The biological role of sulfur in plant nutrition and development

Sulfur is a basic element for building plant proteins and supporting photosynthesis. It directly affects the development of vegetative mass and the formation of a high-quality harvest. In plants, this element exists in two forms: reduced (as part of sulfhydryl groups and disulfide bonds of young growing organs) and oxidized (the initial and final product of protein metabolism).

• Content in dry mass — 0.005–1%
• Sulfur-containing amino acids — 3 types
• Nutrient sources — sulfates of Ca, Mg, K, Na

Plants absorb sulfur from the soil through the root system in the form of the SO₄^2- ion. This process occurs via co-transport with the H⁺ ion or in exchange for HCO₃^- ions. Plants can also assimilate the element through leaves from the atmosphere in the form of SO₂ gas. The main nutrient sources are calcium, magnesium, and potassium sulfates, and on solonetz soils, sodium sulfates.

Sulfur is a component of essential compounds that govern metabolism:

• Amino acids cysteine, cystine, and methionine, which are present in a free state or build proteins.
• Proteins of chloroplasts and cytoplasm, which determine the intensity of photosynthesis.
• Biotin, thiamine, coenzyme A, and glutathione, involved in respiration and fat synthesis.
• Coenzymes NAD, NADP, FAD, which attach to enzyme proteins via a sulfhydryl group and transfer hydrogen ions.
• Ferredoxins, which control the metabolism of non-heme iron.
• Anions SO₄^2-, which maintain the general ionic balance in cells.

Signs and consequences of sulfur deficiency

Insufficient sulfur supply blocks amino acid synthesis, disrupts the formation of chloroplasts, and can cause their degradation. As a result, photosynthesis is inhibited, the growth of above-ground organs is delayed, and stems become thin, lignified, and brittle. Due to vitamin B1 (thiamine) deficiency, root growth intensity also decreases sharply.

Calcium. The calcium content in plants is 0.05–0.5% based on dry mass. In plants, this element is found both in a free and adsorbed state in the form of various salts, which concentrate in vacuoles or cell walls as calcium phosphates, carbonates, and especially calcium oxalate. The sources for plant nutrition are carbonates, sulfates, chlorides, nitrates, and other inorganic calcium salts, as well as calcium humate. It enters the plant as a Ca2+ cation with the transpiration stream. This element is necessary for the plant as early as the seed germination phase. The role of calcium in plant development is multifaceted. It is a component of the nucleus, mitochondria, ribosomes, plastids, cytoplasm, cell wall, and other organelles and cell inclusions, and is necessary to maintain their structure. Calcium is part of membranes, connecting the negatively charged "heads" of phospho- and glycolipid molecules with negatively charged radicals of protein globules. Thus, calcium stabilizes the structure of membranes, which is why its presence is necessary for their normal functioning. When plants are grown in an environment with a calcium deficiency, membrane permeability increases, and they cease to be a barrier preventing the free diffusion of ions. Calcium is also a component of pectic substances that connect cell walls to each other; without it, the plasmalemma does not form. Calcium enhances metabolism, plays an important role in photosynthesis and the movement of carbohydrates, as well as in the processes of nitrogen assimilation by plants, and accelerates the breakdown of storage seed proteins during germination. It regulates the acid-base balance in the cell and changes the colloidal state of the cytoplasm, increasing viscosity and reducing water content. Calcium participates in maintaining the structure of chromosomes, acting as a link between DNA and protein. As a secondary messenger, calcium is involved in transmitting a signal received by the cell to the genome. As a result, many enzymes are activated – dehydrogenases, amylases, phosphatases, kinases, and lipases. Its action is based on participation in the formation of the quaternary structure of proteins, the creation of bridges in enzyme-substrate complexes, and influence on the state of allosteric centers. The regulatory influence of calcium on metabolism also depends on its interaction with the intracellular calcium receptor – the protein calmodulin. The calcium-calmodulin complex activates protein kinases, transport Ca-ATPase, actomyosin ATPase, phosphodiesterase, and other cell enzyme systems. The concentration of intracellular calcium is regulated with the participation of a calcium-containing enzyme – calmodulin. In addition, the Ca-calmodulin complex controls the assembly of microtubules of the achromatic spindle, the formation of the cell cytoskeleton, and the formation of the cell wall. The influence of calcium ions on the assembly-disassembly of cytoskeletal elements explains its necessity for the normal course of mitosis. Calcium is involved in the fusion of Golgi apparatus vesicles during the formation of the phragmoplast* and the new cell wall. Calcium is recognized for its function of neutralizing oxalic acid in plants. The role of this element as an antagonist to other cations is significant. By delaying the excessive intake of some elements into the cell, it simultaneously stimulates the uptake of others.

