PHYSIOLOGICAL ROLE OF TRACE ELEMENTS IN CROPS EN

PHYSIOLOGICAL ROLE OF TRACE ELEMENTS EN

Of the 74 chemical elements found in plant bodies, 11 are major elements (accounting for 99.95%), while the remaining 60 are trace and ultra-trace elements (accounting for 0.05%). Trace elements still play an important role in plant life.

In living organisms, trace elements can exist in many different forms. Many metals, including trace elements essential to plants such as B, Mn, Zn, Cu, Fe, Mo, Co, etc., have been found in the form of organic-mineral complexes. These organic-mineral complexes have fundamental chemical properties such as the following: the properties of a complex differ from those of its constituent components, and a complex can participate in reactions in which its individual components cannot participate.

Currently, the complexes of trace elements such as B, Cu, Fe, Mo, etc. have been studied in detail.

For example, boric acid forms complexes with a wide range of substances that constitute cells, such as fructose, galaclose, glucose, arabinose, mannose, ribose, etc. B forms a complex with ATP. Under the action of light, this complex releases the phosphoric acid group more readily than ATP alone. B may enhance the photosensitivity of ATP.

Many studies suggest that when combined with organic substances, the activity of trace elements increases by hundreds, thousands, or even millions of times compared with their ionic state. For example, in a complex, Fe is bonded not only to four pyrone rings but also to a specific protein, increasing its activity tens of millions of times.

Oparin clearly demonstrated that 1 mg of Fe bound in a complex is equivalent to the catalytic effect of 10 metric tons of inorganic Fe. Likewise, Co in cobalamin (Vitamin B₁₂) is thousands of times more reactive than inorganic Co. An organic-Cu complex can break down H₂O₂ millions of times faster than CuSO₄ or CuCl₂.

The issue of organic-mineral complexes, specifically organic-metal complexes, has become especially significant because of the discovery that inner-complex compounds (chelates) can be used to combat chlorosis caused by Fe deficiency, as well as diseases caused by deficiencies of other trace elements. Fe chelate (Fe-EDTA: Fe-ethylenediamine-tetra-acetic) is used to combat the very dangerous chlorosis in plants caused by Fe deficiency. Other chelate forms followed, such as Cu-EDTA, Zn-EDTA, Mn-EDTA, Mo-EDTA, etc., which are special foliar micronutrient fertilizers.

Recently, EDTA compounds have been found to act like growth regulators. For example, in an experiment with wheat sprouts, EDTA applied at a dose of 10^-5 M produced, after 19 hours, the same effect as 10^-5 M beta-indoleacetic acid (IAA).

1. General role

1.1. Trace elements and enzymes

It can be affirmed that trace elements are fundamental to life because most substance synthesis and metabolism processes are carried out by enzymes, and these enzymes contain trace elements. Approximately 1,000 enzyme systems are currently known, and about one-third of these systems are activated by metals. The metalloenzyme theory has become one of the central issues in both modern biochemistry and physiology. Metals form complexes with proteins that possess new properties. For example, the oxidation of ascorbic acid is accelerated nearly 1,000-fold by the Cu-containing enzyme ascorbin oxidase. Proteins combined with enzymes can form organic complexes with trace elements because many amino acids can form complexes with metals (chelates) through carboxyl or amino groups.

Research groups studying metalloenzyme theory have examined the action of metals as catalysts in binding with proteins or the active groups of enzymes from three perspectives:

• The effect of enzymes on the properties of metals.
• The effect of metals on the properties of
• The combined effect of metals and

However, it should be noted that many metals not only fail to activate enzymes but, on the contrary, inhibit them. This inhibitory effect is commonly observed with metals capable of denaturing enzyme proteins.

The following are some specific illustrations of this issue:

Some metalloenzymes contain only one specific metal in the composition of their active group (apoenzyme), such as Fe, which is an essential component of a series of oxidoreductases whose apoenzyme group is a porphyrin ring, including the cytochrome systems (a, b, c, f)—cytochrome oxidase, peroxidase, etc.; Cu in polyphenol oxidase, ascorbate oxidase, etc.

Some metalloenzymes whose active group is flavin (flavoproteins) commonly contain two or three metals, one of which plays the principal role. Typical examples include nitrite reductase, which contains Mo, Cu, and Mn; hyponitrite reductase, which contains Fe and Cu; nitrogenase, which contains Mo and Fe; nitrate reductase, which contains Mo and Cu; and hydroxylamine reductase, which contains Mn and Mo. In addition to true metalloenzymes, many metals (Na, Mg, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Cd, Cs, etc.) act as nonspecific activators of numerous enzymes. For example, the catalytic activity of carboxylase increases in the presence of Mg or Mn, Co, Fe, Zn, and Cd. Divalent metals (Mg, Zn) can substitute for one another in the activation of certain enzymes. In such cases, the metals generally form weak bonds, known as chelate bonds, with the side chains of enzyme proteins (such as NH groups₄⁺, COO^–, phenol, SH^–…).

