these processes by high temperatures is accompanied by a decrease in cellular proteins, Nucleic Acids, chlorophyll, and chloroplast photochemical activity, as well as membrane disruption, dehydration, and impaired carbohydrate, nitrogen, and Lipid Metabolism. Consequently, short-term heat Shock above 45°С can suppress plant defense responses against viral and certain fungal infections.

The plant response to extreme temperature is determined by the Functional and Structural CHARACTERISTICS OF THE cells. Among the cytoplasmic changes caused by high temperatures, one should note the inhibition of cytoplasmic streaming, increased density and coagulation of the cytoplasm, nuclear changes, and loss of membrane semi-permeability. High temperatures induce pathological respiration and the formation of toxic substances in plants.

Heat resistance is largely determined by the plant's developmental stage: young, actively growing plants are less resistant than older ones or those in a dormant state. Therefore, high temperatures cause the most damage during early developmental stages. For example, during the tillering phase of wheat, spikelet differentiation occurs in the growing point. High temperatures inhibit this process, resulting in fewer spikelets per ear and fewer florets per spikelet, which leads to lower yields. For many plants, heat is particularly dangerous during flowering, as it causes floret sterility and the shedding of Ovaries.

Resistance varies among different plant organs: underground organs are generally less resistant, while shoots and buds are more resilient. During fruit ripening, high temperatures can even be beneficial, provided the plant can withstand them normally. Among tissues, cambial tissues are the most resistant.

To increase heat resistance, P.O. Genkel proposed treating the seeds of certain crops (sugar beets, carrots, tomatoes, melons) with a 0.2% calcium chloride solution before sowing. However, this measure does not always ensure a reliable effect. Hardening plants against high temperatures also fails to yield the desired results.

Methods used for diagnosing drought resistance are also applied to heat resistance. Measures to prevent the harmful effects of high temperatures include the Implementation of shelterbelts and artificial irrigation. For woody plants, trunk whitewashing is recommended.

8. Drought resistance of plants. Classification of plants based on water requirements.

Drought is understood as a prolonged lack of moisture. It is the most frequent adverse environmental factor, especially in the south and east of Ukraine. Drought leads to significant crop yield losses. Its harmfulness depends on the developmental phase and the duration of exposure. Drought causes the most damage during active growth and the formation of Generative organs.

The ability of plants to withstand long periods of moisture deficiency is called drought resistance. There are two types of drought: atmospheric and soil.

Purely atmospheric drought can be observed in spring when air temperature significantly exceeds soil temperature, which at that time is not yet sufficiently moistened by melting snow. In such cases, relative air humidity is quite low (within 10-20%). Atmospheric drought promotes increased transpiration. If excessive water loss by plants is not replenished by root uptake from the soil, a water deficit occurs, and plants begin to wilt.

Prolonged atmospheric drought leads to soil drought, which is more dangerous for plants. It is most often observed in mid-to-late summer. Moisture deficiency in the soil leads to damage to the root system and disruption of the entire plant's water regime, negatively affecting physiological and biochemical processes. In this state, hydrolytic processes intensify within cells: The breakdown of polymeric compounds, primarily Polysaccharides, and in more severe drought, proteins, occurs. This leads to the accumulation of harmful concentrations of ammonia, the disruption of cytoplasmic structures, and a decrease in the intensity of photosynthesis.

Prolonged drought suppresses Plant Respiration, disrupts nutrient transport, and halts growth processes.

Drought tolerance is a genetically determined trait closely linked to a plant's growing environment and its adaptation to water deficit. It reflects a plant's ability to withstand significant dehydration by maintaining a high water potential (suction pressure) within its tissues while preserving the functional activity of cellular structures, as well as through adaptive morphological Features of the stem, leaves, and reproductive organs. Drought-tolerant plants possess a high content of bound water, high cytoplasmic viscosity, and specific anatomical-morphological adaptations. They are characterized by xeromorphism: their upper leaves are smaller than the lower ones, featuring smaller cells, dense venation, high transpiration intensity, and high suction pressure.

The highest drought tolerance is observed in xerophytes—plants of arid habitats. They include several groups of plants that differ in their morphological and anatomical structure (cacti, succulents, narrow-leaved xerophytes such as wormwood and camel thorn; and rough-leaved xerophytes like steppe grasses, feather grass, sheep's fescue, and tumbleweeds).

Plants of temperate climates—mesophytes—are characterized by moderate drought tolerance. This group includes the main agricultural crops. Their resistance is due to adaptations such as the ability to regulate transpiration intensity via the stomatal apparatus, or by shedding leaves and even fruit ovaries. These plants typically have a well-developed root system and high tissue water-retention capacity.

Hygrophytes are plants of humid habitats that are intolerant to drought.

Hydrophytes are plants that live in an aquatic environment.

