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

Lecture Notes

TOPIC No. 6. MINERAL NUTRITION OF PLANTS

Outline

1. Development of the theory of Plant Mineral Nutrition.

2. Research Methods in plant mineral nutrition.

3. Mechanisms of mineral Nutrient uptake by plants.

4. Concept of macro- and microelements.

5. Plant-available forms of phosphorus and sulfur compounds and their role in METABOLISM.

6. Physiological Role of K, Ca, M_>g, and other elements.

7. Physiological role of microelements.

8. Nitrogen nutrition of plants. Nitrate assimilation.

9. Diagnostics of mineral nutrition.

10. Ion Transport of mineral elements in plants.

11. Physiological principles of fertilizer application.

1. Development of The Theory of plant mineral nutrition.

Mineral nutrition is the process by which plants absorb essential mineral elements and incorporate them into their metabolic pathways. Higher plants primarily absorb these elements through their ROOT system, which is why this process is also referred to as root nutrition. It serves as a fundamental factor in regulating plant growth, development, and productivity.

Mineral nutrition is made possible by the autotrophic nature of plants, i.e., their ability to utilize inorganic substances to build their Tissues and Organs. The Study of mineral nutrition has a long and rather contentious history.

Until the discovery of Photosynthesis at the turn of the 19th century, the prevailing belief was that plants constructed their bodies from "earth juices." This view was based on centuries of agricultural experience regarding the vital role of soil in crop production. As early as the 6th–5th centuries BC, manure, ash, and river silt were used to improve soil fertility and crop yields when growing rye, wheat, barley, corn, flax, and various vegetables.

Aristotle (384–322 BC) believed that the soil functioned as a Stomach and Digestive System for the plant, providing it with food in the form of complex substances.

In the late 18th and early 19th centuries, the German agronomist A. Thaer expanded upon Aristotle's interpretation to develop the "humus theory."

In 1563, the French naturalist Palissy presented compelling arguments regarding The Role of fertilizers and Mineral Substances, suggesting that manure would be of no value if it did not contain salt, which remains after its mineralization.

Nearly 100 years later (in 1656), the English chemist Glauber demonstrated through his experiments the positive impact of saltpeter on increasing the yields of various plants.

Original insights into plant nutrition were provided by M. V. Lomonosov, who emphasized Structure/19.html">The Importance of both soil and atmospheric nutrition.

In 1837, the French agrochemist J. B. Boussingault proved through his experiments that plants could be grown in pure sand, provided that mineral salts (ash and saltpeter) were added.

German chemist Justus von Liebig developed the theory of mineral plant nutrition, which posits that soil fertility is fundamentally based on mineral substances. He believed that plants were capable of absorbing nitrogen from the air in the form of ammonia. Furthermore, Liebig rejected the humus theory, dismissing the role of soil as a source of organic matter for Plant GROWTH AND DEVELOPMENT. He was the first to formulate the "law of the minimum," which states that crop yield is limited by the factor that is in the shortest supply. Liebig argued that to prevent soil depletion and maintain fertility, one must adhere to the "law of return," which mandates replenishing the nutrients absorbed by plants. K.A. Timiryazev once noted that The Doctrine of the necessity of nutrient return is one of The most significant achievements of science.

Key tenets of the theory:

· Roots absorb the elements necessary for plant nutrition from the soil in mineral form.

· THE PRINCIPLE OF the indispensability of mineral nutrients, as each element performs a specific physiological role.

· The uptake of mineral nutrients occurs in regulated amounts.

The experiments of W. Knop and J. Sachs were of great importance for The Development of the theory of mineral nutrition, as they proved the possibility of growing plants to full maturity in Water cultures by providing them with nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and iron.

Our compatriots, academicians D.N. Pryanishnikov and P.A. Vlasyuk, made significant contributions to both the theory and practice of mineral nutrition.

2. METHODS FOR STUDYING plant mineral nutrition.

· Laboratory research method.

· Vegetation research method.

· Field research method.

· Production research method.

Laboratory method - involves The Use of various substrates for plant cultivation. Nutrient solutions were developed under laboratory conditions to study The Effect of specific nutrients on plants.

