Mineral nutrition of plants

History

The theory of mineral plant nutrition was confirmed in 1858 in experiments when, under conditions of an artificial nutrient medium of water culture, it was first possible to bring a plant to full maturity. Later, a complete nutrient mixture was proposed for growing plants in sand cultures.

Certain statements about the nature of absorption of substances by living cells were made by Dutrochet as early as 1837. He believed that the penetration of water into the cell and of substances dissolved in it occurs by diffusion through porous cytoplasmic membranes.

Sachs used the concept of “accumulating diffusion,” since he believed that chemical processes occurring in the cell disturb the equilibrium of concentrations of substances inside the cells and in the surrounding solution.

The proponents of the diffusion-osmotic theory include Pfeffer, de Vries, Mayer and other scientists. According to this theory, through the root system, together with water, nutrients are sucked into plants, while water constantly evaporates. Consequently, the entry of nutrients is directly dependent on the intensity of water evaporation by plants. However, the available data on the patterns of substance entry did not fit within the framework of the diffusion-osmotic theory.

K. A. Timiryazev also noted the absence of a definite dependence between the entry of water and nutrients into plants. He wrote that for nutrition, plants do not need the enormous amounts of water that they evaporate. This position was developed in his works by D. A. Sabinin. He showed that with small amounts of substances in the nutrient solution, they are well concentrated in the plant sap.

At the end of the 19th century, Overton advanced the lipoid theory, according to which the penetration of substances into the cell occurs due to their dissolution in the lipid components of cytoplasmic membranes. He noted a correlation between the rate of penetration of basic aniline dyes into plant cells and their solubility in lipids.

Traube and Ruhland proposed the ultrafiltration theory, according to which the penetration of nutrients through the cytoplasmic envelope is determined by the sizes of pores and molecules. Drawet established the dependence of the penetration of the acidic dyes studied on the size of their molecules. But such facts as the absorption of amino acids, phytin and other organic substances with large molecular sizes could not be explained by the proposed theory.

At the beginning of the 20th century, Devaux established the ability of plant cells to rapidly bind cations from highly diluted solutions, which contributed to the development of the adsorption theory. He showed that bound cations, due to mutual equivalent exchange, are displaced back from tissues by other cations, and the intensity of the process depends on concentration and time.

In many studies by D. A. Sabinin and other scientists, the connection between the absorption of substances and the activity of cell vital functions is traced, and the role of the root system in the process of substance absorption is shown. The concentration of substances in the sap depends on the level of provision of plants with nutrients, species characteristics and age of plants. Different physiological activity of cells and tissues causes differences in chemical composition and differences in electrical properties.

The rate of tissue metabolism determines the level of substance absorption. Stewart, Lundegårdh, Burström and other scientists established the connection of respiratory processes in tissues with the process of absorption of mineral salt ions. Hoagland and Broyer showed that increased absorption of substances by plant cells and tissues is observed with good aeration of the nutrient solution, addition of glucose, increased temperature and other conditions promoting activation of respiration.

D. A. Sabinin proved the connection of plant nutrition with the formation and development of individual organs.

The above theories had their significance in developing views on the process of substance entry into plants. They essentially reflect in simplified form various aspects of nutrient element entry. Modern ideas about the entry of mineral nutrition elements include a number of basic concepts from previously advanced theories.

Root

Root structure:

1 — phloem; 2 — xylem; 3 — root hair zone; 4 — elongation zone; 5 — division zone; 6 — root cap; 7 — apical meristem; 8 — primary cortex; 9 — epiblema with root hairs.

Root is a specialized part of the plant that performs the functions of anchoring the plant in the soil and absorbing nutrients: primary assimilation, inclusion in metabolism; distribution and transport of water and mineral substances. Numerous biosynthetic processes occur in the root and some special functions are performed.

The power and character of development of the root system of a plant determine the ability to assimilate nutrients, most of which are absorbed by young, growing parts of the root.

Root growth zone is the zone of division, or meristem. In the meristem, cells are not differentiated into tissues. The elongation zone and the root hair zone are distinguished, having developed elements of xylem and phloem and an epidermis with root hairs. The most intensive absorption occurs by cells of the root hair zone.

The root system of field crops has a huge absorbing surface, reaching a maximum, including active, as a rule, during flowering.

Root hairs greatly increase the absorbing surface of the root. It is generally considered that the root hair zone is the absorption zone. However, absorption of nutrients can occur at a distance of 0.5 m along the root and 1 m from the root tip, that is, where root hairs are absent.

