PLANT PHYSIOLOGY AND BIOCHEMISTRY

Lecture Notes

7. MINERAL NUTRITION OF PLANTS

The Role of Mineral Nutrients

Approximately 68 chemical elements have been identified in plants, 47 of which are present consistently. It is known that the Elemental Composition of living organisms generally mirrors that of the Earth's crust. Their content in a plant depends on the species, as well as soil and climatic conditions.

The most abundant are the organogenic elements (C, H, O, N). On average, carbon accounts for 45% of the dry tissue mass, oxygen for 42%, hydrogen for 6.5%, and nitrogen for 1.5%, collectively comprising 95%.

The remaining 5% consists of ash constituents: P, S, K, Ca, Mg, Fe, Al, Si, Na, and others. When plant residues are burned, the organogenic elements are released as gaseous compounds, while the Mineral Substances remain as ash.

The amount of ash in a plant depends on its capacity to accumulate mineral elements, their availability in the soil, Water supply, and other factors. Ash elements are particularly concentrated in actively functioning living Tissues.

The content of ash elements varies among different plants and even between different Organs of the same plant. For instance, leaves contain 10-15% ash, tree bark approximately 7%, wood about 1%, herbaceous stems 4-5%, and seeds 3%.

The qualitative composition of ash depends on the metabolic processes occurring within tissues and organs. For example, seeds contain higher levels of K, P, and Mg, which are essential for embryo development; storage organs are rich in K, which activates starch synthesis; stems contain more Ca and Si; and leaves are rich in P, Mg, and K (in young leaves) or Ca (in older ones). Elements prevalent in the soil tend to accumulate in higher concentrations. Some species possess The ability to hyperaccumulate specific elements. For example, seaweeds accumulate bromine and iodine; certain cereals (such as maize) accumulate gold, and so on. Based on their concentration in the Organism, elements are classified into Macronutrients (C, O, H, N, P, S, K, Ca, Mg, Na, Cl, Si, Al), micronutrients (Fe, Zn, Mo, Mn, Cu, B, Co, and others), and ultramicronutrients (whose physiological roles remain largely unstudied). The first group constitutes 10 - 10^- % of the plant's fresh mass; the second 10^-5-10^-6 %; and the third 10^-6-10^-12 %.

Normal plant life processes are impaired without macronutrients and micronutrients. At the same time, certain macronutrients like sodium, silicon, and aluminum do not play a significant role in plant physiology.

Macronutrients

These elements perform two primary Functions in the organism: structural and regulatory. The structural role is mainly fulfilled by organogenic elements (C, O, H, N), as well as P and S, which are involved in The formation of Nucleic Acids, Proteins, Lipids, and other cellular components, and Ca and Mg, which are integral to Cell walls, membranes, and METABOLISM/14.html">Chloroplasts.

The regulatory function is carried out by monovalent cations and anions (H⁺, K⁺, Cl^-, Na⁺), which influence Membrane Potential and, together with Ca²⁺ and Mg²⁺ ions, participate in regulating the physicochemical state of cytoplasmic colloids. Each macronutrient performs its own unique, specific functions.

Nitrogen is assimilated by plants in the form of anions (NO₂^-, NO₃^-), the cation (NH₄⁺), and Organic compounds. Structure/19.html">The Importance of nitrogen in plant life cannot be overstated. Its deficiency in the soil disrupts all vital functions, growth, and development. It is a crucial organogenic element, serving as a building block for proteins, nucleic acids, Amino Acids, chlorophyll (essential for Photosynthesis), Hormones, various Vitamins, Alkaloids, and glucosides.

At the same time, it is a highly limiting element. It is not excreted from the organism but is reused multiple times (remobilized)—meaning that as leaves age, nitrogen is released during The breakdown of cytoplasmic proteins and other nitrogenous compounds and translocated to younger PARTS OF THE plant. Externally, this manifests as A change in the color of Aging leaves from green to yellow, starting from the upper, older parts. Similar phenomena are observed when nitrogen is deficient in the soil. Leaves take on a yellow hue with reddish Veins. Furthermore, plant growth is stunted, and the size of leaves and fruits is significantly reduced.