Knop's solution:

Ca(NO₃)₂ - 1 g/L;

KH₂PO₄ - 0.25 g/L;

MgSO₄ - 0.25 g/L;

KCl - 0.125 g/L;

FeCl₃ - traces.

There are also Hellriegel's and Pryanishnikov's solutions.

Certain requirements must be met when preparing nutrient solutions:

· All mineral nutrient elements must be balanced.

· No element can be replaced by another.

· Elements must be present in specific ratios.

· The pH should be 6.5-7.

There are physiologically acidic, alkaline, and neutral salts. From acidic salts, the cation is primarily absorbed (HCl), while from alkaline salts, the anion is absorbed (NaNO₃). From neutral salts, both the cation and the anion are absorbed (NH₄NO₃).

Vegetation experiments are conducted under controlled conditions. The substrate is placed in a container, nutrients are added, plants are planted, and their development is monitored.

Field experiments are conducted under field conditions on an experimental plot.

Production experiments are carried out under industrial production conditions.

3. Mechanisms of mineral nutrient uptake by plants.

The uptake of nutrients by a plant depends on their delivery to the root surface. There are three ways in which contact between nutrients and The Root System is ensured.

The first method is called root interception, where the root system, through continuous growth, occupies increasing volumes of soil containing nutrients, intercepting and absorbing them.

The second is the mass flow of nutrients, which move in the form of soil solution along with water toward the root surface to be absorbed.

The third method is the movement of nutrients toward the roots along a concentration gradient.

When mineral elements are absorbed near the root surface, their concentration decreases. This creates a concentration gradient, which causes new ions to move toward the root system.

All three methods provide the root system with nutrients and facilitate the mobilization of mineral substances in the soil. The mode of movement of an element in the soil solution depends on the ionic form in which it is absorbed by the plant. For example, calcium and magnesium (Са⁺⁺, Мg⁺⁺) reach the root system mainly through mass flow and root interception; K⁺ cations move via diffusion and mass flow; while РО₄³ anions move solely through diffusion.

Today, scientists believe there are several mechanisms for nutrient uptake into The Cell:

1) simple diffusion of a substance through membrane pores along a concentration gradient;

2) passage of a dissolved substance through membrane pores with the solvent flow;

3) diffusion of lipid-soluble substances through the lipid phase of the membrane;

4) Facilitated Diffusion, where substances penetrate the membrane as a complex with membrane carriers;

5) exchange diffusion, where molecules and ions of the external environment and the Cytoplasm are exchanged (via complex formation with membrane carriers) for molecules and ions of the same type, meaning their concentration in the cell remains unchanged;

6) Active Transport, which requires Energy Expenditure to move substances across the membrane;

7) pinocytosis.

Initially, a substance approaches the membrane, interacts with its components in a specific way, and penetrates it. Once inside the cell, the substance is integrated into metabolic processes.

Depending on The Nature of energy expenditure for the Uptake and Transport of mineral nutrients, Two Types of substance entry are distinguished: active and passive. The former is accompanied by the expenditure of metabolic energy and occurs against a concentration gradient. The latter occurs without metabolic energy expenditure. Passive absorption follows the electrochemical activity gradient.

It is believed that There are two unified transport systems in a plant. The first (passive absorption and transport) consists of a continuous hydraulic system of free space (the apoplastic pathway); the second (active absorption and transport) includes the cell protoplasts connected to each other by plasmodesmata (the symplastic pathway).

4. Concept of macro- and microelements.

For the normal Life Cycle of a plant Organism, the following elements are essential: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, copper, zinc, molybdenum, boron, sodium, silicon, cobalt, chlorine, and

others. Many of these are purely mineral and are absorbed by plants primarily from the soil solution in the form of ions. Elements such as carbon, hydrogen, and oxygen enter the plant mainly in the form of СО₂, Н₂О, and О₂.

Almost all organic matter necessarily contains four elements—carbon, hydrogen, oxygen, and nitrogen—known as organogens. The dry matter of plant bodies, remaining after prolonged drying at 102-105 °C, consists of nearly 50% carbon. Oxygen accounts for 42%, hydrogen for 6.5%, and nitrogen for 1.5% of their mass. The remaining elements, belonging to the ash group, account for an average of about 5%.