Russell and Clarkson showed that the movement of phosphates in barley from a zone located further than 40 cm from the root tip remains the same as in the root hair zone.

In Clarkson’s experiment it was shown that root hairs do not have special absorbing properties. Barley grown in water culture with strong aeration of the solution does not form root hairs on the roots, but ion absorption nevertheless occurs quite intensively. Probably the main role of root hairs is to maximize the root surface for supplying plants primarily with phosphorus.

The movement of phosphorus in soil is very slow, but the rate of its absorption by the plant is high. In a short period of time, the roots absorb the phosphorus surrounding them and increase their surface for absorbing subsequent amounts of this element. Other ions have greater mobility and root hairs are less important for their absorption. In soil, due to chemotropism, the root grows and moves toward a higher concentration of nutrients.

Depending on the biological characteristics of the crop and growing conditions, the root system develops with different power. On poor soils and in arid conditions, in search of food and water, plants create a relatively large mass of roots. The use of fertilizers reduces the ratio of root mass to aboveground part of the plant, but increases the total mass and depth of distribution of the root system.

Cell membrane of the root

Mosaic model of the cell membrane:

a — lipids; b — proteins.

The membrane determines the ability of the root cell to selectively absorb ions. On its surface, reactions of metabolism and energy occur. The cell contacts the soil environment by means of the cytoplasmic membrane, or plasmalemma.

The cell membrane consists of two layers of phospholipids, joined by hydrophobic ends. In certain regions of the phospholipid layer, molecules of carrier proteins are embedded. The mosaic structure of the cytoplasmic membrane consists of positively and negatively charged regions, through which adsorption of cations and anions from the external environment occurs (Bergelson, 1975).

Structure of lipid aggregates:

a — gel-like; b — lamellar (layered); c — cylinders of lipids in water; d — cylinders of water in the lipid phase.

Phospholipids form several types of liquid-crystalline structures.

Phospholipid molecules consist of polar “heads” — hydrophilic groups, and nonpolar “tails” — long hydrocarbon hydrophobic chains. Phospholipids dissolve poorly in polar solvents (water) due to the presence of nonpolar “tails” and in a nonpolar medium (oil) due to polar “heads.” A monomolecular layer at the interface creates a limitation of substance permeability (Bergelson, 1975).

Phosphatidylcholine molecule.

An example of a phospholipid molecule is phosphatidylcholine.

The thickness of the bilayer membrane is 10–12 nm. The presence of charged “heads” in the lipid micelle of the membrane creates a potential difference at the interface between lipid and water, owing to which positive ions are attracted to the membrane surface, while negative ones are repelled.

Monomolecular layer of lipids and micelles in oil and water.

Pores and walls of channels in the membrane are built from membrane proteins. In the cytoplasmic membrane, absorbed ions are involved in metabolic reactions. Some membrane proteins possess enzyme activity. Membrane proteins in the form of protein globules have a hydrophobic anchor, due to which the molecules can interact with lipids. Proteins can be bound to the polar heads of membrane lipids through bridges formed by cations of divalent metals.

Method of attachment of negatively charged proteins and polar heads of membrane lipids through bridges formed by ions of divalent metals.

The protein globule moves in the liquid bilayer of the membrane. Molecules of dissolved substance continuously move in the solution.

If the membrane separates two solutions with different concentrations, then depending on its permeability, as a result of diffusion, either the solvent or the dissolved substance will pass and equalization of concentration will occur. At the beginning of this process, net movement of pure substance through the membrane occurs (I), and then dynamic equilibrium is established (Clarkson, 1978).

The figure below shows how at first net movement of water molecules (light circles) into chamber A occurs, since the membrane is impermeable to the dissolved substance. After reaching a steady state (II), a certain difference in solution levels between chambers A and B is established. The magnitude of the level difference is proportional to the initial difference in concentration of the dissolved substance (osmotic potential), (Clarkson, 1978).

Distribution of solute molecules as a result of diffusion through a permeable membrane.

Movement of water molecules through a semipermeable membrane separating two chambers initially filled with dissolved substance (black circles) at different concentrations.

The passage of nutrients through root cell membranes by the mechanism of passive transport is possible in the presence of hydrophilic openings (pores) in the membranes. Nonspecific passive transport of ions and molecules occurs due to diffusion through membrane pores for hydrophilic substances, whereas for neutral molecules — by dissolution of the penetrating substance in the membrane (for substances soluble in oils).