Calcium plays an important role in restoring and maintaining the balance of the soil solution, which ensures the normal intake

The phragmoplast is a characteristic structure in the equatorial plane of a dividing cell, formed by the remnants of the mitotic spindle connective tissues between two nuclei in the telophase of mitosis. It serves as the basis for the formation of the cell wall between daughter cells.

of mineral nutrition elements into the root system. In the case of calcium deficiency, young meristematic tissues and the plant root system are the first to suffer. Dividing cells fail to form new cell walls, resulting in multinucleate cells. Calcium deficiency leads to the swelling of pectic substances, which causes the mucilaginous degradation of cell walls and cell destruction. As a result, roots, leaves, and individual parts of the stem rot and die. Plants experiencing calcium starvation show reduced resistance to adverse environmental conditions. A deficiency of this element during the cultivation of crops is more frequently observed on solonetzic soils and soils of light texture.

Physiological role of magnesium and conditions of its deficiency

Magnesium is a critically important element for yield formation, without which key plant life processes stop. It directly governs photosynthesis, nitrogen uptake, and energy metabolism. If a crop lacks magnesium, the synthesis of proteins and carbohydrates is blocked, which instantly affects growth and produce quality.

• Content in dry matter — 0,05–0,5 %
• Share in chlorophyll mass — about 3 %
• Bound magnesium in a cell — up to 50 %
• Dangerous Ca:Mg ratio in soil — more than 11

In a plant cell, magnesium exists as free or adsorbed ions, as well as in chelated form. Its main role is associated with photosynthesis: the element is a component of the chlorophyll molecule and cannot be replaced by anything else. Furthermore, magnesium is necessary for the synthesis of carotenoids and the direct precursor of chlorophyll — protoporphyrin IX.

Magnesium serves as an activator for a multitude of respiration and photosynthesis enzymes: ribulose-1,5-bisphosphate carboxylase, phosphokinase, ATPase, enolase, and enzymes of the Krebs cycle. It catalyzes the formation of ATP, helping to store energy, and regulates its transfer between photosystems I and II. In reactions of photosynthetic water decomposition, magnesium ensures the transfer of hydrogen to an acceptor according to the scheme: DPN + ATP = Mg2+ + TPN + ADP.

The element directly influences protein and carbohydrate metabolism. It stabilizes the structure of ribosomes by binding RNA and protein — without magnesium, their subunits disintegrate, and protein synthesis ceases completely. With magnesium deficiency, the formation of plastids and mitochondria is disrupted; simple sugars accumulate in the tissues, but their conversion into starch and other polysaccharides is inhibited. At the same time, due to the high activity of peroxidase, oxidative processes intensify, and the content of ascorbic acid and invert sugar falls.

Symptoms of magnesium deficiency appear first on old leaves, as the plant redirects the element to young reproductive organs. The leaf blade lightens, and interveinal chlorosis begins — the veins remain green, while the tissue between them turns yellow, browns, and dies. Acute deficiency leads to "mottling" and curling of leaves. Lack of magnesium also delays flowering, makes flowers pale, and reduces the number of female flowers in monoecious crops.

Iron: the balance between chlorosis and toxicity

Iron directly governs plant respiration and photosynthesis. Upon its deficiency, the synthesis of chlorophyll precursors is disrupted, causing acute chlorosis to develop on young leaves. In advanced cases, the leaf blade may turn completely white, which leads to a sharp drop in productivity. Most cultivated soils contain few available forms of iron; therefore, a deficiency of this element is a frequent problem in the field and greenhouse.

The main part of iron is concentrated in chloroplast proteins. It is a component of cytochromes, ferredoxin, and enzymes (cytochrome oxidase, peroxidase, catalase), where it functions through reversible oxidation and reduction. In grain legumes, iron-containing proteins — leghemoglobin and nitrogenase — are responsible for atmospheric nitrogen fixation by nodules. In cells, the element is stored in chloroplasts as the protein ferritin, which supplies the cell with iron when necessary.

• Dry matter content — 0.01–0.08%
• Absorption forms — Fe2+, Fe3+, chelates

Sodium: Osmotic regulator and potassium partner

Sodium regulates osmotic pressure, maintains acid-base balance, and controls the water regime of plants, especially halophytes. It acts as a cofactor in photophosphorylation processes, increasing the net productivity of photosynthesis. In natriophilic crops, this element accelerates the outflow of carbohydrates from leaves to generative organs.