Upon combustion of plants, organic elements are released into the atmosphere in gaseous form as corresponding compounds: H₂O, CO₂, H₂S, and NH₃, while the remainder, in the form of oxides, constitutes the mineral part of the plant, known as ash. Ash content varies across different organs and parts of plants. It is lowest in wood—about 1%, in roots and stems—4-5%, in leaves—10-15%, in flowers—15-20%, in bark—about 7%, and in seeds—3%. The amount of ash also differs among various plants. For example, in potato leaves it is 5-13%; in beets—11-12%; in turnips—8-15%. The composition of mineral elements in plants is determined by growing conditions, age, and developmental phase.

All chemical elements are divided into three groups based on their quantitative content, as proposed by Vernadsky.

Ten of them, which are required in the largest quantities (from 10% to 0.01% of the plant's dry mass), belong to the group of macroelements. These elements include: carbon, hydrogen, oxygen, nitrogen, potassium, phosphorus, magnesium, calcium, and sulfur.

Elements contained in plants in significantly smaller amounts (from 0.001% to 0.00001%) make up the microelement group. Among them are iron, boron, manganese, zinc, molybdenum, cobalt, and several others.

The third group consists of ultramicroelements. Their content in plant ash does not exceed 10⁶-10^-12%. These include lead, silver, lithium, mercury, arsenic, and others.

Such a division of elements into these three groups is quite arbitrary, as their quantities can vary significantly. A Classification based on their biological significance and physiological Functions is more appropriate.

Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur are the primary components of organic matter, while elements such as potassium, calcium, magnesium, manganese, and chlorine perform regulatory roles, participating in osmotic regulation, electron balance, and membrane permeability.

5. Forms of phosphorus and sulfur compounds available to PLANTS AND THEIR role in metabolism.

Phosphorus. It belongs to the organogenic elements. In the form of phosphoric acid residues, phosphorus is a constituent of such structural substances as Nucleic Acids, Nucleoproteins, and phosphatides, as well as certain storage substances, intermediate metabolic products, and high-energy compounds—ribulose diphosphate, phosphoglyceric acid, phosphoglyceraldehyde, and adenosine phosphates (AMP, ADP, ATP, etc.). Phosphorus is a component of Coenzymes involved in photosynthesis and Respiration (NAD, FAD, NADP, CoA, etc.).

Structural phosphorus compounds play a special role in building nuclei (as part of DNA) and Introduction/36.html">Biological Membranes. Phosphorus compounds are directly involved in Energy Metabolism during physiological processes such as the uptake and transport of mineral nutrients, and the transformation and translocation of storage Organic compounds.

Sugar phosphates are present in all Plant Tissues and play a leading role in the processes of sugar transformation during respiration (e.g., Glycolysis), the Dark Phase of photosynthesis, and other metabolic processes.

Phosphorus is absorbed by roots from the soil in the form of the higher oxide PO₄³. Along with this form, plants can also take up HPO₄² and H₃PO₄ ions. Phosphorus nutrition is of great importance during the initial period of growth and development, as it accelerates these processes and promotes better nutrient assimilation and moisture absorption. Mineral phosphates, which accumulate in the cell sap, participate in The formation of Buffer solutions that regulate the cell's pH level. A specific property of phosphorus is its ability to accelerate the reproductive Development of Plants and shorten the growing season. Phosphate metabolism in plants depends on many conditions. In the absence of water, the processes of photosynthetic and Oxidative Phosphorylation are disrupted. Nitrogen starvation leads to a decrease in phosphorus uptake by the root System and Its translocation to above-ground organs. Low temperatures reduce the intensity of phosphorus absorption and metabolism, causing stunted growth and reduced plant productivity.

The phosphorus requirements of different plants vary. In particular, cereals absorb more phosphorus than legumes.

Sulfur. The Physiological Role of sulfur as an organogenic element is due to its presence in the form of sulfhydryl (R - SH) and disulfide (R - S - S - R) groups within important structural compounds: certain Proteins, Enzymes, Vitamins, and several physiologically active substances.

Sulfur enters plants primarily in mineral form—as sulfate ions—but can also be absorbed as part of Certain Amino Acids.