According to calculations, the area of pores in the membrane is no more than 0.1% of its surface.

In the case of electrically charged particles, the process of passing through the membrane depends on the difference in concentration and electrical potential.

Root nutrition of plants

At present, the composition, amount and forms of compounds in which mineral substances enter plants have been sufficiently well determined. These data were obtained thanks to numerous experiments on growing plants in water and sand cultures and on hydroponics, where record-high yields are obtained on nutrient mixtures of mineral salts.

Plants can also use organic compounds for nutrition, for example, amino acids, organic acids, sugars, sugar phosphates. The nitrogen of amino acids in plants undergoes deamination, and the released ammonia is included in nitrogen nutrition processes.

By the end of the 20th century, the study of the process of mineral nutrition of plants began to be studied not only under conditions of deficiency of one or another nutrient, but also with increased supply. This is connected with the need to determine the conditions of mineral nutrition under which the potential productivity of the plant is achieved.

A number of characteristics of agrotechnical practices depend on the function of different parts of the root: the depth of soil cultivation and application of fertilizers. Thus, the root of cereal crops has three main zones:

• growth and elongation zone 1.5 mm long, due to cell division of which in the apical meristem root growth occurs;
• root hair zone, or absorption zone, having root hairs up to 1 mm long, the length of the zone itself is 1–2 cm;
• lateral root zone.

In field conditions, the main importance in mineral nutrition of plants belongs to the root hair zone, also called the absorbing zone.

Using labeled carbon ¹⁴C, it was established that carbon dioxide from roots to leaves is transferred in 10–15 min. The rate of movement of photosynthesis products from leaves to the root system is 40–100 cm/h. The rate of transfer of nutrients, including those applied with fertilizers, from the root to plant parts is even higher. Thus, when barley roots were immersed in a solution containing labeled phosphorus ³²P, its appearance in leaves was noted after 5 min; from the roots of fourteen-day-old maize it entered the leaves after 2 min. Similar results were observed in experiments with other crops.

The rate of nutrient absorption changes with root age. For example, as maize plants age from 20 to 80 days, the rate of absorption of nitrogen, phosphorus, potassium, calcium and magnesium decreases tens of times.

Study of the function of seminal and nodal roots made it possible to determine the important role of the tillering node in the distribution of water and mineral substances. The tillering node consists of loose porous parenchyma tissue, which promotes easy movement of nutrients. The roots of the tillering node are characterized by high absorbing capacity and play a significant role in plant nutrition, more than primary (seminal) roots. Their role especially increases during tillering, accompanied by enhanced branching of nodal roots.

Absorption of nutrients depending on the nature of energy expended can be active or passive.

Active absorption occurs with expenditure of metabolic energy. Passive absorption occurs due to the energy of thermal diffusion or solar energy.

Cycle of substances and metabolic role of roots.

In all movements and transformations of nutrients, the protein plasma of the cell plays a role, having a dual nature — basoid and acidoid. In the protein plasma, the acidoid and basoid parts are arranged mosaically in such a way that during the movement of nutrients from cell to cell, gradual exchange and interaction of cations and anions with positively charged (basoid) and negatively charged (acidoid) parts of the protein molecules of the plasma occur.

Active absorption and movement of ions occurs through a system including cell protoplasts connected to each other by plasmodesmata. In the case of passive absorption, ions, reaching the root surface by mass flow or diffusion, enter the free space of the root and move through the plant with the transpiration stream. Plant cells have loose cellulose envelopes connecting to each other and forming a continuous system — the apoplast. Through this system, due to transpiration of water by leaves, movement of substances dissolved in it occurs.

Ions in the free space can also move by diffusion — from a zone with higher concentration to a lower one. The process is relatively slow; for example, the dye fluorescein diffuses 5 mm in 1 h, 25 mm in 24 h, and 50 cm in a year. For this reason, the importance of diffusion in the movement of soluble substances is not substantial.

Ions upon contact with the root are adsorbed by cell walls. The adsorption process is exchange in nature. The high intensity of exchange, the rate of entry and movement of substances is explained by adsorption exchange between the root system and soil colloids, as well as the soil solution.

