The physiological role and biochemical functions of boron in plants

Boron. The boron content in plants is 1–20 mg/kg of dry matter. It enters plants in the form of an anion. Boron uptake is possible both as a result of passive diffusion into the roots and under metabolic control. Its movement from the roots to the above-ground organs is performed exclusively passively along with the transpiration stream.

Boron, due to the specific structure of its atomic electron shell, can easily form compounds with almost all chemical elements, as a result of which it participates in the formation and maintenance of the structure of intermolecular and supramolecular complexes of biopolymers, primarily proteins, nucleic acids, lipids, and polysaccharides. Complexes of these biopolymers are the basis of the most important cell components – ribosomes, the membrane apparatus, chromatin, and cell walls.

The importance of boron in the formation of plant cell walls is evidenced by the content of xylose and arabinose – components of hemicelluloses in the vascular system – only in plants that received additional boron. In plants, boron forms complexes with such organic compounds as:

• d-fructose
• α-galactose
• α- and d-glucose
• glycerol
• α-mannitol
• pyridoxine
• salicylic acid

This element easily interacts with methyl alcohol – one of the components of pectin substances.

The functions of boron in the plant organism are primarily related to carbohydrate metabolism; sugar transport across membranes; the synthesis of DNA, RNA, and phytohormones; cell wall formation and tissue development. The formation of boron-carbohydrate complexes influences the orientation of cellulose micelles in the cell wall, which increases its elasticity. Its role is especially significant in the transport of sugars across membranes, as the borate-polysaccharide complex is more mobile than polar sugar molecules.

Boron has a significant influence on the photosynthetic activity of plants: it promotes an increase in the number and size of chloroplasts, the content of chlorophylls and carotenoids in leaves, the intensity of photosynthesis in the morning and evening hours, and a reduction in its midday depression, which is associated with an increase in the water-holding capacity of cells. Boron takes part in the processes of fertilization and fruiting: it is necessary for the formation of viable pollen and promotes its rapid germination and the growth of pollen tubes.

An important role is assigned to this element in phenolic metabolism. Boron, as an effective complexing agent, binds phenol into a non-toxic compound. This element is not a component of enzymes, but due to its ability to form complexes with polyhydroxyl compounds, it influences the rate and direction of enzymatic reactions. In particular, boron affects the activity of catalase, invertase, indoleacetic acid oxidase, peroxidase, polyphenol oxidase, pectase, and glucosidase. Boron stimulates the formation of nodules on the roots of legumes. Under its influence, the fixation of atmospheric nitrogen by these plants increases.

With a boron deficiency, the synthesis of nucleic acids is disrupted, as well as the formation, transformation, and transport of carbohydrates, and the formation of reproductive organs; the death of meristematic cells and the degradation of the plant vascular system occur. Boron cannot be reutilized because it does not move from older plant organs to younger ones.

A characteristic sign of boron deficiency is the appearance of black necrotic spots on young leaves and apical buds. In plants experiencing boron deficiency, caffeic and chlorogenic acids, which inhibit growth, accumulate; cells differentiate poorly, the development of the vascular system is disrupted, leaves become thin, and flowers do not form. Cytoplasm coagulates in sieve tubes, therefore the outflow of sugars through the phloem is sharply inhibited, and they accumulate in the leaves. With a boron deficiency, vessels become twisted and compressed, which disrupts the transport of water and mineral nutrients to growth cells. A lack of carbohydrates, water, and mineral salts causes the death of stem and root growth points. In plants experiencing boron deficiency, roots are poorly supplied with oxygen and immunity is reduced.

Symptoms of boron deficiency for individual crops are as follows: sugar and forage beet develop heart rot, table beet develops internal black spot disease, the growth point of flax dies off due to bacterial infection, sunflower exhibits browning of the apex and cessation of young leaf growth, potato shows an increased incidence of common scab on tubers, tobacco shows apical dieback, fruit crops exhibit corky spots under the skin of small fruits, stone fruits are affected by gummosis and suberization, and grape nursery plants exhibit necrosis of the wood vessels. These diseases are particularly manifested in hot, dry years.

Cobalt. The cobalt content in plants is 0.02–12 mg/kg of dry mass. It enters plants in the form of a cation, chelate compounds, and vitamin B12. In the plant organism, 50% of cobalt is found in ionic form, 20% is in the form of cobamide compounds and as part of vitamin B12, and 30% consists of unidentified highly stable organic compounds. Cobalt in an organic complex – vitamin B12 – is 100,000 times more active than inorganic cobalt.