The special significance of sulfur lies in its role as a component of coenzyme A, which enables its diverse and high activity due to the thioester bond. At the site of this bond, an acetyl group can be attached to form acetyl-CoA (CH₃CO~S-CoA), which participates in the fundamental reactions of lipid Biosynthesis and the Condensation of monomeric molecules into polypeptide chains. It serves as a hub for carbohydrate and Fat Metabolism in the cell.

Sulfur is a component of the essential amino acid Methionine, which in turn is a constituent of the Active Site of many enzymes. It is the methionine residues within a protein molecule that determine its hydrophobic properties. Sulfur determines the activity and direction of many metabolic processes. Within The amino acid Cysteine, it exists as a sulfhydryl group and can serve as a reserve for the formation of high-energy bonds. Sulfhydryl groups act as activators for A number of Proteolytic Enzymes. In redox processes, the reaction of converting sulfhydryl groups to disulfide groups and vice versa plays an important role.

Furthermore, sulfur is found in the Essential Oils of the Brassicaceae family.

In the soil, sulfur exists in both organic and inorganic forms. The organic form, consisting of PLANT AND ANIMAL residues, predominates. The inorganic form is primarily found as calcium, magnesium, and sodium sulfates. It is this form of sulfur (SO₄²) that is available to plants. Less oxidized (SO₂) or more reduced (H₂S) forms are toxic to plants.

Sulfur absorbed as sulfates is reduced within the plant and assimilated during Protein Synthesis.

Sulfur metabolism in plants is linked to Nitrogen metabolism. Under conditions of excessive nitrate nitrogen nutrition, The oxidation of sulfhydryl groups is enhanced, leading to a disruption of Protein metabolism.

The industry does not produce specialized sulfur fertilizers. Sulfur is included as an additive in phosphorus, and partially in potassium and potassium-magnesium fertilizers. Elemental sulfur is sometimes used.

6. Physiological role of K, Na, Ca, Mg, and other elements.

Potassium. It is assimilated from the soil in the form of K⁺ ions. In plant organisms, it exists in an unbound state.

Physiological role:

· Participates in photosynthesis, specifically in electron transport within the ETC.

· Contributes to the release of O₂.

· Participates in Photophosphorylation.

· Promotes Hydration of the cytoplasm.

· Involved in stomatal opening.

· Acts as a coenzyme for over 60 enzymes, influencing carbohydrate and protein metabolism.

· Capable of remobilization.

· Enhances resistance to adverse environmental conditions.

· Found in highest concentrations in young plants and developing tissues.

Sodium. Under potassium deficiency, sodium exerts a positive effect on the growth and development of certain plant species. Significant amounts of this element can be accumulated by halophytes growing in saline soils. Sodium is capable of replacing potassium in the soil exchange complex, thereby increasing the latter's availability to plants.

The physiological role of sodium is sometimes analogous to that of potassium. For instance, both elements are interchangeable in their effect on cellular osmotic properties, meaning sodium also promotes cytoplasmic hydration. However, while potassium activates enzyme systems by facilitating the assembly of protein subunits, sodium, conversely, stimulates their dissociation. It Supports osmotic properties, water uptake, and stimulates cell elongation during growth. "Natriophiles" include beets, alfalfa, cabbage, and tomatoes. "Natriophobes" include corn, rice, buckwheat, and soy.

In the vast majority of plants, sodium is not considered an essential element. Nevertheless, as noted, its presence in the soil has a positive effect on the mobility and uptake rate of potassium and enhances certain physiological processes. Excess levels of bioavailable sodium in the soil are harmful, as they disrupt the necessary cation balance within the plant.

Calcium. Calcium is a vital element for mineral nutrition. One of its primary properties is The ability to influence physicochemical parameters of the cytoplasm, such as density and permeability. Calcium acts as an antagonist to monovalent cations, especially hydrogen. Beyond its regulatory role, it serves a structural function, forming The basis of the middle lamellae that cement the cell walls of adjacent Cells.