Phosphorus is assimilated by plants primarily as the orthophosphate anion (PO₄^3-), as well as in the form of sugar and alcohol phosphates. Plants whose roots secrete weak acids can assimilate phosphorus from phosphorite flour and other poorly soluble phosphorus compounds such as Ca₃(PO₄)₂, AlPO₄, and FePO₄. Such plants include lupine, beans, and buckwheat. The positive effect of phosphorus fertilizers is more pronounced when sufficient N and K are present. Phosphorus, like N, is highly mobile and is remobilized multiple times within the plant. By being part of Nucleic Acids and lipids, it performs a structural function. Additionally, it is a necessary component of Nucleoproteins (FAD, NAD), high-energy compounds (ATP), and sugar phosphates (trioses, pentoses, hexoses). Consequently, phosphorus plays an active role in the synthesis and transformation of organic substances, particularly CARBOHYDRATES.

Phosphorus deficiency disrupts photosynthesis and Respiration, leading to an increased breakdown of complex organic compounds. Externally, this appears as a shift in leaf color from green to a bluish- or violet-green, followed by desiccation. Leaves turn yellow, blacken at the edges, and fall off. Concurrently, growth processes in both the aerial and subterranean parts of the organism are inhibited. Phosphorus transformation in plants is most active during organ growth and the increase of living Cytoplasm, especially during seed germination and maturation, when P is stored as phytin—the calcium-magnesium salt of Inositol hexaphosphoric acid — C₆H₆(ОН₂РО₃)₆.

Sulfur is assimilated as sulfate ions from salts such as Na₂SO₄, K₂SO₄, CaSO₄, MgSO₄, Fe₂(SO₄)₃, as well as from certain organic compounds (Sulfur-Containing Amino Acids). It is highly mobile and is efficiently remobilized.

Sulfur is a component of Enzymes, where it links Coenzymes (NAD, FAD) and prosthetic groups (e.g., Fe) to the protein moiety. A significant portion of plant sulfur is found in sulfur-containing amino acids in the form of sulfhydryl (R-SH) groups, which participate in forming the Tertiary and Quaternary structures of proteins by converting into sulfide groups (disulfide bridges: R-S-S-R, R₁-S-S-R₂). Cysteine contains a sulfhydryl group, while cystine and Glutathione—a powerful antioxidant playing a key role in redox processes—contain disulfide groups.

The SH group is a constituent of coenzyme A, which is the initial link in the Biosynthesis of Fatty acids, The Citric Acid Cycle (Krebs cycle), and The Glyoxylate cycle. In plants, sulfur is also part of mustard and garlic oils (found in Brassicaceae and Liliaceae). These oils repel many leaf-eating insects, thus playing a protective role.

Sulfur deficiency leads to chlorosis—initially, leaf veins turn yellow, followed by the appearance of reddish spots on the leaf blades, eventually resulting in leaf necrosis.

Potassium is assimilated by plants from soluble salts—chlorides, sulfates, and nitrates. It is also easily remobilized, translocating to younger organs and tissues. In plants, potassium exists primarily in a free, ionic form. Only a small fraction is loosely bound to cytoplasmic proteins.

Potassium increases the Hydration of cytoplasmic colloids, enhancing their water-holding capacity and permeability. This creates favorable conditions for the active synthesis of proteins and other organic compounds. Furthermore, potassium activates approximately 60 enzymes, including starch synthetase and phosphokinase, thereby influencing ATP metabolism. Additionally, potassium regulates stomatal opening and activates the Transport of Assimilates throughout the plant. Clearly, vital processes such as photosynthesis, respiration, the formation of complex polymers (carbohydrates), and The transport of organic substances are impossible without K.

With K deficiency, the growth of young plants ceases; leaves turn yellow, then brown, dry out at the edges, or curl and wrinkle.

Calcium is absorbed in the form of a cation from its soluble salts. Plants whose roots secrete weak acids can assimilate calcium from minerals such as chalk and limestone.

Calcium is relatively immobile and is not reutilized in plants; instead, it accumulates in the form of poorly soluble salts (gypsum, calcium oxalate) in older leaves. It alters the acidity of the soil solution, thereby influencing the uptake of other elements by the roots. Ca plays a role in maintaining The structure of membranes and Chromosomes and is a component of cell walls in the form of calcium pectate. It influences cellular metabolism by activating certain respiratory enzymes (succinate dehydrogenase), photosynthesis, and Phosphatases (α-amylase, among others). Unlike potassium, calcium increases the viscosity of the cytoplasm and reduces its hydration, thereby inhibiting cellular metabolism.

