Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

6. Plant Mineral Nutrition
6.2 Macronutrients

These elements perform two main Functions in the Organism: structural and regulatory. The former is primarily driven by organogenic elements (C, O, H, N, S), which are involved in The formation of Nucleic Acids, Proteins, Lipids, and other cellular components, as well as Ca and Mg, which are part of The Cell

wall, membranes, and METABOLISM/14.html">Chloroplasts. The latter 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. Furthermore, each macronutrient performs specific functions unique to itself.

Nitrogen is assimilated by plants in the form of anions (NO^2-, NO^3-), cations (NH⁺), and Organic compounds. The Significance of nitrogen in plant life is immense. If it is deficient in the soil, all critical functions, as well as Plant GROWTH AND DEVELOPMENT, are disrupted. It is an essential organogenic element that serves as a building block for proteins, nucleic acids, Amino Acids, chlorophyll (without which Photosynthesis is impossible), Hormones, numerous Vitamins, Alkaloids, and Glycosides. At the same time, it is a highly scarce element. It is not excreted from the organism but is reused multiple times (reutilized); that is, during leaf senescence, it is released through The breakdown of cytoplasmic proteins and other nitrogenous compounds and flows into the younger PARTS OF THE plant. Externally, this manifests as a color change in Aging leaves—from green to yellow, starting from the upper, older parts. Similar phenomena occur when soil nitrogen is deficient. Leaves acquire a yellow tint with reddish Veins. Additionally, plant growth slows down, and the sizes of leaves and fruits decrease significantly.

Phosphorus is assimilated by plants mainly in the form of the orthophosphoric acid anion (PO^4-), as well as sugar and alcohol phosphate esters. Plants whose roots secrete weak acids can assimilate phosphorus from phosphorite flour and other sparingly soluble phosphorus compounds—Ca3(PO₄)₂, AlPO₄, FePO₄. Such plants include lupines, beans, and buckwheat, among others. The positive effect of phosphorus fertilizers is more pronounced in the presence of sufficient amounts of N and K. Possessing high mobility, phosphorus, much like N, is repeatedly reutilized within the plant. As a constituent of Nucleic Acids and lipids, it performs a structural function. Moreover, it is a necessary component of Nucleoproteins (FAD, NAD, NADP), high-energy compounds (ATP), and phosphate esters—trioses, pentoses, and hexoses.

As a result, phosphorus actively participates in the synthesis and transformation of organic substances, particularly CARBOHYDRATES. In case of phosphorus deficiency, photosynthesis and Respiration processes are disrupted, and the breakdown of complex organic compounds intensifies. Externally, this appears as a shift in the green color of the leaves to blue- and purple-green, followed by desiccation. The leaves turn yellow, blacken at the edges, and drop off. In addition, the growth of both aboveground and underground plant parts is delayed. The TRANSFORMATION OF PHOSPHORUS within the plant occurs actively during organ growth and the increase of living Cytoplasm, during seed germination (which is particularly important), and during seed ripening, when P is stored as phytate—a calcium-magnesium salt of Inositol hexaphosphoric acid—C₆H₆(OH₂PO₃)₆.

Potassium is assimilated by plants from soluble salts—chlorides, sulfates, and nitrates. It is also easily reutilized, flowing into young Organs and Tissues. In plants, potassium is predominantly found in a free, ionic form. Only a small fraction of it is loosely bound to cytoplasmic proteins. Potassium enhances the Hydration of cytoplasmic colloids, their Water-retaining capacity, and permeability. In this way, it creates conditions for the active synthesis of proteins and other organic compounds. Furthermore, potassium activates about 60 Enzymes, including starch synthetase and phosphokinase, thereby influencing ATP metabolism. At the same time, potassium regulates stomatal opening and closing and activates the movement of assimilates throughout the plant. Evidently, vital processes such as photosynthesis, respiration, the synthesis of complex organic substances (carbohydrates), and The transport of organic compounds are impossible without K. When K is deficient, the growth of young plants stops, leaves turn yellow, then brown, dry out at the edges, or curl and wrinkle.