Different plant organs, like different plant species, exhibit varying levels of drought resistance. For example, young growing leaves are relatively more resistant than mature or old leaves due to the influx of assimilates. During prolonged drought, plastic substances can be translocated to young leaves from reproductive organs. Drought is particularly damaging during the formation of reproductive organs. In early developmental stages, it can lead to flower sterility, while in later stages, it reduces both the quality and quantity of the yield. Spring and winter cereals are most sensitive to moisture deficiency during the booting and heading phases; millet and sorghum during panicle emergence and grain filling; legumes during flowering; potatoes during flowering and tuber formation; sunflowers during HEAD formation and flowering; and melons during flowering and ripening.

Drought tolerance in plants is diagnosed using both field and laboratory methods. Field methods include the direct approach, where various plant species and varieties are grown in natural arid conditions to determine their degree of drought resistance. This method is reliable but time-consuming. Laboratory methods exist for artificially creating (simulating) moisture deficits. For example, plants can be grown in controlled environments where they are subjected to soil moisture deficiency or streams of dry, heated air. Indicators of drought resistance include the water-retention capacity of plant tissue, cytoplasmic density, and others.

To combat drought, irrigation is combined with the application of mineral fertilizers. Irrigation should be managed so that plants can utilize all the water without experiencing moisture stress. To determine the correct timing and volume of irrigation, it is necessary to monitor soil moisture (which should remain above the plant wilting coefficient), the state of leaf Stomata, and to measure tissue water potential, cell sap concentration, etc.

The best method of irrigation is sprinkling (overhead irrigation), which not only moistens the soil but also improves the microclimate.

9. Salt tolerance of plants and potential ways to enhance it.

Many soils contain excessive amounts of soluble salts that are harmful to Plant GROWTH AND DEVELOPMENT. This excess is mostly observed in zones with insufficient moisture, where soil leaching and salt removal processes are minimal. Salinization can also be caused by the rise of salt solutions from deeper soil horizons, creating high salt concentrations in the root zone. Similar salinization is promoted by the systematic application of high doses of mineral fertilizers, especially in the form of crude potash salts.

Excessive soil salinity leads to an increase in the Osmotic Pressure of the soil solution, making it difficult for the root system to absorb water. Furthermore, an excess of soluble salts is toxic to the plant.

Soils containing 0.25% of readily soluble salts are classified as slightly saline, while those with a concentration of up to 0.5% are considered moderately saline (solonchakous). Highly saline soils (solonchaks) contain over 0.5% salts.

Based on their reaction to soil salinity, plants are divided into halophytes and glycophytes. In natural conditions, A large number of species grow on saline soils, having adapted to high salt concentrations. This group, the halophytes, can grow in a nutrient medium containing 3-5% sodium salts. Among halophytes, three main groups of plants can be distinguished based on their Anatomical and physiological properties:

1) euhalophytes—salt-accumulating plants with fleshy stems and leaves, whose cells are characterized by a very high osmotic potential that exceeds the osmotic potential of the soil solution. Such plants freely absorb various cations and anions from saline soils;

2) crinohalophytes—salt-excreting plants. Their protoplasm is characterized by high permeability to salts, effectively filtering and passing them through. As a result, the salt content within the cells themselves remains constant.

Plants of this group possess specialized secretory cells—vesicular hairs on the leaves—in which salts accumulate. Once fully saturated with salt, these hairs rupture, leaving the salt on the leaf surface. New hairs grow in place of the dead ones;

3) glycohalophytes—salt-excluding plants, in which salt permeability through the Cell Cytoplasm is very low. Their cell sap has high osmotic pressure, driven by a high concentration of Organic compounds, especially carbohydrates. Such plants include wormwood and others.

According to A. Schimper's theory (1898), the harmful effect of high salt concentration is primarily manifested through the high osmotic pressure of the soil solution, which defines the "physiological" dryness of saline soils. Later, B.P. Strogonov and P.A. Genkel proved that the disruption of plant life processes in such soils is a consequence not only of osmotic pressure but also of the Toxic effects of salts. The degree of plant damage depends on the composition of ions in the environment. The HCO₃ anion is the most toxic, while chloride and sulfate anions are less toxic. Salt tolerance is primarily determined by The properties of the cytoplasm and its sensitivity to the toxicity of specific salts.

Most cultivated plants have low or no resistance to salinization.

The Physiological Effect of salinization manifests as changes in the osmotic properties of cells, the destruction of Cytoplasmic membranes, and a decrease in enzyme activity, which leads to a disruption in the relationship between photosynthetic and oxidative phosphorylation. Salinity causes disturbances in protein metabolism, resulting in the accumulation of free Amino Acids and the formation of toxic compounds (cadaverine, putrescine, ammonia).

In general, plants are least resistant to salinity during the Initial Stages of ontogenesis. Their resistance changes with age.