The accumulation of sodium in tissues occurs unevenly. Excess ions are usually retained in the root system, while in shoots, leaves, and generative organs, its level remains stable for a long time. With a sodium deficiency, plants begin to accumulate nitrates, especially if the level of nitrogen nutrition in the soil is high.

• Average content — 0.02% of dry matter
• Concentration range — 0.008–2.5%
• Absorption form — Na+ cation

The demand of crops for sodium is closely related to the availability of potassium. Based on their response to these elements, plants are divided into four groups:

• Require sodium in case of potassium deficiency: alfalfa, barley, oats, tomatoes, Brussels sprouts, carrots.
• Have a low demand for sodium in case of potassium deficiency: corn, red clover, lettuce, onions, potatoes.
• Have a small demand for sodium if potassium is sufficient: peas, wheat, cruciferous crops.
• Strongly need sodium if potassium is sufficient: celery, beet, turnip.

Chlorine: Regulator of water balance and participant in photosynthesis

Chlorine enters plants with the transpiration stream in the form of ions and accumulates in the cell sap. In tissues, it does not form part of stable compounds but exists in a free ionic state, with only a small portion weakly adsorbed by the cell protoplasm. The average chlorine content in plants is 0.01% of dry matter. This element directly affects tissue hydration, protoplasm swellability, and, together with potassium, maintains cellular electroneutrality.

During photosynthesis, chlorine activates water photolysis enzymes and stimulates oxygen evolution by isolated chloroplasts. It participates in energy metabolism by activating oxidative and photophosphorylation, and also regulates turgor when sufficiently available. The element improves oxygen uptake by roots for respiration and accelerates the incorporation of nitrogen into amino acids under the influence of chloride anions. The role of chlorine in photosynthesis is also confirmed by its necessity for the autotrophic nutrition of Chlorella. The main sources of this element in the soil are:

• atmospheric precipitation and sea spray;
• soil minerals;
• fertilizer;
• plant residues.

Chlorine deficiency leads to leaf chlorosis, but this is only detected in strictly controlled experiments with water cultures. Cabbage, beets, carrots, lettuce, and tomatoes cannot develop without this element. In practice, agronomists are more often faced with an excess of chlorine, especially in arid conditions.

When there is an increased concentration of chlorine in the soil solution, plants develop chlorine toxicosis. An excess of chlorides causes leaf burn in sensitive species and disrupts their metabolism. The group of distinctly chlorophobic crops includes:

• strawberries;
• potatoes;
• gooseberries;
• tomatoes;
• currants;
• beans.

Aluminum: Enzyme activator and nucleic acid stabilizer

Aluminum is required by plants in strictly limited quantities, and its average content is 0.02% of dry matter. Roots absorb this element from the soil solution in the form of ions. The absorption process takes place in two consecutive stages:

1. Accumulation of aluminum ions in the root free space, where they interact with cell walls, the surface of protoplasts, and precipitate as hydroxides or phosphates.
2. Penetration of a portion of the ions through the membrane barrier into the cells, where they interact with the protoplasm and accumulate in the nuclei and mitochondria.

Aluminum is vital for plants only in microdoses, exceeding which makes it toxic. The element acts as a specific activator of succinate dehydrogenase, pectin polygalacturonase, and ascorbate oxidase enzymes, although it is not a component of the latter. In addition, together with copper and iron, it activates the function of pyridoxal enzymes.

In low concentrations, aluminum stimulates the growth of the root system and above-ground plant organs. It increases the area of the assimilation apparatus and raises the intensity of photosynthesis. In crops of grain legumes, this element activates symbiotic nitrogen fixation: under its influence, the number and size of nodules increase.

The element plays a special role in the metabolism of hydrophytes, stimulating seed set in them. Without aluminum in the nutrient medium, the development of ferns is also impaired—their spores lose the ability to form a normal gametophyte. Accumulator plants have the greatest need for this nutrient element.

Symptoms of deficiency and the danger of aluminum excess

When aluminum is lacking, the leaf development of some plants is impaired. This deficiency is most acutely manifested in tea plantations in the form of pronounced chlorosis. However, increasing the availability of the element in the soil should be done carefully, as its excess is extremely dangerous.

• Stimulating concentration in solution — 1·10-4 M
• Indicator of deficiency — chlorosis of tea leaves
• Result of toxicity — phosphorus starvation