Plant Physiology - Musiyenko, M. M. 2001

Root Nutrition of Plants
Physiological Effect of Trace Elements on Plants

Physiologically, Trace Elements constitute a rather diverse group of substances. Despite their different Functions, they are grouped together because all of them are required in very small quantities for normal development. Among the numerous trace elements, boron, copper, manganese, molybdenum, and zinc are of particular importance to plants. Currently, research on trace elements is conducted in the following directions:

·studying the problems of uptake, transport, and assimilation of trace elements;

·elucidating their role in enzymatic reactions, the METABOLISM of growth regulators and Vitamins, and the Structural Organization of Cells, Organelles, and Biopolymers.

Based on these studies, systems for optimizing the mineral Nutrition of agricultural crops are being developed, taking into account the interactions between macro- and trace elements, and criteria for plant nutrient supply across various soil and climatic zones are being identified.

Researchers investigate The impact of trace elements on environmental pollution by toxic elements, mainly metals—particularly in heavily industrialized areas—and study their migration within the soil-plant-animal-product-human ecological system.

Molybdenum

Molybdenum is a rare element. In the Earth's crust, its Abundance is 100 times less than that of manganese, 60 times less than zinc, 10 times less than copper, and 5 times less than boron. It enters the plant in the form of MoO₄^-- ions, accumulating primarily in young Organs. By the end of the growing season, most of it becomes concentrated in mature seeds. Legumes are characterized by a unique ability to accumulate molybdenum. Molybdenum activates Nitrogen metabolism and participates in nitrate reduction as an essential component of nitrate reductase. It is part of the active center of Nitrogenase and is a mandatory component for The Biosynthesis of phytoglobin. However, The Role of molybdenum is not limited to its Participation in the Primary processes of nitrate reduction and Biological Nitrogen Fixation; it also encompasses the final link in nitrogen metabolism—Protein Synthesis. Specifically, it activates amination and Transamination reactions, as well as ENZYMATIC REACTIONS OF NUCLEIC ACID METABOLISM, and affects Ribosomes, which directly execute protein synthesis using Messenger RNA and activated Amino Acids.

Under molybdenum deficiency, spotting appears on the leaves, although the Veins themselves remain light green. The chlorotic tissue swells, the leaf margins roll inward, and necrosis subsequently develops on the margins and tips.

Manganese

The average manganese content in plants is up to 0.001%. Manganese removal with crop yield on carbonate soils ranges from 0.1 to 0.7 kg/ha, while on acidic soils it can reach 0.5–5 kg/ha. Therefore, manganese fertilizers (such as manganese slag) are widely used to increase the yields of sugar beets, winter wheat, vegetables, and fruit crops. Manganese enters the plant as Mn²⁺ ions. It concentrates in the leaves and actively influences Cell growth processes as a cofactor of RNA polymerase II, which is responsible for mRNA synthesis in The Nucleus, and as a cofactor of auxin oxidase, which degrades indole-3-acetic acid.

Manganese is a strong oxidizing agent; thus, it plays a vital role in the redox reactions of Respiration (the Krebs cycle) and Photosynthesis (Water photo-oxidation, The Calvin Cycle). The manganese-water-oxidizing complex of Photosystem II acts as an intermediate that accumulates charges during water oxidation, while the coordination shell around the manganese ion serves to stabilize highly active intermediates.

The specific demand for manganese ions by A large number of Enzymes, particularly oxidases, should also be taken into account. When a plant utilizes nitrate nitrogen, manganese acts as a reducing agent, whereas under ammonium nutrition it acts as a strong oxidizing agent. In both cases, the intensity of redox processes and the synthesis of organic substances in the plant increase.

Manganese starvation leads to The Development of mottled chlorosis and spotting in plants, slows down growth, and causes necrotic spots to appear. On acidic soils, the availability of manganese to plants increases, whereas on alkaline soils, microflora converts a portion of the manganese into forms that are barely accessible for plant uptake. Applying manganese fertilizers to the soil improves its agronomic properties and promotes better utilization of ammonium, nitrate, and other fertilizers by plants.