When organic and mineral acids are in excess within plants, calcium forms insoluble salts with them, which are then shed during leaf fall. This neutralizes their harmful effects on the plant. Calcium cations mitigate the NEGATIVE IMPACT OF excessive amounts of other cations on the plant, which is particularly noticeable in saline soils.

A deficiency of Ca has a particularly severe effect on roots: they cease longitudinal growth, becoming thickened and mucilaginous, while the stem tips eventually die off, halting overall growth.

Magnesium is absorbed from magnesium salts in the soil, as well as from dolomitic limestone. It exhibits sufficient mobility within the plant and is therefore easily reutilized.

In tissues, magnesium exists in both ionic and bound forms. It is a constituent of chlorophyll and magnesium pectates found in cell walls. Magnesium binds the large and small subunits of the ribosome, thereby maintaining its functional activity. In its ionic form, it activates phosphokinases and enzymes of The Calvin Cycle. A magnesium deficiency in a plant disrupts the synthesis of proteins, chlorophyll, and carbohydrates, and reduces the intensity of Glycolysis.

An external sign of magnesium deficiency is chlorosis of the young leaves. The green leaves become yellow, red, or purple along the edges and between the veins (marbled chlorosis). This phenomenon is first observed in the lower leaves and subsequently appears in the upper ones.

Chlorine is always present in plant tissues. However, it is not an essential element without which plants would fail to develop or suffer significant impairment. Chlorine is required by species that have evolutionarily adapted to chloride salinity (sugar beet, spinach, buckwheat) and by halophytes. Cl^- ions participate in the photo-oxidation of water.

Silicon is necessary for plants of the Poaceae, Cyperaceae, and Equisetaceae families for the incrustation of cell walls in epidermal tissues, as well as for diatoms to form their frustules (external skeletons). Old wood contains high amounts of silicon. It has no major physiological significance, and its role in biochemical processes has not been established.

Sodium. Despite the fact that Na is present in plant ash in significant quantities and that its chemical properties are very similar to those of K, it plays a much smaller physiological role. It is believed that Na is primarily required to maintain the osmotic potential of Cells in plants growing on saline soils.

Thus, each macronutrient performs specific functions within the plant organism. Therefore, none of them can be replaced by another. In other words, a full set of essential macronutrients is required for the normal GROWTH AND DEVELOPMENT of the organism. The ratio of their content is determined by the plant species, the stage of development, growing conditions, etc.

The primary nutrients—N, P, and K—have a noticeable effect on the appearance, growth rates, and Development of Plants, thus playing a morphogenetic role. An excess of N enhances the formation and growth of vegetative organs but delays flowering, as well as the ripening of fruits and seeds.

Potassium, like N, promotes vegetative growth, whereas phosphorus, conversely, accelerates plant development, flowering, and fruiting. Consequently, the demand for these elements changes during The process of Plant Growth and Development. In the Cytology/cytology/16.html">Early stages of ontogenesis, a plant requires more N and K for the intensive formation of assimilating organs, whereas before the initiation of reproductive organs, it requires more P, as Energy Metabolism intensifies prior to flowering.

Thus, with the help of N, K, and P, one can either accelerate the timing of flowering and fruiting or delay them and stimulate the formation of vegetative mass (in forage grass crops, cultivated meadows, and pastures). By changing the N to K ratio in the soil, it is possible to regulate the formation of female and male flowers in monoecious and dioecious plants.

Micronutrients

These elements are components of Cofactors and prosthetic groups of enzymes and control vital processes such as photosynthesis, respiration, substance transformation, and plant growth and development, thereby performing a metabolic function. Furthermore, in their ionic state, micronutrients activate the operation of various cellular enzyme systems and perform a regulatory function. At the same time, each micronutrient is distinguished by a specific mode of action.

Iron is required by the plant in larger quantities than other micronutrients, which is why it is sometimes classified as a macronutrient.