Thanks to well-developed root hairs, which with a dense network entwine the soil and its colloidal particles, a huge area of contact between the root system and soil is created, which determines the effectiveness of adsorption of nutrients. Thus, the number of root hairs, whose lifetime is several days, reaches 425 per 1 mm² of root surface in maize roots, on average for most crops — 200–500.

The exchange process of adsorption consists in ion exchange of nutrients in the form of ions, such as K⁺, Ca²⁺, Mg²⁺, NH₄⁺, NO₃^-, H₂PO₄^-, SO₄^2-, for H⁺, HCO₃^- ions. The latter are released by the root surface during respiration. The root system is capable of releasing a large amount of carbonic acid; thus, plants of mustard release 2.25 t CO₂ per 1 ha over 85 days of vegetation.

The resulting carbon dioxide, reacting with water, forms a solution of hydrogen ions and bicarbonate ions (“carbonic acid”):

As a result of the respiratory process, H⁺ and HCO₃^- ions appear on the surface of root hairs. The former are exchanged for soil cations K⁺, Ca²⁺, Mg²⁺, NH₄⁺, while anions NO₃^-, H₂PO₄^-, SO₄^2- are exchanged and displace the HCO₃^- anion into solution.

On the surface of the root hair, cations and anions interact with the basoid (basic) and acidoid (acidic) parts of the cell plasma and are included in the process of transfer in the plant.

Exchange absorption of nutrient ions by the root system can occur not only for H⁺ and HCO₃^- ions, but also for ions of organic and mineral compounds released by roots. For example, plant roots are capable of releasing citric, malic, oxalic and other organic acids, which dissociate into H⁺ and organic anions.

Closer contact of the root system with the soil absorbing complex promotes intensification of absorption processes. The best nutrition conditions are created with a sufficient amount of ions in the soil solution and in an adsorption-bound state.

Adsorption absorption of nutrients is characterized by the absorption capacity of cations and anions of root hairs, just as in soil colloids. The magnitude of the absorption capacity of roots depends on plant species, nutrition conditions and other factors. Thus, the cation absorption capacity per 100 g of dry matter of roots for legumes is 40–60 mmol, potato and tomato — 35–38 mmol, cereals — 9–29 mmol.

The cation absorption capacity of roots increases with nitrogen nutrition, which is apparently connected with the synthesis of protein substances. A significant part of the cation absorption capacity falls on hydrogen ions (H⁺).

Anion absorption is similar to cation absorption. The main role belongs to the bicarbonate ion HCO₃^-. In many crops, anion exchange quantitatively exceeds cation exchange, which indicates the presence of a larger number of colloids in the active part of roots with a positive charge.

The main barrier to ion absorption is the surface membrane, or plasmalemma. The ability of plants to absorb or exchange ions with the external environment depends on the properties of membranes.

Attempts to influence root membranes to regulate ion entry into roots have a certain significance in managing mineral nutrition of plants. In this regard, membrane-active compounds that affect ion transport in plants are of interest. These include valinomycin, gramicidin (antibiotics), 2,4-dinitrophenol, dimethyl sulfoxide. The latter is most promising, since it has a milder action, increasing the conductivity of the sulfolipid layer of membranes.

The mechanism of absorption of mineral substances by leaves is analogous to the mechanism of absorption by roots. The first stage of ion absorption from solution is exchange adsorption — the process proceeds very quickly on the absorbing surface. In roots and leaves, absorption of salts from solution depends on the pH of the medium, concentration of the solution, salt composition, duration of contact of the solution with the absorbing surface, and also the age of the absorbing organ.

Mechanisms of nutrient transport to plant roots

Absorption of nutrients is a complex physicochemical and metabolic process including:

• diffusion;
• adsorption;
• metabolic transfer of substances against the electrochemical gradient.

Diffusion is important in the movement of soluble nutrients in the soil to roots. Exchange adsorption — in the entry of nutrients into the plant through the root system. Nutrients that have entered interact with the cell protoplasm by metabolic or nonmetabolic means.

Metabolic absorption and movement of nutrients depend on aerobic respiration, temperature and soil aeration.

Nonmetabolic, or passive, absorption may not be connected with the vital activity of plants, therefore it depends little on temperature and other conditions of plant life. An example of nonmetabolic absorption is pinocytosis, that is, capture of part of the nutrient solution, in which cells of young roots absorb ions and drops of solution.

Three mechanisms of delivery of nutrients to the root surface are distinguished:

• root interception;
• mass flow;
• diffusion.