This element influences the state of biocolloids and their degree of hydration. It accumulates in older plant tissues and organs in the form of oxalates, binding oxalic acid, which is highly toxic to the cytoplasm in its free state. Thus, in this context, calcium AIDS in detoxifying oxalic acid. Simultaneously, this fact indicates that calcium cannot be transported from older organs to younger ones, as all salts formed by calcium with organic acids are water-insoluble. The transport of free Ca²⁺ ions occurs via the xylem conducting system. Calcium content is higher in above-ground plant organs than in underground ones. This element is a component of cellular organelle complexes—nuclei, Chloroplasts, and Mitochondria—in the form of inorganic phosphates and ions. By interacting with negatively charged phospholipid ions, calcium stabilizes membranes and reduces their passive permeability. It is a component of phytin and Pectins.

Almost all reactions activated by potassium are inhibited by calcium. At the same time, calcium activates certain enzymes, such as electron-carrier enzymes in the photosynthetic Electron Transport Chain.

Not capable of remobilization.

Magnesium. Like calcium, this element enters the plant as a cation (Мg²⁺), but unlike calcium, it accumulates primarily in young plant organs and tissues. Magnesium is a constituent of the chlorophyll molecule; therefore, photosynthesis cannot occur without it. It is a multifunctional element. Many of its functions are similar to those of potassium and calcium. It is believed that magnesium facilitates the uptake and translocation of phosphorus within the plant. Along with phosphorus, it is a component of phytin and thus participates in cellular energy metabolism. Magnesium is also present in pectic substances. It plays an important role in Carbohydrate Metabolism, activates phosphatase enzymes involved in the synthesis and Hydrolysis of glucose-phosphate compounds, and serves as a cofactor for many enzymes, including those in The Calvin Cycle.

Magnesium links the large and small ribosomal subunits and maintains their functional activity.

7. Physiological role of micronutrients.

Micronutrients are characterized by high Specificity and essentiality. Through their participation in enzymatic reactions, they influence metabolism, accelerate plant development, and increase resistance to fungal and bacterial diseases, as well as to adverse external factors (drought, low or high air and soil temperatures).

Micronutrients are Components of the prosthetic groups of enzymes and control vital processes such as photosynthesis, respiration, metabolic transformations, and others. In other words, micronutrients are activators of activators, i.e., enzyme activators. Despite the similarity in the functions of micronutrients, each is characterized by a specific mode of action.

Iron. It has the ability to change valence, which allows it to transport electrons and serve as a component of various enzymes (oxidases, catalases). In the cytochrome system, it participates in chlorophyll synthesis and activates chlorophyll synthase enzymes. Deficiency manifests as chlorosis.

Albinism - a genetic disorder of chlorophyll synthesis.

Etiolation - the failure of chlorophyll formation due to the absence of light.

Boron. While this element has not been identified as a direct component of enzymes, its involvement in numerous enzymatic processes is well-established. It exerts a positive influence on Cell Division, as well as carbohydrate and protein metabolism. Boron is most abundant in the parenchyma tissue of young plant leaves. It facilitates NUCLEIC ACID METABOLISM and influences the differentiation and division of tissues into xylem and phloem elements, making it particularly critical during the Cytology/cytology/16.html">Early stages of plant ontogenesis. Furthermore, it is essential for the formation of root nodules in legumes.

Boron is immobile and cannot be translocated from older plant organs to young, growing tissues. Consequently, boron deficiency primarily manifests as the disease and dieback of the apical parts of plant stems.

Copper. Copper is directly involved in photosynthesis and respiration. Significant amounts of this micronutrient are concentrated in chloroplasts as a component of plastoquinone, which plays a vital role in electron transport during photosynthesis. Additionally, copper is a constituent of several oxidases, such as polyphenol oxidase and cytochrome oxidase. It protects chlorophyll from degradation and accelerates The biosynthesis of Tryptophan.

According to the concept proposed by B.A. Rubin, by forming part of a wide range of respiratory enzyme systems, copper provides plants with resistance to adverse environmental factors, including Temperature fluctuations and phytopathogens, and enhances Salt Tolerance.