1.2. Trace elements, growth regulators, and vitamins

The role of Zn in the biosynthesis of indole-type compounds is known, and serine biosynthesis is inhibited. Zn also acts in coordination with the gibberellin group. Mn supports the activity of the auxin group. Mn has a specific effect on auxin oxidase activity. B also has a positive effect on auxin biosynthesis. B also promotes the transport of growth regulators.

The relationship between trace elements and vitamins has also been studied. It has been found that Mn, Cu, Zn, and many other trace elements are concentrated in organs containing large amounts of vitamins. Cobalt is present in vitamin B₁₂. Boron is associated with vitamin C biosynthesis; the elements Mn, B, Zn, Mo, and Cu are associated with the biosynthesis of B-group vitamins (B₁, B₂, B₆, B₁₂).

1.3. Trace elements and metabolic processes

Trace elements have profound and multifaceted effects on photosynthesis. Chlorophyll biosynthesis requires not only Fe and Mg, but Mn and Cu are also concentrated in chloroplasts. The elements Co, Cu, Zn, and Mo have beneficial effects on chlorophyll stability. Zn and Co have beneficial effects on carotenoid synthesis. In general, trace elements positively affect the content and state of plant pigment groups, as well as the number and size of chloroplasts. Trace elements are structural components or activators of enzymes directly involved in both the light and dark phases of photosynthesis and therefore have a marked effect on photosynthetic intensity and the composition of photosynthetic products. The roles of Fe-containing enzymes and proteins (cytochromes, ferredoxin) and Cu-containing proteins (plastocyanin) in the electron transport chains of the two photosynthetic reactions are now well understood, as is the role of Mn in the splitting of H₂O, releasing O₂. During the light phase, if Mn is deficient, the Hill reaction cannot occur, the release of O₂ is inhibited, and the amount of H₂O₂ will become toxic to the cell. In the dark phase of photosynthesis, trace elements participate in the metabolic enzymes of the C₃, C₄, CAM cycles, etc. B, Mn, Zn, Cu, Co, and Mo help promote the transport of photosynthetic products from the leaves to storage organs. Trace elements also help limit the decline in photosynthetic intensity when plants experience drought, high temperatures, or senescence.

Trace elements have direct effects on respiration. Many elements, particularly Mg and Mn, are powerful activators of enzymes that catalyze both the anaerobic breakdown (glycolytic pathway) and aerobic breakdown (Krebs cycle) of organic substrates during respiration. Trace elements are essential structural components of oxidoreductases directly involved in the most important respiratory reactions (Fe-containing cytochrome systems and Cu-containing polyphenol oxidase and ascorbate oxidase). Many trace elements directly affect oxidative phosphorylation (ATP formation), meaning that they affect the useful energy efficiency of respiration.

1.4. Trace elements and plant stress tolerance

• Salt tolerance

The elements that affect plant salt tolerance include Mn, B, Zn, Al, Cu, Mo, etc. They reduce the permeability of the protoplasm to Cl; increase the rate of P, Ca, and K uptake; and increase the accumulation of substances with protective effects (such as globulin and albumin). Applying B, Mn, Al, and Cu to plants or spraying them onto leaves increases the viscosity and content of hydrophilic colloids in leaves under saline soil conditions, thereby increasing the amount of bound water and the water-retention capacity of the leaves. B, Mn, and Al affect salt tolerance because they increase the content of soluble carbohydrates in the leaves, ensuring sufficient osmotic pressure to supply water to cells and stabilizing the colloidal system of the protoplasm. Under moderately saline conditions, Mn, Co, Mo, and Cu greatly increase the stability of protein-bound chlorophyll in chloroplasts, thereby increasing the tolerance of the chlorophyll-protein system.

• Drought tolerance

Drought promotes hydrolytic processes in plants, weakens protein synthesis, and leads to the accumulation of large amounts of free amino acids, which inhibits plant growth. Al, Co, and Mo have positive effects on drought tolerance because they can maintain high levels of protein synthesis under these unfavorable conditions. B, Zn, Cu, Mo, Co, Al, etc. have beneficial effects on the synthesis, metabolism, and transport of carbohydrates from leaves to storage organs, which is one of the main factors enhancing plants’ drought and heat tolerance, particularly during critical periods.

2. The role of certain important trace elements
2.1. The role of boron (B)

B is a cofactor of many enzyme systems. When B is deficient, the growing points of stems, roots, and leaves gradually die because B plays a major role in carbohydrate metabolism. When B is deficient, large amounts of sugar accumulate in the leaves, causing the growing tip to lack carbohydrates and resulting in an excess of NH₃ because carbohydrates are very effective acceptors of NH₃. Recently, it has been suggested that growing points die because nucleic acid metabolism is disrupted.