The physiological role of cobalt in plants is primarily associated with its participation in redox processes occurring within the cell. Cobalt is involved in isomerization reactions. Specifically, it catalyzes the conversion of glutamine to methylaspartic acid and methylmalonyl-coenzyme. Furthermore, cobamide coenzymes take part in the migration of hydrogen radicals within pyrrole nuclei and in intramolecular hydrogen transfer reactions during the conversion of diols into aldehydes. In the form of a cation, cobalt catalyzes the oxidation of carotene, organic acids, and unsaturated fatty acids. It participates in enzymatic reactions such as carboxylation and decarboxylation, hydrolysis of peptide bonds and phosphoric esters, and the transfer of phosphoric groups. Cobalt increases the activity of the enzymes phosphatase, arginase, lecithinase, aminopeptidase, nitrate reductase, hydrogenase, ascorbate oxidase, catalase, and peroxidase. Arginase, aminopeptidase, lecithinase, and nitrate reductase are enzymes of nitrogen metabolism, which predetermines the influence of cobalt on this vital metabolic process. By altering the activity of catalase, peroxidase, polyphenol oxidase, and alkaline phosphatase, it regulates, to a certain extent, the intensity of respiration in plants. By influencing the activity of hydrolytic enzymes like protease and lipase, cobalt stimulates physiological and biochemical processes in germinating seed.

It has been established that cobalt exerts a stimulating effect on DNA to a certain extent by participating in the spiraling and despiraling processes of its molecules. Simultaneously, this element influences the decomposition of peroxides during their formation, activates the synthesis of ribonucleic acids in plants, and participates in changing the permeability of the plasmalemma, thereby facilitating the selective absorption of ions from the external environment by the root system.

Cobalt contributes to the saturation of leaves with chloroplasts. It positively influences chlorophyll content and increases its stability. The effect of cobalt on chlorophyll accumulation is due to its positive impact on the stability of the chlorophyll-protein-lipid complex. By increasing the amount of chlorophyll and its photosynthetic activity, cobalt enhances the overall intensity of photosynthesis. It promotes a more intensive outflow of assimilates from leaves and the conducting system to stems, roots, and reproductive organs.

Cobalt has a positive effect on the growth of aerial organs and the root system of the plant. In this regard, it promotes:

• earlier flowering;
• a reduction in the duration of the plant growing season.

The mechanism of cobalt's influence on plant growth and development is based on its connection with the hormonal balance of the cell, primarily in the auxin-ethylene link.

Cobalt promotes more intensive uptake of nitrogen, phosphorus, potassium, and manganese by plants. The necessity of cobalt for nitrogen fixation is linked to the positive effect of vitamin B12 on the quantity and quality of leghaemoglobin contained in the nodules of legumes. With the help of cobamide coenzymes, the biosynthesis of protein compounds in general, and the synthesis of the nitrogen-fixing enzyme nitrogenase in particular, is activated in the nodules, which ultimately strengthens the nitrogen fixation process. The enhancement of nitrogen fixation by cobalt can also occur through its positive influence on hydrogenase. Hydrogenase, as is known, is an enzyme that carries out the activation of hydrogen. Cobalt plays an important role in energy metabolism, and under its influence, the amount of ATP increases.

In the case of cobalt deficiency, physiological and biochemical processes and plant growth are weakened; productivity decreases, and harvest quality deteriorates. The main symptoms of its deficiency in plants are:

• weak growth;
• interveinal leaf chlorosis;
• high sterility of flowers;
• low plant productivity.

Symptoms of cobalt deficiency in plants are similar to signs of nitrogen starvation. External signs of cobalt deficiency are clearly manifested mainly in legumes. With an excess of cobalt in the growth medium, the growth of the root system is sharply restricted, and the leaves become chlorotic.

Manganese. The average manganese content in plants is presented in the table:

Average content	Typical concentration range
0.001 % or 10 mg/kg dry mass	from 2 to 400 mg/kg

Manganese enters plants in the form of a cation and an anion of manganates. Its movement through conducting paths is an active process, as less manganese moves through xylem elements than through phloem elements. In plants, this element is mainly found in the form of free cations; however, its complex compounds with organic molecules are also possible.