Zinc. Zinc is a component of various enzymes and plays an active role in redox processes; it enhances respiration and stabilizes it under high temperatures. It is also involved in carbohydrate and protein metabolism and positively influences the formation of Auxins and chlorophyll. When adequately supplied, zinc can bind with sulfur-containing and Other Amino Acids, such as cysteine, Histidine, tryptophan, Lysine, and Serine. Zinc regulates DNA and RNA content in cells, boosts photosynthetic activity, and facilitates the uptake of nitrogen, potassium, silicon, and manganese by plants.

Molybdenum plays a significant physiological role in the synthesis, transformation, and translocation of CARBOHYDRATES within plants. By exerting a major influence on the development of Azotobacter in the soil, it plays a notable role in the biological Fixation of Atmospheric nitrogen and in the nitrogen metabolism of the plant organism. Molybdenum is a component of nitrate reductase and participates in the reduction of nitrates in the root system and leaves. A deficiency in molybdenum leads to a decrease in total and protein nitrogen content and slows down nitrate reduction, thereby inhibiting Amino acid synthesis. Molybdenum is also an essential metal component of enzymes involved in tissue respiration.

The mobility and bioavailability of soil molybdenum depend not only on its total content but also on the degree of soil cultivation and the use of fertilizers. Physiologically acidic fertilizers, especially when applied to unlimed, low-buffer sandy soils, exacerbate molybdenum deficiency. An excess of heavy metals in the soil—such as lead, copper, zinc, and others—has a similar effect.

Manganese. There is a prevailing view that the supply of manganese to plants determines the growth and morphogenesis of plant organs. A deficiency of this element leads to a delay in the cell elongation phase, particularly within the root system.

Research has revealed The Influence of manganese on the transport of indole-3-acetic acid and on seed germination. It activates the movement of this compound from seeds to sprouts and promotes the formation of "indole-3-acetic acid-deoxyribonucleotide" complexes, which influence plant morphogenesis.

During seed germination, manganese facilitates hydrolysis and the translocation of phosphorus compounds from the seed to the stem and roots, accelerating the biosynthesis of organic phosphorus compounds in the plant. Its deficiency causes a significant deficit of indole-3-acetic acid and disrupts the transport of phosphorus to the aerial PARTS OF THE plant.

The level of manganese supply and the intensity of its uptake are closely linked to soil pH. When the soil solution reaction is near-neutral or alkaline (pH 6.5–7.5), the solubility and bioavailability of this element decrease.

Chlorine. Chlorine is present in plants in very small quantities. It influences chloroplast function (photosynthetic phosphorylation) and participates in excretion processes.

In addition to those discussed, plants require other micronutrients such as cobalt, vanadium, chromium, and others.

8. Nitrogen Nutrition of Plants. Nitrate Transformation.

Nitrogen. Discovered by Rutherford in 1732. The physiological significance of nitrogen is primarily defined by its role as a constituent of amino acids, proteins, nucleic acids, and many vital organic compounds that form the basis of protoplasm.

On Earth, the majority of nitrogen exists in a gaseous state as molecular nitrogen 14, accounting for about 80% of the total air volume. Atmospheric nitrogen reserves are estimated at approximately 8 tons per square meter of the Earth's surface. However, this vast quantity of nitrogen is inaccessible to plants. The intermediaries facilitating The conversion of atmospheric molecular nitrogen into an accessible form are soil nitrogen-fixing microorganisms. Through free-living soil nitrogen fixers, as well as symbiotic nitrogen fixers associated with perennial and annual legumes, 130–140 kg of nitrogen can be accumulated per hectare of soil annually.

Other forms of nitrogen in the air include trace amounts of ammonia and nitrogen oxides, which are formed during electrical discharges. These forms of nitrogen enter the soil via atmospheric precipitation. Their total quantity is very small, and therefore they are not of significant importance in Plant nitrogen nutrition.

In soil, nitrogen is primarily found in Three types of compounds: ammonium salts, nitrate salts, and organic compounds in the form of various plant and animal residues and their decomposition products (amino acids, Peptides, amines). Plants primarily assimilate nitrate and ammonium forms of nitrogen, the reserves of which constitute no more than 1–2% of the total nitrogen in the soil. Therefore, even chernozems rich in organic matter contain no more than 200 kg of available nitrogen in the arable layer per hectare. This amount is sufficient to obtain high grain yields. In sod-podzolic soils with low organic matter content, the levels of readily available nitrogen are 3–4 times lower.