Based on the degree of salt tolerance, cultivated plants are divided into low-tolerance (wheat, buckwheat, flax, cucumbers, beans, apple, cherry), medium-tolerance (oats, millet, corn, sunflower, rye, potatoes, onions, carrots, tomatoes, grapes, alfalfa), and high-tolerance (barley, mustard, clover, cabbage, sugar beet).

Salt tolerance is determined by Direct and Indirect methods. This type of resistance can be assessed by seed germination energy, germination percentage, etc. Laboratory methods include determining The rate of stomatal opening and closing in salt solutions, the degree of chlorophyll bleaching, and others.

To combat soil salinity and mitigate its harmful effects, soil gypsuming and other chemical reclamation methods are employed.

One of the primary control methods is the development of salt-tolerant crop varieties. Only plants with an intensive metabolism of organic acids, sucrose, and aspartic and glutamic amino acids—which can neutralize ammonia through amide formation—are capable of adapting to high salinity levels.

P.O. Genkel proposed pre-sowing treatment of seeds for certain agricultural crops with solutions of NaCl, MgSO₄, and Na₂CO₃ salts to increase resistance to chloride, sulfate, and soda salinity, respectively. It is believed that this Procedure hardens the seeds against salinity, resulting in reduced permeability of cytoplasmic membranes and a significant increase in the threshold of toxic salt effects.

10. Plant resistance to atmospheric pollution, pesticides, and toxic chemicals.

The intensive development of industry and agriculture, along with active human Interference, is accompanied by significant environmental changes. Gaseous atmospheric pollution is increasing annually, and various chemical compounds are accumulating in soil and water. All of this hinders normal development, causes diseases, and even leads to the extinction of many rare PLANT AND ANIMAL species, resulting in ecological imbalance, reduced crop yields, and diminished product quality.

The combustion of oil, coal, and various organic wastes releases large quantities of Hydrocarbons, nitrogen and sulfur oxides, carbon dioxide, and other substances into the atmosphere, as well as solid components like dust and soot. Smoke and gases not only poison the environment but also affect the climate. Typically, in areas adjacent to large industrial facilities, air humidity and light levels are below normal, while temperatures are higher than in non-polluted areas.

Industrial pollution of the environment with toxic compounds causes significant damage to agriculture and the ecological system as a whole.

To develop preventive measures for plant protection based on monitoring air, soil, and water pollution, it is necessary to clearly understand the toxic effects of pollutants on the plant world, the functional changes they may induce, and the pathways through which they enter the plant organism.

Sulfur dioxide, nitrogen oxides, hydrogen halides, and other harmful gaseous compounds penetrate plants from the air through gas exchange, as well as via rainfall and the deposition of fog or dust On the surface of shoots. The bulk of toxic gases enters the leaf through the stomata. Upon reaching the intercellular spaces, they come into contact with the spongy mesophyll parenchyma, then diffuse through the Plasmalemma into the cell protoplast, where they trigger various chemical, biochemical, structural, and functional changes. The rate of gas uptake is determined by the plant's resistance, while the degree of impact depends on the properties and chemical forms of the pollutant.

In the cells of damaged plants, a decrease in the pH of cell sap is observed, Enzymes such as peroxidase are noticeably activated, carbohydrate-Nitrogen metabolism is disrupted, cytoplasmic streaming and cell elongation are inhibited, chlorophyll degradation and chloroplast structure destruction occur, and photosynthesis is depressed; furthermore, the permeability and regulatory activity of stomatal guard cells are impaired. All these changes subsequently lead to disrupted Plant Growth and development, reducing the intensity of transpiration by 1.5–2.0 times. As a result, crop yields are significantly reduced and their quality deteriorates. In winter crops, frost resistance decreases sharply.

Toxic gases also adversely affect the Development of the root system, as they significantly reduce total root mass and physiological activity.

General phenotypic signs of plant damage include necrosis and chlorosis of leaves, followed by their death and premature shedding. However, it is difficult to identify the specific toxicant based on these external signs alone, as the overall pattern of changes is quite non-specific to any particular toxic substance. The effect of a toxicant depends on its dose, which is the product of the toxicant's concentration and the duration of exposure.