The physiological role of manganese in plant life is determined to a significant extent by its participation in enzyme activity. This element is part of the active groups of a number of enzymes that catalyze various links of metabolic reactions in plant cells, where it serves as a connecting link between a coenzyme, pyrophosphate, and a substrate.

There are about 25 metal-enzyme complexes activated by manganese known to science. This element performs the following functions:

• increases the activity of oxidative enzymes such as ascorbate oxidase and peroxidase;
• plays an important role in activating Krebs cycle reactions;
• participates in dehydrogenation and decarboxylation reactions.

Manganese: A regulator of nitrogen nutrition and photosynthesis

Manganese directly influences crop productivity by managing critical stages of carbohydrate and nitrogen metabolism. It is necessary for breaking down sugars into pyruvic acid and ensures the translocation of finished assimilates from leaves to roots, stems, and reproductive organs. Without this element, photolysis of water and reduction of carbon dioxide during photosynthesis are impossible. Moreover, manganese supports the structure of chloroplasts, protecting chlorophyll from rapid degradation under direct sunlight.

Depending on the form of nitrogen fertilizer, manganese changes its role in the plant. During ammonium nutrition, it acts as an oxidizing agent, and during nitrate nutrition, as a reducing agent, helping to reduce nitrates to ammonia. If manganese is excluded from nutrition, the plant begins to absorb other elements sporadically, which completely disrupts their balance. The element is also necessary for the formation of ascorbic acid, protein synthesis, and the maintenance of DNA and RNA structures.

At the cellular level, manganese activates key enzymes of respiration and sugar breakdown: malate dehydrogenase, oxalosuccinate decarboxylase, pyruvate decarboxylase, and isocitrate dehydrogenase. It participates in condensation reactions of organic acids and is a component of arginase, phosphotransferase, and hydroxylamine reductase. Additionally, manganese enhances the effect of indoleacetic acid on growth processes, mitigating the inhibitory influence of malic and succinic acids.

Copper: The basis of respiration and energy transfer

Copper enters plants through the root system in the form of cations or chelated compounds. About 2/3 of the total copper in a plant organism is bound in stable organic complexes, which are concentrated in the mitochondria. This element is a component of enzymes that manage the dark reactions of photosynthesis and respiration. They perform oxidation by transferring electrons from the substrate to molecular oxygen.

Parameter	Content in the plant
Copper fraction in dry matter	2–20 mg/kg

Copper is a direct component of the following essential enzymes:

• laccase (catalyzes the oxidation of hydroquinone and orthodiphenol laccol);
• ascorbate oxidase (catalyzes the oxidation of ascorbic acid);
• polyphenol oxidase (involved in the addition of an electron to phenols);
• uricase (oxidizes uric acid);
• tyrosinase (synthesizes the pigment melanin from the amino acid tyrosine).

The specific role of copper in photosynthesis is due to its presence in the low-molecular-weight protein plastocyanin. This protein functions as an electron carrier between photosystems PS II and PS I. Copper deficiency blocks these processes, and the absence of the enzyme tyrosinase leads to albinism — the complete loss of green coloration and the plant's inability to photosynthesize.

Physiological role of copper: From photosynthesis to protection against lodging

Copper works in the plant as an active catalyst for metabolic processes. Due to its ability to easily transition from a divalent to a monovalent state (Cu²⁺ ↔ Cu⁺), this element participates in redox reactions as an electron donor and acceptor. Copper increases the activity of a whole group of enzymes: catalase, nitrate reductase, peroxidase, carbonic anhydrase, hexokinase, aldolase, phosphorylase, and phosphoglucomutase. This directly affects the intensity of respiration, nutrient uptake, and stress resistance of crops.

• Share of leaf copper in chloroplasts — about 75%
• Share of copper in the polar lipid fraction — more than 20%
• Molybdenum content in dry matter — 0.2–2 mg/kg
• Molecular weight of nitrate reductase — 200–300 kDa

The micronutrient directly participates in building the photosynthetic apparatus. Located in the chloroplasts, copper stimulates the synthesis of chlorophyll and carotenoids, increasing net photosynthetic productivity. In nitrogen metabolism, it regulates the primary assimilation of mineral nitrogen through nitrite and nitrate reductase enzymes, and also participates in the breakdown of amino acids and protein synthesis. Furthermore, copper activates phosphorylase, helping the plant to accumulate sugar phosphate esters during the initial stages of development and more rapidly incorporate mineral phosphorus into organic compounds — phospholipids and nucleotides.