With B deficiency, RNA and ATP levels in the growing points of stems decrease markedly due to reduced energy metabolism.

B can also increase dehydrogenase activity. B also ensures the amount of O₂ for roots. B increases protein synthesis in plants, so it also helps prevent excessive vegetative growth and lodging. B increases cation uptake during nutrition and promotes P transport within the plant.

When B is deficient, the rate of Ca uptake decreases, disrupting cell wall formation.

Many studies have found that B affects pigment synthesis, photosynthesis, mineral nutrition, N metabolism, pollination, and fruit setting in plants.

Sources of B fertilizer include H₃BO₃, Mg₃(BO₃)₂, borax: Na₂B₄O₇.10 H₂O

2.2. Role of Copper (Cu)

Cu is a component of the oxidase enzyme system. Cu deficiency is associated with N nutrition. Cu has a major effect on protein synthesis and participates in the initial stage of nitrate assimilation. The role of Cu in protein synthesis is associated with nucleic acid metabolism (RNA decreases under Cu deficiency). Cu contributes actively to the formation and stability of chlorophyll. Cu strongly affects the metabolism of carbohydrates, phosphatides, nucleoproteins, vitamins, and growth hormones. When applying nitrogen fertilizer, especially NH₄⁺ the demand for Cu also increases.

In addition to preventing excessive vegetative growth and lodging, Cu also improves drought and cold resistance and increases the water-retention capacity of tissues.

A common source of Cu fertilizer is CuSO₄. Waste products from pyrite production may also be used to fertilize crops.

2.3. Role of Zinc (Zn)

Zn is an essential component of the enzyme carbonic anhydrase, which catalyzes the reaction:

H₂CO₃ = CO₂ + H₂O

Zn deficiency causes carbonic acid to accumulate, impeding oxidation and disrupting metabolism. Zn participates actively in oxidation-reduction processes. It is a component of alcohol dehydrogenase, glutamate dehydrogenase, and lactate dehydrogenase, and participates in the metabolism of compounds containing HS groups.

Zn plays an important role in the metabolism of P, carbohydrates, proteins, and nucleic acids. Under Zn deficiency, inorganic P accumulates in tissues, impeding oxidative phosphorylation. Zn deficiency increases reducing sugar and free amino acid levels, while sucrose and starch decrease because protein synthesis is inhibited; consequently, RNA and DNA decrease, while ribonuclease enzyme activity increases.

Zn promotes the synthesis of growth hormones, especially auxin. Zn plays an active role in pollen development, particularly in egg cells and embryos. Zn deficiency makes maize, soybeans, timber and fruit trees, sugarcane, flax, grapes, and tomatoes susceptible; oranges and tangerines develop small, yellow-mottled leaves, while albinism appears in maize.

The main fertilizer source is ZnSO₄ applied to alkaline and sandy loam soils.

2.4. Role of Manganese (Mn)

Mn deficiency usually causes a marked reduction in photosynthesis. Mn is believed to participate in the reaction that releases O₂ during photosynthesis (the photolysis of water).

With Mn deficiency, most of the Fe in cells is converted into the reduced Fe²⁺ form, which harms the plant. If Mn is excessive, Fe is converted into the Fe³⁺ form, which is physiologically inactive and causes plants to yellow. Therefore, plants grow normally only when the Mn/Fe ratio is appropriate (from 1/2 to 1/3).

Mn affects the activity of enzyme systems that strongly break down carbon compounds, such as peptidase, phosphatase, and decarboxylase.

Mn also aids N uptake, especially in the NO₂form. The main fertilizer source is MnSO₄.

2.5. Role of Molybdenum (Mo)

Mo is essential for many plants. Symptoms of Mo deficiency include leaf yellowing due to nitrogen deficiency, slow plant growth, and substantial accumulation of NO₃^–in tissues. With Mo deficiency, legumes have few, small, gray nodules. Mo deficiency has been identified in more than 40 plant species. Mo is essential for microorganisms capable of fixing N₂ such as Azotobacter, Clostridium pasteurianum, blue-green algae, and bacteria living symbiotically with legumes.

Mo is a component of the nitrate reductase enzyme that catalyzes nitrate reduction. Mo participates in amino acid and protein synthesis, particularly increasing the ratio of protein-N to total N.

Mo affects the synthesis and transport of carbohydrates, the synthesis of pigments and vitamins (especially vitamin C), and the assimilation of P, Ca, and several other elements. Ca and Mo have complementary effects, so applying Ca to acidic soils increases the availability of stored Mo.