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

6. Mineral Nutrition of Plants
6.3 Trace Elements

Trace elements are integral components of enzymatic Cofactors and prosthetic groups, regulating vital processes such as Photosynthesis, Respiration, substance conversion, and Plant GROWTH AND DEVELOPMENT, thereby performing a metabolic function. Furthermore, in their ionic state, trace elements activate various cellular enzyme systems and fulfill a regulatory role. In addition, each trace element exhibits a distinct Specificity of action.

Iron is required by plants in larger quantities than other trace elements, 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, these salts precipitate and become unavailable to plants. The heme form of iron is part of Cytochromes, catalase, and peroxidase, whereas the non-heme form is a component of ferredoxin, nitrate reductase, and Fe-S Proteins—compounds essential for photosynthesis, respiration, and nitrate reduction. Evidently, due to the high demand for iron and its critical importance for plants, it is stored in Tissues as ferritin. In the case of iron deficiency, chlorosis develops in young leaves because chlorophyll Biosynthesis relies on the iron-containing enzyme cytochrome oxidase.

Copper is absorbed by plants in the form of cations derived 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 (0.02-0.05% CuSO₄ solution). Copper is a component of phenol oxidase, ascorbic acid oxidase, and presumably cytochrome oxidase, as well as plastocyanin (an electron carrier in the photosynthetic Electron Transport Chain). Thus, it participates in redox processes, photosynthesis, and respiration. Additionally, copper stabilizes the bond between chlorophyll and chloroplast proteins, thereby preventing its degradation under adverse conditions (such as drought or low temperatures). Copper deficiency is particularly pronounced in plants growing on drained peat soils. A characteristic sign of copper deficiency in cereals is the sudden whitening and drying of leaf tips ("white plague") and unfilled ears (blank panicle). In vegetable and fruit crops, copper starvation causes leaf curling, deformation, and reduction in size, as well as a gradual color change from dark to light green (while the Veins retain their color). In trees, leaves become chlorotic, apical buds die off, and pustules appear on the bark of shoots, which subsequently wither. If copper salts are applied to the soil, affected trees recover and grow well.

Zinc is assimilated by plants only in a mobile, soluble form—predominantly as zinc sulfate, the content of which increases at low soil pH and decreases under moisture deficit. Consequently, plants on acidic sod-podzolic, gray forest, and peat soils do not suffer from zinc deficiency, although its availability to plants declines in drought years. 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 dehydrogenases (such as GAPDH, malate dehydrogenase), Phosphatases, carboxylases, and Carbonic anhydrase—an enzyme that catalyzes the reversible conversion of carbonic acid into Water and carbon dioxide, which is required for the dark reactions of photosynthesis. Zinc-containing enzymes activate reductive processes and thus create 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 unfavorable factors. An important physiological function of zinc is its involvement in the Synthesis of the Growth Hormone auxin, which is why it naturally accumulates in young tissues and the embryo. Zinc deficiency inhibits growth processes; in fruit trees, it shortens internodes, leading to rosette formation and small fruit size, while in vegetable crops, it causes chlorosis and leaf spotting.

Manganese is absorbed by plants from its soluble salts in the form of divalent cations. Its availability decreases under high soil calcium content and during prolonged dry, hot weather. Foliar application (0.06-0.1% KMnO4 solution) allows manganese to be directly assimilated by leaves. Within plants, manganese exists as ions of various oxidation states (Mn²⁺, Mn³⁺, Mn⁴⁺), their ratio shifting depending on the cellular redox status. Mn positively influences chlorophyll biosynthesis, stabilizes its binding to proteins, and thereby enhances resistance to degradation under adverse conditions. Furthermore, manganese participates in the photooxidation of water during photosynthesis and in the reductive Reactions of the Calvin cycle. It activates Krebs cycle Decarboxylases and nitrate reduction. In the event of an excess, manganese acts as an iron antagonist, substituting for it in the Functional groups of iron-containing enzymes. Symptoms of manganese deficiency are nonspecific and generally manifest as Various Forms of chlorosis. In fruit trees, In addition to chlorosis, branch tips may die back and wither.

Molybdenum is absorbed as an anion from readily soluble salts, such as sodium and ammonium molybdates, the latter of which can also be used for foliar feeding (0.06-0.1% solution). Molybdenum is unavailable to plants in acidic soils, which therefore require liming. Inside the plant, molybdenum is present in organically bound forms and is a constituent of nitrate reductase, the enzyme that reduces nitrate nitrogen to the ammonium form, which is directly utilized in Amino acid synthesis. Furthermore, molybdenum inhibits The activity of acid phosphatase—an enzyme that hydrolyzes phosphorus compounds—thereby increasing the levels of Nucleic Acids and organic phosphates (such as ATP and sugar phosphate esters) in the plant. Molybdenum is essential for Protein Synthesis. Free-living and symbiotic nitrogen-fixing microorganisms have a particularly high demand for Mo, as it forms part of the enzyme complex that reduces atmospheric molecular nitrogen to ammonia. A deficiency of Mo disrupts The Development of the leaf apparatus; petioles and leaves elongate and narrow, lose turgor, and wilt starting from the margins. In certain species, yellow-green and pale-orange spots appear on the leaf surface between the veins.