The source of iron for plants is its soluble salts. However, in an alkaline environment, they precipitate and become unavailable to plants. The heme form of iron is a component of Cytochromes, catalase, and peroxidase; the non-heme form is part of ferredoxin, nitrate reductase, and FeS proteins—compounds without which photosynthesis, respiration, and nitrate reduction would be impossible. Apparently, due to the high demand for iron and its special significance for plants, it is stored in tissues in the form of ferritin.

When iron is deficient, plants develop chlorosis of the young leaves, as chlorophyll biosynthesis occurs with the participation of the iron-containing enzyme cytochrome oxidase.

Zinc is assimilated by plants only in a mobile, soluble form—mainly as zinc sulfate, The amount of which increases at low soil pH values and decreases during moisture deficits. Therefore, on acidic sod-podzolic, gray forest, and peat soils, plants do not experience zinc deficiency. However, in dry years, its availability to plants decreases. Zinc is also absorbed by leaves during foliar feeding (0.03-0.05% ZnSO₄ solution).

The Physiological Role of zinc is diverse. It is a component of more than 30 enzymes, including PGA dehydrogenases, Carbonic anhydrase, phosphatases, and carboxylases. Carbonic anhydrase is an enzyme that carries out the reversible dissociation of carbonic acid into water and carbon dioxide, which is necessary for the Dark Phase of photosynthesis. Zinc-containing enzymes activate reductive processes, thereby creating conditions for the intensive Biosynthesis of Proteins and nucleic acids. Therefore, plants treated with zinc salt solutions are more resistant to drought, high temperatures, and other adverse factors.

An important physiological function of zinc is its Participation in the Synthesis of the Growth Hormone, auxin. It is therefore no coincidence that it accumulates in young tissues and the embryo. When zinc is deficient, the growth process is inhibited; in fruit trees, internodes shorten, and leaf rosettes and small fruits form, while in vegetable crops, chlorosis and leaf spotting appear.

Molybdenum is assimilated in the form of an anion from highly mobile salts—sodium and ammonium molybdates. The latter can also be used for foliar feeding (0.06-0.1% solution). Molybdenum is unavailable to plants in acidic soils. Such soils must be limed.

In tissues, molybdenum binds to organic substances. It is a component of nitrate reductase, which reduces the nitrate form of nitrogen to ammonium. The latter is directly utilized for the synthesis of amino acids. Furthermore, molybdenum inhibits The activity of acid phosphatase, which hydrolyzes phosphorus compounds, thereby increasing the content of nucleic acids and organic phosphates (ATP, sugar phosphates, etc.) in the plant. Molybdenum is necessary for Protein Synthesis. Not only higher plants but also small organisms have a requirement for Mo. This is especially true for free-living and symbiotic nitrogen-fixing microorganisms, in which it is part of the enzyme complex that carries out the reduction of atmospheric molecular nitrogen into ammonia.

A deficiency of Mo in a plant disrupts The Development of the leaf apparatus. In this case, petioles and leaves elongate and narrow, lose turgor, and wilt, starting from the edges. In some species, yellow-green and pale-orange spots appear on the leaf surface between the veins.

Manganese is absorbed by plants from its soluble salts in the form of divalent oxide. Its bioavailability decreases in soils with high calcium content, as well as during prolonged hot and dry weather. Through foliar application (a 0.06–0.1% KMnO₄ solution), manganese is absorbed directly by the leaves.

In plant tissues, manganese exists as ions with varying oxidation states (Mn²⁺, Mn³⁺, Mn⁴⁺). Their ratio fluctuates depending on the redox processes within The Cell. Mn positively influences chlorophyll biosynthesis, stabilizes its binding to proteins, and thereby increases resistance to degradation under adverse conditions. Furthermore, manganese participates in the photo-oxidation of water during photosynthesis and in the reductive Reactions of the Calvin cycle. It activates Krebs cycle Decarboxylases and nitrate reduction. In cases of excess, manganese acts as an iron antagonist, substituting for iron within the Functional groups of iron-containing enzymes. Symptoms of manganese deficiency are non-specific and manifest as various types of chlorosis. In fruit-bearing plants, beyond chlorosis, the tips of branches may die back and desiccate.