In the process of nutrition, not only the outer walls of rhizodermis cells are important, but also the mucilage layer covering the root surface. Mucigel — a layer of mucus around root hairs and young parts of the root, participating in the process of contact exchange.

Various nutrient ions participate unequally in transport mechanisms. For example, phosphorus and potassium are delivered to roots mainly by diffusion, while calcium and magnesium — by mass flow.

Mass flow acquires important significance at high concentration of nitrates in the soil solution (about 10 mol/L). At low nitrate concentrations, diffusion predominates.

The concentration of nitrates in the root-adjacent soil layer is 3–4 times higher than in soil outside the root layer, which is especially noticeable at high nitrogen doses — up to 120 kg/ha. Such an increase in nitrate concentration in soil adjacent to the root is explained by the fact that nitrates move to the root by mass flow, but the rate of water absorption by roots is higher than the rate of nitrate absorption, which leads to accumulation of nitrates in the root zone.

Cell envelopes constitute the main part of the apoplastic compartment of tissues through which substances move. Before ions are absorbed by the plasmalemma and involved in the symplast, they must pass through the free space of the apoplast of root cells.

Diffusion

Diffusion — absorption by the root of a nutrient, accompanied by a change in concentrations at the root surface, which creates a concentration gradient, which is the driving force of the process. The rate of diffusion through soil depends on soil type and the nature of ion absorption by the soil. For ions not adsorbed by soil, such as nitrates, the concentration gradient, that is, the ratio between ions in solution and ions adsorbed on the surface of soil particles, can reach 1. For ions strongly absorbed by soil, for example, phosphates, this ratio can be 10-4.

In order for the root to absorb a nitrate ion, it must be at a distance of no more than 1 cm from it, and for phosphate — no more than 0.1 cm. At the same time, roots of annual plants are often located from each other at an average distance of 0.5 cm.

Theory of nutrient entry

Metabolism between the plant and the environment occurs through surface-located cells of the root system and aboveground organs. Plant cell walls are easily permeable, since the radius of mineral substance ions is 0.4–0.6 nm, and the average radius of channels in the cell wall is 5–20 nm. However, if cell walls were the only barrier between the plant root and the external nutrient solution, then equalization of substance concentrations would occur due to diffusion. But in plant organisms, nutrients are found in higher concentrations than in the external nutrient solution. The entry of individual substances and their concentration occurs differently and does not correspond to the concentration ratio in the nutrient solution. This is explained by the presence of the plasmalemma, which prevents loss of substances and at the same time ensures penetration of water and nutrients.

The entry of substances against a concentration gradient is associated with energy expenditure.

There are a number of theories of substance entry into the cell. Transport of mineral salts into the cell is provided by two mechanisms — passive flow of substances along the electrochemical gradient and their active transfer in the direction against the electrochemical gradient.

Electrochemical gradient — distribution of mineral salt ions between the cell and the external environment under the action of a difference in electrical potentials and a difference in concentrations. Movement of ions along the electrochemical gradient is passive, while against it is active.

The ratio of transport mechanisms of mineral salts changes in plant ontogenesis and depends on a number of conditions. For example, passive entry of ions predominates at high salt concentrations in the external environment, which develops under conditions of salinization or local application of fertilizers.

Free space. Ions pass through the cell membrane to the plasmalemma as a result of diffusion or together with an aqueous solution.

Diffusion — movement of molecules of gases, liquids or dissolved substance in the direction of a concentration gradient and depends on the difference in concentrations and the area through which the flow of substance and ions occurs.

Free space — part of the total volume of root system tissues into which ions enter and from which they are released as a result of diffusion. The share of free space accounts for about 4–6% of the total root volume. Free space is localized in the loose primary envelope of cell walls outside the protoplast, outside the plasmalemma.

Free space is divided into water space, from which ions pass by diffusion into water, and Donnan space, from which ions are released into salt solution by exchange. The envelope of root cells consists of a loose surface layer with a system of pores or channels about 20 nm in diameter. In relation to the external solution, root cells are negatively charged. The walls of pores or channels of free space, carrying a negative charge, attract and retain cations and repel anions. In the central part of such a channel, the concentration of cations is equal to the concentration in the external solution. This part belongs to the water space.

The exchange, or Donnan, space is located closer to the cell walls. Here concentration of cations occurs due to the negative charge of the walls.

Course of ion absorption (with radioactive label) after transfer of roots from pure water to salt solution:

a — normal course (nonmetabolic + metabolic absorption); b — at 0 °C or in the presence of respiration inhibitors (only nonmetabolic respiration); a' — metabolic respiration.