Stem strength and lodging resistance also depend on copper. The copper-containing enzyme polyphenol oxidase regulates the balance of auxins and natural growth inhibitors of a phenolic nature. When adequately supplied with copper, plants better tolerate adverse weather conditions and maintain an upright stem stand.

Molybdenum: transport and key functions in nitrogen metabolism

Molybdenum enters plants through the roots as a molybdate anion or as part of chelate complexes. This process requires energy expenditure against the concentration gradient. The element accumulates primarily in the phloem and vascular parenchyma. Unlike most other micronutrients, molybdenum is practically not reutilized (not redistributed from old organs to young ones), yet plants can accumulate it in large quantities without signs of toxicity.

In the plant organism, molybdenum is part of three enzymes: nitrate reductase, xanthine dehydrogenase, and formate dehydrogenase. It is also an important component of nitrogenase, which ensures the fixation of atmospheric nitrogen by nodule bacteria and free-living microorganisms. Additionally, molybdenum activates the enzymes malate dehydrogenase and succinate dehydrogenase.

The key enzyme of nitrogen metabolism, nitrate reductase, is a complex protein containing heme and molybdenum. It consists of two subunits that sequentially transfer electrons from NADH or NADPH to reduce nitrates:

Nitrate reductase subunit	Composition	Function performed
First	FAD, heme	Electron transfer from NADH to heme (reactions A and B)
Second	Molybdenum	Nitrate binding and electron transfer from heme (reactions C and D) with a change in molybdenum valence

The main biochemical functions of molybdenum are reduced to the following processes:

• reduction of nitrates to ammonia and subsequent biosynthesis of amino acids;
• fixation of molecular nitrogen by free-living microorganisms and nodule bacteria in symbiosis with grain legumes;
• participation in the biosynthesis of nucleic acids and proteins.

Molybdenum and selenium: metabolism regulation and stress protection

Molybdenum plays a key role in nitrogen uptake. Only in the presence of this element is the breaking of triple bonds in the nitrogen molecule possible. Molybdenum enhances nitrogen uptake by plants and accelerates the synthesis of amides, amino acids, and proteins. This contributes to an increase in the chlorophyll content in leaves and increases the intensity of photosynthesis, which ultimately improves the frost resistance and drought tolerance of crops.

In the plant organism, molybdenum participates in redox reactions, acting as an important link in the electron transport chain. By changing the local pH of the environment, molybdate ions regulate chemical reactions. The element also inhibits acid phosphatase, protecting phosphorylated compounds from premature hydrolysis, and neutralizes the toxic effect of mobile aluminum in acidic soils.

When molybdenum is deficient in the nutrient medium, the following disorders develop:

• disruption of chlorophyll synthesis and delayed photosynthesis;
• excessive accumulation of nitrates in tissues;
• limited development of nodules on the roots of grain legumes.

Deficient plants appear underdeveloped and acquire a pale-yellow color.

Selenium activates respiratory and glycolysis enzymes, reduces mutation frequency, and stimulates cell division. The element is necessary for the biosynthesis of formate dehydrogenase, activates fumarase, and has a positive effect on the activity of nitrate reductase. Selenium also participates in chlorophyll formation, the synthesis of tricarboxylic acids, and the metabolism of long-chain fatty acids. Due to this, the plant's resistance to water stress, drought, and salinity is increased.

The main source of selenium is the soil, from where roots absorb it via active transport and ion exchange processes. If the content of the element in the soil solution is low, plants are able to absorb it directly from the atmosphere. With excessive intake, selenium binds to organic acids and is removed from the plants. In tissues, it is found in the form of elemental selenium, selenates, selenites, selenopeptides, and analogs of sulfur-containing amino acids.

Parameter	Selenium content in dry matter
Standard content in plants	0.01–10.0 mg/kg

The maximum amount of selenium is accumulated by cereal crops, while its concentration in root and tuber crops is significantly lower. Selenium is capable of substituting sulfur in methionine and cysteine, forming selenomethionine and other non-protein amino acids. In accumulator plants, this process occurs particularly actively. Selenium also replaces sulfur in the composition of certain enzymes (galactosidase, oxytocin, putidaredoxin), activates papain and glyceraldehyde 3-phosphate dehydrogenase, regulating protein glycolysis and hydrolysis processes.