Copper

The role of copper in plant life is quite specific: it cannot be replaced by any other element. The copper content in soil ranges from 0.5 to 20 mg/kg, while in plants it reaches up to 0.2 mg per 1 kg of mass, with leaves exhibiting the highest concentration. It enters The plant cell in ionic form (Cu²⁺, Cu⁺) and has been detected in various Proteins, including Key Enzymes of metabolic cycles, as well as in proteins with yet unknown functions. For instance, they are found in low-molecular-weight proteins of phloem exudates, various extracts, and subcellular and protein fractions. Copper can bind to Amino Acids and amines, and The formation of complex compounds of copper with organic acids and Phenolic Compounds is also possible. About 70% of total leaf copper is located in Chloroplasts, primarily in the form of plastocyanin. Because copper easily changes its valence by donating or accepting a single electron (e), it is a component of the Electron Transport Chain in Mitochondria and chloroplasts.

A number of catalytic copper proteins have been discovered in plants, including polyphenol oxidase, ascorbate oxidase, tyrosinase, etc. The most widespread enzyme is polyphenol oxidase, found in chloroplasts, mitochondria, and Peroxisomes. It participates in The oxidation of polyphenols and Tannins, and in the biosynthesis of secondary metabolites (e.g., Lignin). Diamine oxidase is a fairly common amine oxidase in plants. Alongside aliphatic diamines—putrescine and cadaverine—this enzyme also oxidizes aliphatic monoamines.

Copper affects phytoglobin synthesis and The activity of enzymes associated with biological nitrogen fixation. A whole range of copper-containing enzymes catalyze the reduction of O₂ to H₂O₂ or H₂O (polyphenol oxidase, mono- and diamine oxidases, ascorbate oxidase, cytochrome c oxidase). Polyphenol oxidase participates in the oxidation of polyphenols and tannins during tea leaf Fermentation, and its action explains the darkening of fruits and vegetables upon drying.

The detection of copper within cytochrome c oxidase and plastocyanin reveals new possibilities for studying the role of copper in photosynthesis and chlorophyll biosynthesis at the level of protochlorophyll transformations. Assumptions have been made regarding the possible participation of copper in systems associated with O₂ evolution during water photo-oxidation. There is also evidence supporting the role of copper in nucleic acid metabolism, owing to its rather strong chelating and complex-forming capacity.

The established direct correlation between the germination energy of cereal and legume seeds and their copper content suggests its involvement in the metabolism of growth-promoting physiologically active substances. Copper is also required for Ethylene synthesis. The positive effect of copper on lodging resistance is likewise likely related to its influence on the level of phenolic inhibitors, a reduction of which leads to stem elongation and lodging. Additionally, copper increases drought, heat, and frost resistance. The level of copper availability also affects the growth processes and absorptive capacity of the cereal ROOT system. Higher doses (100 µg of copper per liter) inhibit both root elongation and root Hair growth.

Copper is applied practically in crop production, particularly on poor peat-bog soils. Under copper deficiency, young leaves rapidly wilt and dry up without visible signs of chlorosis. An abnormal, intense leaf drop is frequently observed. It should be noted that copper can also exert toxic effects as a pollutant, particularly in aquatic ecosystems.

Zinc

The total zinc content in soil ranges from 20 to 100 mg/kg, while in plants it is 15–60 mg per 1 kg of dry mass. Zinc removal with crop yields varies between 50 g and 2 kg per hectare. The availability of Zn²⁺ cations to plants depends on soil pH, organic matter content, and phosphates. Water-soluble humic substances form soluble organic complexes containing zinc in the soil. Zinc is part of a significant number of diverse enzymes required for the functioning of hexokinase, enolase, triosephosphate dehydrogenase, aldolase, and Alcohol dehydrogenase. Carbonic anhydrase binds zinc quite tightly; the metal cannot be removed from its active center by dialysis or other Methods. As is well known, carbon dioxide entering The Cell dissolves in water to form H₂CO₃. The enzyme carbonic anhydrase catalyzes the release of CO₂ from H₂CO₃ for subsequent use in the dark reactions of photosynthesis.

The Effect of zinc on growth is directly related to its participation in auxin metabolism, particularly that of indole-3-acetic acid. It is known that IAA is chemically closely linked to Tryptophan. It has been proven that tryptophan is synthesized in leaves when sufficient zinc is present. A dependency of tryptophan synthetase activity on zinc has also been noted.