Copper is absorbed by plants as cations from its soluble salts, which are formed by inorganic (sulfates, chlorides, nitrates) and organic acids. It is readily assimilated through foliar spraying with solutions of these salts (a 0.02–0.05% CuSO₄ solution).

Copper is a component of phenol oxidase, ascorbate oxidase, and is believed to be a part of cytochrome oxidase, as well as plastocyanin (an electron carrier in the photosynthetic Electron Transport Chain). Thus, it plays a vital role in the redox processes of photosynthesis and respiration. Additionally, copper stabilizes the association between chlorophyll and chloroplast proteins, preventing its breakdown under stress conditions such as drought or low temperatures.

Copper deficiency is particularly pronounced in plants grown on drained peat soils. A characteristic sign in cereals is the sudden whitening and drying of leaf tips (known as "reclamation disease" or "white tip") and the development of empty, unfilled ears. In vegetable and fruit crops, copper starvation causes leaf curling, deformation, and size reduction, accompanied by a gradual color shift from dark to light green (while the veins retain their original color). In trees, leaves become chlorotic, terminal buds die, and blisters appear on the bark of shoots, which eventually wither. If copper salts are applied to the soil, affected trees typically recover and resume normal growth.

Boron is absorbed from the soil as boric acid anions (Н₃ВО₃), borax (Na₂B₄O₇x10H₂O), and magnesium borate (MgB₂O₄x3Н₂О). For foliar feeding, a 0.03–0.05% boric acid solution is used.

Unlike other elements, boron is not a structural component of enzymes and does not directly influence their activity. It is believed that the Physiological Effect of B is based on its ability to form highly reactive complexes with other cellular organic compounds (carbohydrates, organic acids, coenzymes, phenols, etc.). This determines boron's positive impact on the metabolism of proteins, nucleic acids, carbohydrates, and Auxins. Furthermore, sugars complexed with B cross membrane barriers more easily, which significantly accelerates their transport through The Vascular System. Simultaneously, B enhances the water-holding capacity of cytoplasmic colloids, thereby increasing plant drought resistance.

When boron is scarce, meristematic tissues and reproductive organs are the first to suffer. Consequently, boron deficiency leads to a reduction in the number of flowers and fruit sets, as well as developmental abnormalities in fruits and seeds. D. Shkolnik and M. Vlasyuk (1974–1976) attribute this to the formation of defective pollen, which disrupts Fertilization and fruit set processes. In ROOT crops, boron deficiency causes browning and dry rot of the core, while in table beets, it leads to black spot disease.

Dicotyledonous plants are the most susceptible to boron deficiency. In these plants, Carbohydrate Metabolism shifts toward The production of Phenolic Compounds, specifically growth inhibitors that suppress protein synthesis, Cell Division, and organ growth.

Cobalt is absorbed by plants from its soluble salts, as well as from pyrite cinders (metallurgical waste products applied to the soil). Applying cobalt to limed sod-podzolic and peat soils yields positive results.

Within the plant, cobalt is incorporated into organic compounds, such as vitamin B₁₂, and also exists in ionic form. In its bound form, it positively influences chlorophyll synthesis and the Stability of the pigment-protein complex, while in its ionic form, it activates numerous enzymes, thereby accelerating plant growth and development and increasing dry matter content.

Leguminous plants, which live in Symbiosis with nitrogen-fixing Bacteria, have a high demand for cobalt. Co has a dual effect on the process of symbiotic Nitrogen Fixation. On one hand, as a component of vitamin B₁₂, it participates in the Formation of the pink pigment leghemoglobin, which is essential for binding free oxygen during nitrogen fixation. On the other hand, it activates enzymes that reduce molecular nitrogen to ammonia. Therefore, in the event of Co deficiency (especially when accessible nitrogen forms are absent in the soil), legume growth and biomass accumulation are stunted. For most other plants, the requirement for Co is several times lower than for other micronutrients, and typically, no specific deficiency symptoms are observed.

Overall, virtually no physiological process in a plant organism can occur without the participation of various micronutrients. Their deficiency in soil, water, and consequently in living organisms leads to so-called deficiency diseases, which are accompanied by impaired vital functions, various deformities and developmental anomalies, and in some cases, may even cause plant death.