The process of entry of substances into free space occurs quickly with an increase in concentration in accordance with the saturation curve. The process is characterized by weak selectivity; under the action of inhibitors or lowered temperatures its magnitude remains constant. The process is reversible.

The negative charge of the membrane is associated with negatively charged carboxyl groups of protopectin and plasmalemma built into cell walls. At this stage of absorption, exchange adsorption is important. For example, a root kept in a solution of calcium ions and transferred to a solution containing potassium ions loses adsorbed calcium as a result of exchange adsorption.

The network of intercellular spaces present in plant tissue provides additional transport. Loose cellulose envelopes of cells create many microregions for adsorption and passive flow of substances through the tissue.

Substances that have entered the free space as a result of diffusion may undergo primary exchange adsorption. At the same time, they can be displaced from the free space into the external solution or into the internal part of the cell. The adsorption process consists in binding by cell envelopes of mineral salt ions and retaining them by forces of electrostatic attraction.

Consequently, free space is a part of the nutrient medium and at the same time an absorption zone. Further, depending on the intensity and type of metabolism, selective absorption occurs. The entry of nutrient elements into free space should more correctly be considered as the first preparatory stage of absorption.

Cellulose envelopes of plant cells are united by a system of vessels called the apoplast. As a result of transpiration of leaves and water flow, water and mineral salts are sucked into the intercellular spaces in the roots. To reach the xylem vessels for transport to aboveground organs, nutrients must overcome a considerable distance in the radial direction across the root. This path of ion movement along the apoplast is interrupted by Casparian strips — waterproof regions in which intercellular walls contain suberin — a substance that does not pass aqueous solutions. However, some ions enter the conducting vessels through extracellular pathways — “passage cells.”

In the growing root tip, the endodermis does not have time to form Casparian strips, therefore ions can penetrate along the extracellular pathway. It should be noted that only a small amount of ions enters the plant along the apoplastic pathway, and its role in the balance of nutrient elements under normal life conditions is probably insignificant.

After ions overcome the cell membrane (plasmalemma), their further movement occurs along a unified system of cells — the symplast — from cell to cell due to protoplasts connected by bridges through plasmodesmata, which ensure contact of cell plasmalemmas. The plasmodesma mechanism allows regulation of the rate of transport of substances along the symplastic pathway, which is 2–4 cm/h.

Stages of entry:

1. Saturation of the apoplast free space with ions by exchange adsorption, diffusion, passive adsorption.
2. Overcoming of the membrane barrier by ions and their transition into the symplast.
3. Radial movement of ions in root tissues and vascular-fibrous bundles.
4. Inclusion of ions in metabolism.
5. Vertical movement of ions along aboveground parts of the plant.
6. Entry into photosynthetic cells, utilization and reutilization, outflow.
7. Transport of assimilates and ions downward along the phloem to roots.

Carrier theory and ion pumps

Nutrients enter the root in the form of ions with their obligatory transfer through the cell plasmalemma. This transition can be passive, that is, along the electrochemical gradient, and active — against the electrochemical gradient. The latter — the mechanism of active transport of ions through the phospholipid membrane — is of greatest importance.

According to the carrier theory, an ion overcomes the membrane in the form of a complex with a carrier molecule. On the inner side of the membrane, the complex dissociates and releases the ion inside the cell. Ion transfer is carried out with the help of carriers of various types.

The driving force of transport with the participation of a carrier is the chemical gradient of substance concentrations in the case of passive transfer and the electrochemical potential. In this case the process is called facilitated diffusion. Functioning of the facilitated diffusion system leads to equalization of gradients and establishment of equilibrium in the system.

During facilitated diffusion, substances move along the concentration gradient at high speed.

In fungi, algae and bacteria, some antibiotics can serve as carriers, for example, valinomycin, whose molecule contains 6 carbonyl groups. The complex of valinomycin with potassium is strongly hydrophobic and readily soluble in the hydrocarbon part of the membrane. Therefore, valinomycin greatly increases the permeability of membranes to potassium, forming a “potassium hole” in them.

Ion permeability of phospholipid membranes increases under the action of certain antibiotics produced by bacteria and fungi, for example gramicidin and nystatin.

Transport of substances into root cells is stimulated by the fact that in the cytoplasm ions are involved in biosynthetic processes, as a result of which the concentration of ions inside cells decreases.