For SO₂, the permissible concentration in the air should not exceed 80 µg/m³ for long-term exposure and 240 µg/m³ for short-term exposure; for hydrogen fluoride, 0.50 µg/m³ and 1.0 µg/m³, respectively; and for HCl, 100 µg/m³ and 200 µg/m³. However, HCl does not pose a major threat, as this pollutant is found in concentrations that cause plant damage only in isolated areas. The toxicity of gases depends on environmental factors, especially air humidity. For example, at high concentrations of SO₂ in the air, increased humidity leads to the formation of sulfurous and subsequently sulfuric acid, which sharply increases the toxicity of SO₂. Gaseous sulfur (SO₂) that has penetrated through the stomata contacts the moist surface of leaf cells and transitions into an aqueous solution. Depending on the pH of the cell solution, sulfites, bisulfites, and Other Compounds are formed. SO₃^2- and HSO₃^- are toxic to many biochemical and physiological processes. The degree of phytotoxicity decreases upon the transition of SO₃ to SO₄^2-. This oxidation in plant cells can occur through enzymatic and non-enzymatic pathways. In most plants, metabolically active young leaves accumulate more sulfur than older ones. Accumulation occurs faster in light than in darkness. The oxidation of SO₃^2- can take place in chloroplasts under light or in mitochondria. Furthermore, it is stimulated by cellular enzymes such as peroxidase, cytochrome oxidase, ferredoxin-NADP-reductase, as well as by metals and ultraviolet light.

Sulfur dioxide adsorbed by leaves does not remain localized at the entry points. It has high mobility and can be transported from leaves to roots and into the surrounding environment. SO₂ adsorbed by leaves can be utilized in metabolic sulfur reduction processes.

Short-term exposure to low concentrations of sulfur dioxide leads to an enhancement of photosynthetic processes in many plants. Conversely, high concentrations and prolonged exposure to low concentrations inhibit photosynthesis. This effect manifests in carboxylation reactions, electron transport, and photosynthetic phosphorylation reactions.

When dicotyledonous herbaceous plants and broad-leaved trees are damaged, the appearance of necrotic areas between leaf Veins is typical; in monocotyledonous plants, a faint yellowish color or ivory-colored necrosis occurs, starting at the leaf tips and spreading across the entire leaf blade.

Necrosis of leaf tips and margins is a typical symptom of fluoride damage in broad-leaved plants. Necrotic tissue may be separated from healthy tissue by a narrow red-brown band formed due to the deposition of resins and Tannins.

Numerous studies have proven that atmospheric pollutants lead to a certain suppression of plant growth and development even without visible symptoms of damage. Their effect manifests primarily at the biochemical level, then spreads to the ultrastructural and cellular levels, and only thereafter do visible symptoms of damage develop.

Plant sensitivity to individual harmful gases depends not only on external but also on internal factors. For example, cereal crops have increased resistance to SO₂ at certain critical Stages of development—the three-leaf stage and before flowering. For dicots, the period between flowering and the onset of ripening is critical.

Substances and compounds such as ozone, nitrogen oxides, and fluorides have a significant impact on physiological processes in plants.

Under The Influence of ozone, the permeability of plant tissues to water, glucose, and individual ions changes, as does the permeability of Mitochondrial and Chloroplast membranes; photosynthesis is inhibited, with a decrease not only in The activity of The electron transport system but also in chlorophyll content.

The effect of gaseous NO and NO₂ compounds at concentrations that do not lead to visible damage causes a decrease in the intensity of photosynthesis. Inhibition of photosynthesis by nitrogen oxides may result from the competition between CO₂ assimilation and nitrate reduction processes, which occur in chloroplasts using the universal reductant NADPH. Under the influence of NO, chloroplast membranes swell, and any disruption in Membrane Structure affects the intensity of photosynthesis.

Gaseous fluorides (HF) enter the plant through leaf stomata and can affect their functioning. Fluorides can influence CO₂ fixation, weaken chlorophyll synthesis, inhibit Hill reaction activity, and stimulate or suppress respiration depending on the duration of exposure. Changes in respiration under the influence of fluorides occur due to the Swelling of mitochondrial membranes and the leakage of proteins from them, as well as changes in the activity of oxidative enzymes. NaF disrupts ribosomal systems.

Substances that pollute the atmosphere reduce plant resistance to low temperatures and drought.

Significant soil and water pollution is facilitated by the improper use of chemical agents in controlling agricultural pests, diseases, and weeds. Some pesticides do not decompose for several years or even decades and therefore can accumulate in soil and water, eventually entering food products.

Indeed, as early as the 1940s, highly effective chlorinated hydrocarbons were widely used to combat certain insect species. Hundreds of thousands of tons of DDT were applied over vast areas; the substance proved resistant to degradation, accumulating in large quantities in the soil. From there, it was leached by groundwater into water bodies, absorbed by plants and animals, and eventually entered the human food chain, causing severe poisoning. While the use of DDT has long been banned, its lingering presence in water and soil remains a persistent environmental concern.

The adaptation of many pest species to toxic agents necessitates the continuous search for and synthesis of new, more effective chemical compounds to control insects, weeds, and various agricultural plant diseases. Consequently, soil and water are subject to constant contamination by these new chemicals, which include not only pesticides and insecticides but also certain growth regulators. Many of these compounds are poorly soluble in water, yet they are relatively easily absorbed by living organisms, where they accumulate. This process of bioaccumulation also facilitates the buildup of radioactive isotopes of certain biologically essential elements.