The classification is based on the ratio between reserve carbohydrates and proteins (C:N) in plant seeds. Ammonia is most successfully assimilated by plants with a predominant carbohydrate content. An example is cereals, where the C:N ratio in the grain is approximately 6:1. Cereal sprouts, when kept in the dark, easily assimilate ammonia until the seed's carbohydrate reserves are almost entirely depleted. In certain legumes (peas, vetch), the C:N ratio is lower, and their nutrition with ammonia in the dark is possible only if the physiological acidity of the nutrient solution is neutralized (by adding CaCO₃ to the solution). Sprouts from seeds with even lower carbohydrate content (e.g., lupine) are unable to assimilate ammonia in the dark at all.

Plants assimilate the nitrate form of nitrogen better from a slightly acidic medium (pH around 5), while the ammonium form is absorbed more intensively from a neutral medium (pH around 7). As previously noted, to utilize ammonium nitrogen, a plant must have a sufficient supply of carbohydrates to incorporate it into Cellular metabolic processes. Otherwise, the accumulation of ammonia in plant cells is lethal.

The content of nitrogen in the soil in a form available to plants is determined not only by the intensity of microbiological processes of organic nitrogen mineralization and Nitrogen Fixation but also by its uptake by the plant root system and leaching by water from the arable layer. Furthermore, denitrification processes occur, driven by aerobic microorganisms that reduce nitrate nitrogen to molecular nitrogen. The latter escapes from the soil into the atmosphere. This process is particularly active in waterlogged soils with poor aeration (e.g., in rice paddies).

Nitrogen absorbed by a plant is incorporated into organic compounds only in the form of ammonia. According to current data, The process of reducing absorbed nitrates is catalyzed by flavoprotein enzymes. It is accompanied by A change in the valence of the nitrogen atom:

Class="center">

Nitrate reductase is an enzyme induced by the cell in response to the influx of nitrate ions. It is localized in the cytoplasm. Its activity is particularly high in the cells of young organs and meristematic tissues. The activity of nitrite reductase is 5–20 times higher than that of nitrate reductase. Consequently, nitrites do not accumulate but are rapidly reduced to ammonia. The site of nitrite reductase localization in leaves is the chloroplasts, and in roots, it is the proplastids.

Nitrate reduction in various plants can occur in both roots and aerial organs. In this regard, three main groups of plants are distinguished.

The first group consists of plants in which the nitrate form of nitrogen is almost completely reduced in the roots and transported to the aerial organs as organic compounds. Most woody plants belong to this group.

The second group includes plants that reduce nitrates in their leaves. This category comprises several members of the Amaranthaceae family (such as beets and orache), as well as cotton, cocklebur, and others.

The third group of plants is the most numerous. Plants in this group are characterized by their ability to reduce nitrates in both leaves and roots. As a rule, nitrate reductase activity is higher in the leaves than in the roots.

The process of nitrate reduction in leaves is closely linked to photosynthesis, which serves as a source of ATP—energy required for the synthesis of enzymes (nitrate reductase) and nitrate transport—and as a source of organic acids that act as substrates for ammonia assimilation.

Ammonia, absorbed by the root system from the soil or produced via nitrate reduction within the plant, reacts with keto acids to form Amino Acids and amides. The primary role in ammonia assimilation processes in plants belongs to the biosynthesis of Glutamic Acid and Glutamine.

The nitrogen nutrition of legumes has specific features due to their ability to form symbiotic relationships with microorganisms. During this process, nodules form on the plant roots, housing Bacteria capable of fixing atmospheric molecular nitrogen. They accumulate enough nitrogen to meet not only their own requirements for this element but also the needs of the host plants. In turn, the plants provide the bacteria with "food" in the form of organic compounds—products of photosynthetic activity (primarily Disaccharides and Monosaccharides).

The nitrogen-fixing activity of bacteria depends on the presence of nitrogen in the soil, primarily nitrates. When soil nitrogen levels are high, the physiological activity of nodule bacteria decreases. Increased soil acidity has a similar inhibitory effect.