Zinc deficiency inhibits Cell Division rates, leading to morphological leaf alterations, disruptions in cell elongation phases, and impaired tissue differentiation. The primary symptom of zinc deficiency is interveinal chlorosis. Later, necrosis appears on the leaves, acquiring a purplish hue. A characteristic feature of zinc deficiency is rosette formation—a reduction in SHOOT length and small-leavedness. Like other trace elements, zinc is used for foliar feeding. The effect of zinc fertilizers on agricultural crops was first examined by P.A. Vlasyuk back in the 1930s.

It has been found that elevated doses of phosphorus and nitrogen exacerbate the symptoms of zinc deficiency. Zinc deficiency leads to a significant accumulation of soluble nitrogen compounds—amides and amino acids—which disrupts protein synthesis. The application of zinc enhances the synthesis of sucrose and starch, as well as the total content of CARBOHYDRATES and protein substances. Zinc fertilizers increase Plant resistance to drought, heat, and cold.

Cobalt

In plants, cobalt content ranges from 0.05 to 11.6 mg per 1 kg of dry matter. Legumes are characterized by elevated concentrations of cobalt, whereas cereals show lower levels. In legumes, cobalt accumulates in root nodules, which is associated with its specific role (alongside molybdenum) in nitrogen fixation processes. Cobalt also accumulates in Generative organs, indicating its involvement in Fertilization processes. Within the plant, it occurs as ions, as part of the porphyrin structures of cyanocobalamin (vitamin B₁₂), and as unidentified organic complexes. Vitamin B₁₂ is synthesized in plants even under sterile conditions. Cobalt, both in ionic form and as part of tetrapyrrole structures, performs specific functions in enzymatic reactions that regulate nucleic acid metabolism. Cobalt ions stimulate the Incorporation of Amino acids into intact ribosomes and positively affect The transfer of aminoacyl-tRNA to ribosomes. The necessity of cobalt for the growth of microorganisms in media where nitrates serve as the sole nitrogen source has been established. Cobalt takes an active part in redox reactions, stimulates the Krebs cycle, and exerts a positive effect on respiration and Energy Metabolism.

The deficiency of this element is particularly pronounced in lower plants, legumes, and sugar beets, which are the most responsive to its application.

Boron

Boron enters the plant as the boric acid anion (ВО₃^3-), with its content reaching up to 0.1 mg per 1 kg of dry matter. Depending on the yield, monocotyledonous plants remove 20–60 g of boron per hectare, while dicotyledonous plants remove 50–300 g. Sod-podzolic, gray forest, and brown forest soils are typically poor in boron. Complexes with the tetraborate anion are formed with sugars, keto acids, phenolic compounds, vitamins, and Coenzymes. They carry a negative charge and exhibit enhanced reactivity. Boron enhances the growth of pollen tubes and pollen germination, and increases the number of flowers and fruits. The most typical manifestation of boron deficiency disorders is the dieback of growing points. Certain changes in water exchange, particularly Transpiration, are also observed. Its deficiency further affects the synthesis, transformation, and transport of carbohydrates. The presence of boron positively influences the sugar content in fruits and starch accumulation in potatoes. Boron is of particular importance in limed, acidic, and podzolic soils, as it improves their agronomic properties. Sugar beets, flax, clover, tobacco, alfalfa, and sunflowers are the most sensitive to boron, whereas cereals are less sensitive.

Boron has a positive effect on the uptake of soil nutrients by plants. Therefore, applying small doses of boron to the soil increases the efficiency of fertilizers. In agricultural practice, boron fertilizers are widely used, notably borax, boric acid, boron-magnesium sulfate, and ground raw boron ores.

Vanadium

From a biological standpoint, a certain interrelation exists between vanadium and molybdenum in nitrogen fixation processes. It has been proven that vanadium stimulates atmospheric Nitrogen Fixation and the growth of Azotobacter in cultures where bound nitrogen has not been added. Vanadium exerts a positive effect on the activity of nitrate reductase and catalase, increases chlorophyll content in leaves, and enhances photosynthetic intensity.