Calcium is absorbed as a cation from its soluble salts. Plants whose roots secrete weak acids can assimilate calcium from minerals such as chalk and limestone. Ca has low mobility and is not reutilized in plants; instead, it accumulates in the form of sparingly soluble salts (gypsum, calcium oxalate) in older leaves. It alters the acidity of the soil solution, influencing the uptake of other elements by plants. Ca participates in maintaining the Structure of Cell 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), photosynthetic enzymes, and Phosphatases (a-amylase, among others). Unlike potassium, calcium increases cytoplasmic viscosity and decreases its water content, thereby suppressing cellular metabolism. In the event of an excess of organic and mineral acids in plants, Ca forms insoluble salts with them, which are eliminated during leaf fall. This neutralizes their harmful effects on the plant. Calcium cations mitigate the NEGATIVE IMPACT OF an excess of other cations on the plant, which is particularly noticeable in saline soils. Calcium deficiency severely affects the roots—they fail to grow in length, merely becoming thicker and covered in mucus, while the stem tips eventually die off and growth ceases.

Magnesium is absorbed from soil magnesium salts as well as from dolomitic limestone. It exhibits considerable mobility within the plant and is therefore easily reutilized. In tissues, magnesium exists in both bound and ionic forms. It is a structural part of chlorophyll and magnesium pectates found in cell walls. Magnesium links the large and small ribosomal subunits, maintaining their functional activity. In its ionic form, it activates phosphokinases and Calvin cycle enzymes. A deficiency of magnesium in the plant disrupts the synthesis of proteins, chlorophyll, and carbohydrates, and reduces the intensity of Glycolysis. The external sign of magnesium deficiency is interveinal chlorosis of young leaves. The edges of green leaves and the areas between veins become yellow, red, or purple (marble-like chlorosis). Initially, this phenomenon is observed in the lower-tier leaves, and subsequently in the upper ones.

Sulfur is assimilated in the form of sulfate ions from salts such as Na₂SO₄, K₂SO₄, CaSO₄, MgSO₄, and Fe₂(SO₄)₃, as well as from certain organic compounds (Sulfur-Containing Amino Acids). It exhibits significant mobility and is readily reutilized. Sulfur is a component of enzymes, where it links Coenzymes (NAD, FAD) and prosthetic groups (e.g., Fe) to the protein moiety. A substantial portion of plant S is found in sulfur-containing amino acids in the form of sulfhydryl (R-SH) groups, which participate in the formation of Tertiary and Quaternary protein structures by converting into sulfide groups (disulfide bridges, R-S-S-R, R₁-S-S-R2). Cysteine possesses a sulfhydryl group, while cystine and Glutathione—a powerful reductant playing a crucial role in redox processes—contain a disulfide group. The SH group is a constituent of coenzyme A, which serves as the initial link in the Biosynthesis of Fatty acids, The Citric Acid Cycle (Krebs cycle or Tricarboxylic Acid Cycle), and The Glyoxylate cycle. In plant bodies, sulfur is part of mustard and garlic oils (found in Brassicaceae and Liliaceae). These oils repel many leaf-chewing insects, thus playing a protective role. When sulfur is deficient, chlorosis occurs—initially, leaf veins turn yellow, later the blades become covered with reddish spots, and the leaves eventually die.

Chlorine is always present in plants, yet it is not an element without which plants fail to develop or suffer severe disorders. Chlorine is essential for plants that have evolutionarily adapted to chloride salinity (sugar beets, spinach, buckwheat) and for halophytes. Cl^- ions participate in the photooxidation of water.

Silicon is required by plants of the Poaceae, Cyperaceae, and Equisetaceae families for the incrustation of epidermal cell walls, as well as by diatoms for the formation of their frustule (external Skeleton). High concentrations of silicon are found in old wood. It lacks 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 its chemical properties closely resemble those of K, it plays a considerably smaller physiological role. It is believed that Na is necessary primarily for maintaining the osmotic potential of plant Cells growing on saline soils.

Thus, each macronutrient performs specific functions in the plant organism; therefore, none of them can be substituted by another—meaning that a complete set of macronutrients is required for normal growth and Development of the organism.

The ratio of their content is determined by the plant species, developmental stage, growing conditions, etc. The primary nutrient elements—N, P, and K—noticeably influence the appearance, growth rates, and Development of Plants, exerting a morphogenetic role. An excess of N enhances the formation and growth of vegetative organs while delaying 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 course of Plant Growth and development. During Cytology/cytology/16.html">Early stages of ontogenesis, for the intensive formation of assimilating organs, the plant requires more N and K, whereas prior to the initiation of reproductive organs, it requires more P, as Energy Metabolism intensifies leading up to the flowering period. Thus, with the help of N, K, and P, one can either accelerate the timing of flowering and fruiting or delay it while boosting vegetative mass production (in fodder grass crops, cultivated meadows, and pastures). By altering the N to K ratio in the soil, one can regulate the formation of female and male flowers in monoecious and dioecious plants.