Boron is absorbed from the soil as boric acid anions (Н₃ВО₃), borax (Na₂B₄O₇ • 10Н₂О), and magnesium borate (MgB₂O₄ • 10Н₂О). For foliar feeding, a 0.03-0.05% boric acid solution is used. Unlike other elements, boron is not a component of enzymes and does not directly affect their activity. The physiological action of boron is believed to stem from its ability to form highly reactive complexes with other cellular Organic compounds, including CARBOHYDRATES, organic acids, Coenzymes, and phenols. This underlies boron's positive effect on the METABOLISM of proteins, nucleic acids, carbohydrates, and Auxins. Additionally, sugars complexed with B cross membrane barriers more easily, significantly accelerating their transport through The Vascular System. At同時に, B increases the water-holding capacity of cytoplasmic colloids, thereby conferring drought resistance to plants. 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 pods, as well as impaired development and various abnormalities in fruits and seeds. M. Ya. Shkolnyk and P. O. Vlasyuk (1974–1976) attributed this to The formation of defective pollen, which disrupts Fertilization and fruit set. In ROOT crops, boron deficiency causes browning and dry rot of the core, while in table beets, it results in black spot. Dicotyledonous plants are the most susceptible to boron deficiency, which shifts their Carbohydrate Metabolism toward the accumulation 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 industry wastes applied to the soil). Applying cobalt yields positive results on limed sod-podzolic and peat soils. 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 affects chlorophyll synthesis and the Stability of the pigment-protein complex, whereas in its ionic form, it activates numerous enzymes, thereby accelerating Plant Growth and development and increasing dry matter content. Leguminous plants living in Symbiosis with nitrogen-fixing Bacteria have a particularly high requirement for cobalt. Co exerts a dual effect on symbiotic Nitrogen Fixation: on the one hand, as part of Vitamin B12, it participates in the formation of leghemoglobin, the pink pigment necessary for binding atmospheric nitrogen; on the other hand, it activates enzymes that reduce molecular nitrogen to ammonia. Consequently, a shortage of Co (especially in the absence of available nitrogen forms in the soil) halts growth and biomass accumulation in legumes. In contrast, the cobalt requirement of most other plants is several orders of magnitude lower, meaning deficiency symptoms are rarely observed in them.

Overall, no physiological process in plant organisms can take place without the participation of specific trace elements. Their deficiency in soil, water, and consequently in living organisms leads to so-called deficiency diseases, which are accompanied by impaired vital Functions, various malformations and developmental anomalies, and sometimes even plant death.

Heterotrophic Nutrition in plants. Autotrophic organisms independently synthesize organic matter from inorganic precursors, whereas heterotrophic organisms feed on preformed organic matter. Among plants, some are heterotrophs—namely, parasites and carnivorous plants. In The life cycle of an autotrophic plant, there are periods when it is nourished by previously stored organic compounds, such as during the germination of seeds and tubers, or the growth of shoots from rhizomes.

Organic food is broken down into easily assimilated compounds through Digestion. There are Three types of digestion: intracellular, membrane, and extracellular. Intracellular digestion occurs in the Cytoplasm, vacuoles, Plastids, and spherosomes. Membrane digestion is carried out by enzymes located in cell membranes. Extracellular digestion takes place via the secretion of hydrolytic enzymes into the external environment.

Parasites are plants that have either completely (e.g., broomrape) or largely (e.g., dodder) lost their capacity for photosynthesis. Broomrape seeds germinate in response to root exudates from the host plant. The broomrape seedlings grow toward the host root, and the tip of the broomrape embryonic root penetrates it, transforming into a haustorium (sucker) while secreting Cell wall-degrading enzymes. The broomrape obtains all necessary nutrients from the host plant. Dodder is a twining plant whose root end withers while its stem wraps around the host stem, attaching to it via disc-like haustoria that press closely against the host bark. A group of Cells from the center of the disc grows inward until it reaches the host's vascular system, from which dodder extracts water and nutrients.

To date, over 400 species of angiosperms are known to capture small insects and utilize them as an additional source of nutrients. Most of these plants grow in nitrogen-poor bogs. Based on their trapping mechanism, carnivorous plants are divided into two groups. In passive trapping, insects adhere to leaves whose glands secrete a sticky mixture, or they fall into pitcher- or urn-shaped traps brightly colored and scented with sweet, aromatic nectar. In active trapping, insects are secured by sticky mucus and enclosed by the movement of leaves or glandular hairs, or they are drawn into trapping bladders along with water due to a negative pressure (vacuum) maintained within them. An insect trapped in the bladder is digested by secretions from glands containing acids, proteases, phosphatases, RNases, and lipase. The products of digestion are absorbed by these same glands, which are connected to the plant's vascular system.

In mature cereal grains, the embryo is not in direct contact with the endosperm tissues that store nutrient reserves. The DIGESTION AND ABSORPTION of these reserves are facilitated by a modified cotyledon known as the scutellum. Proton pumps located in the epithelial Cells of the scutellum secrete protons into the endosperm. Furthermore, organic acids and acid Hydrolases—such as amylases, proteases, and glucanases—are transported from the scutellum into the endosperm. By the third day of germination, a layer of living cells within the endosperm, known as the peripheral aleurone layer, becomes active. These cells also secrete organic acids and acid hydrolases into the endosperm. As a result, the storage reserves of the endosperm are solubilized, absorbed by the scutellum, and subsequently transferred to the vascular bundles.