Zinc: uptake and enzymatic functions

Zinc enters plants from the soil solution in the form of cations and chelate compounds. The root system absorbs it both passively and through active transport mechanisms. The physiological role of zinc in the plant organism is determined by its mandatory presence in a wide range of enzymes that regulate key life processes.

Parameter	Zinc content in dry matter
Standard content in plants	15–60 mg/kg

Effect of zinc on hormonal balance, metabolism, and crop resistance

Zinc directly controls nitrogen metabolism in plants. With its deficiency, protein synthesis is blocked, causing harmful non-protein compounds — amides and free amino acids — to accumulate in tissues. Sufficient zinc supply activates the synthesis of protein substances, improves protein conformation, and strengthens the bond between chlorophyll and protein. This protects chloroplasts from premature decay, increases the content of green pigment, and stimulates photosynthesis. Furthermore, the element is critical for fertilization processes and the proper development of the embryo.

The most important practical function of zinc is the regulation of auxin levels. In the case of a deficiency of this micronutrient, the content of plant growth hormones decreases sharply. This is caused by two main biochemical reasons:

• Impaired biosynthesis of indole-3-acetic acid (IAA) due to disruptions in the formation of its precursor, tryptophan, reduced activity of tryptophan synthetase, and low content of vitamin B6.
• Increased oxidative decarboxylation, which leads to the direct destruction of IAA.

The micronutrient regulates the phosphorus balance in the plant. Zinc shortage triggers uncontrolled phosphorus uptake by roots and its excessive transport to aerial organs, where it accumulates in inorganic form. Optimal zinc nutrition constrains this excess flow and helps the plant efficiently utilize phosphorus. Zinc also increases crop immunity, improving drought tolerance, winter hardiness, and resistance to fungal and bacterial infections.

Vanadium and iodine: nitrogen fixation and stimulation of life processes

Vanadium is one of the essential elements that regulate respiratory gas exchange, photosynthesis, as well as carbohydrate, lipid, and nucleic acid metabolism. It activates nitrate reductase, stimulating protein accumulation in tissues. Its role in nitrogen fixation is especially high: vanadium is necessary for nodule bacteria and free-living microorganisms to assimilate atmospheric nitrogen. The highest amount of the element is accumulated by legumes, cereals occupy an intermediate position, and fruit and berry crops contain the least.

At the cellular level, vanadium protects the genetic structure by inhibiting ribonuclease and preventing RNA degradation. It activates the removal of histones from nuclear chromatin, creating favorable conditions for biochemical processes during seed germination. The micronutrient also stimulates photosynthetic activity and increases chlorophyll content. The majority of vanadium is concentrated in roots and leaves, while stems and seeds contain significantly less.

• Average vanadium content — 1.0 mg/kg of dry matter
• Accumulation in legume nodules — 3–4 mg/kg of dry matter
• Average iodine content — 0.42 mg/kg of dry matter

Iodine is also a physiologically active component of the plant organism. Its accumulation in tissues varies widely depending on the species of the crops and the general ecological situation. Data on the limit levels of accumulation of this element are provided in the table below.

Indicator of iodine accumulation in plant dry matter	Value, mg/kg
Minimum value	0.01
Maximum value	2.50

Iodine distribution in plants is extremely uneven: depending on the organ, its concentration can differ by hundreds of times. The main part of the micronutrient accumulates in the underground part — roots and rhizomes. In aerial organs, the highest amount of the element is concentrated in leaves, whereas stems and grain contain it in minimal quantities.

• Fluctuation in iodine concentration — up to 250 times
• Main accumulation zones — roots and rhizomes
• Primary aerial accumulator — leaves

Biochemical mechanisms of iodine influence on the crop

Iodine is a key structural component of the cell and takes a direct part in metabolism. It is an essential part of amino acids, peptides, polypeptides, and proteins. Furthermore, this trace element is a component of the biologically active hormone thyroxine (tetraiodothyronine).

Acting as an active electron donor or acceptor, iodine ions create a strong electromagnetic disturbance. This allows them to indirectly influence enzyme activity by regulating redox, transport, and synthetic processes. The element also alters protoplasm viscosity and the structural properties of proteins, which directly improves the plant water regime.

Iodine significantly influences the efficiency of photosynthesis. It not only regulates the water status in tissues and activates pigment synthesis, but also directly participates in electron transfer reactions. By influencing nitrogen, carbohydrate, and redox metabolism, the trace element accelerates growth processes and increases the total crop productivity.