Thanks to the nitrogen-fixing activity of nodule bacteria, more than 100 kg of atmospheric nitrogen per hectare can be assimilated in Symbiosis with legumes. Therefore, it is essential to create the most favorable conditions for the development and physiological activity of these microorganisms. To this end, bacterial preparations, such as nitragin, are applied.

9. Diagnosis of mineral nutrition.

Diagnosis of plant mineral nutrition requirements is performed through visual and chemical methods.

Visual diagnosis is based on external symptoms indicating a deficiency of a specific element.

Chemical diagnosis is more advanced. It is based on color reactions resulting from the interaction of plant sap with specific Reagents.

10. Ion transport of mineral elements in plants.

The movement of substances in plants is ensured by two main processes:

1) the Transpiration stream, or upward flow, which carries water and dissolved nutrients from the roots to the shoots;

2) the flow of assimilates, or downward flow, which transports substances produced during photosynthesis from the leaves to plant parts located below (stems, roots, etc.) and above (stem tips, shoots, fruits, etc.) the leaf. The upward flow occurs via the xylem, which includes specific conductive tissue elements—vessels and tracheids. The downward flow occurs via the phloem, which consists of sieve tubes and companion cells.

The transport of mineral elements throughout the plant occurs via the following pathways:

· Radial.

· Long-distance.

· Short-distance.

Radial transport. Before reaching the conductive system, nutrients pass through the radial zone of the root.

The first barrier to ion entry into the plant is The Cell wall, which consists of Primary and secondary layers. Suberin has hydrophobic properties, making the cell wall impermeable to water. Such suberized walls are found only in endodermal cells and are known as Casparian strips.

The Cellulose walls of all cells and the intercellular space of the plant form the apoplastic system, or apoplast.

Apoplastic ion movement through the exodermis and mesodermis walls is blocked by Casparian strips. Only a certain portion of ions reaches the xylem vessels via the apoplastic pathway due to the presence of passage cells in the endodermis. The majority of nutrients in plant tissues are transported via the symplastic pathway.

The symplast consists of the protoplasts of all cells connected to each other by plasmodesmata.

Ions that have entered through the protoplast Plasmalemma can migrate via plasmodesmata without energy expenditure. Plasmodesmata penetrate the cortical cells, Casparian strips, endodermis, and the parenchyma of the central cylinder in both radial and longitudinal directions. The number of plasmodesmata varies widely depending on the age of the tissues. They are closely associated with The Endoplasmic reticulum, which facilitates symplastic transport.

Long-distance transport. This occurs via the xylem, which consists of vessels, tracheae, and tracheids. It is an upward flow, moving from the central cylinder to the leaves.

Short-distance transport. This occurs within the leaf, from the central vein to the mesophyll cells.

11. Physiological principles of fertilizer application.

The use of mineral fertilizers is the primary method for increasing crop yields. By providing nitrogen, phosphorus, potassium, and other elements often deficient in soil, these fertilizers enable cultivated plants to better utilize solar energy and soil-climatic factors, resulting in higher yields. The lack of these essential elements cannot be compensated for by any other agrotechnical measures.

The effectiveness of fertilizers depends on the plant's nutritional requirements and the soil's capacity to meet them. D. N. Pryanishnikov figuratively illustrated this interdependence as a triangle, with the plant, soil, and fertilizer at the vertices, placing the plant at the apex. This emphasizes that the plant and its yield are the cornerstone of all agronomic and biological research in agricultural practice. There are three generally accepted rules for nutritional diagnostics.

- Throughout the growing season, the level of supply of essential nutrients for the future crop is monitored. The basis of nutritional diagnostics is The chemical composition of plants during vegetation. Along with chemical analysis, the stages of Plant Growth and development must also be taken into account.

- Several nutrient elements must be determined, including at least the three primary ones: nitrogen, phosphorus, and potassium.

- Comparison of chemical diagnostic data with consideration of PLANT GROWTH CHARACTERISTICS, climatic conditions, and agrotechnical practices.

Furthermore, when developing a Fertilization system, the following must be considered:

· The physiological role of micronutrients.

· The demand for these nutrients at specific stages of Organogenesis.

· The nutrient uptake required for the targeted crop yield.