Dependence of K⁺ absorption by barley roots on solution concentration.

The course of the curve indicates two different transport mechanisms (according to Epstein).

Two ion transport systems have been established. The first system is characterized by higher selectivity and is engaged under natural conditions at low external ion concentration of less than 1 mmol. An increase in ion concentration in the external solution leads to saturation of the first system and engages the second, less selective one.

The first system is localized in the plasmalemma, the second — possibly in the plasmalemma, but more likely in the tonoplast.

Hypothetical mechanism of an electroneutral ion-exchange pump allowing transfer of ions against the electrochemical gradient:

A — activation; B — reorientation; C — relaxation; o — external medium; i — internal medium.

Protein globules with a diameter exceeding the thickness of the cell membrane can serve as carriers. In this case, movements of the globule around its axis ensure transfer of ions from the outer side to the inner. An example of such transfer is the ion pump represented by transport potassium-sodium ATPase. ATPases are so called because they are capable of splitting molecules of adenosine triphosphate (ATP).

The released energy is used for transport of substances, and transport ATPase is reversibly phosphorylated. The molecular mass of transport ATPase is 200,000–700,000. Simultaneously with phosphorylation and dephosphorylation of transport ATPase, binding and release of the ion occurs and at the same time conformational changes of the ATPase molecule occur, allowing the ion to be transferred into the cell.

The active site of the ATPase molecule consumes metabolic energy, and a cation M+ attaches to the exchange site. In the activated pump, the exchange site changes its orientation and becomes facing the external environment. Under these conditions, the strength of its electric field changes and cation C+ binds to it preferentially. Relaxation — transition from activated state to nonactivated. Then the initial orientation of the pump is restored: the exchange site with cation C+ becomes facing the internal environment, where M+ binds preferentially to the exchange site, while C+ is released.

Ion pump — active transport of ions into cells at the expense of ATPases. Its task is to maintain a constant ion composition inside the cell.

Transport ATPases in animal cells were discovered in 1957, in plant cells — in 1964. ATPase carries out pumping of ions out of cells and entry of K⁺ ions. Besides the potassium-sodium pump, there is also a proton pump, pumping out H⁺ ions, which creates a negative charge of cells.

Transport of substances against the electrochemical gradient requires a constant supply of energy. The need for energy explains the dependence of nutrient absorption on metabolic processes of respiration and photosynthesis, during which macroergic compounds are formed.

The rate of ion transport by carriers is determined by the rate of their turnover, which in turn depends on temperature, oxygen content, presence of inhibitor, number of binding sites of the carrier, and occupancy of active sites.

The carrier theory made it possible to explain selectivity of absorption, interaction of ions and inhibition of the process. The theory assumes the presence of a control mechanism regulating the entry of substances into the cell by the principle of feedback. According to Pitman, the maximum content of ions in root cells is 80–90 mg-eq/g fresh mass. For different plants this value does not depend on the concentration of ions in solution and is achieved upon absorption from 10 mM solution in 10–15 h, from 1 mM solution — in 20 h and from 0.1 mM solution — in 36 h.

Transport of some substances through the membrane can be carried out with the help of low-molecular fat-soluble carriers. The carrier, having captured an ion, penetrates through the phospholipid membrane and releases the ion inside the cell. Transport of substances into the cell through membranes can occur through pores in the plasmalemma. Such pores may exist both temporarily and permanently. Their diameter is presumably 0.5–0.8 nm. For comparison: the diameter of a hydrated potassium ion is 0.34 nm.

The presence of single channels in plant cells responsible for calcium ion transport has been established.

Transfer of substances through hydrophilic channels or pores can proceed according to channel selectivity. Transfer by a relay mechanism — passage of substances through the membrane, in which an ion or molecule is successively transferred from one carrier to another. In this case, carrier molecules are embedded in the membrane successively and relay ions or molecules. If the carrier moves with the transported ions, the transfer mechanism is called shuttle.

Symport and antiport

Hydrogen ions are pumped out of the cell through the proton pump, which, pumping protons out of the cell, creates concentration and electrical gradients.

At the same time, alkalization occurs inside the cell, as a result of which the carrier can transport back into the cell under the action of the electrochemical gradient both protons back and other anions. Such a transfer mechanism is called symport. The opposite process — pumping out of the cell of proton H⁺ and delivery into the cell, to preserve electroneutrality, of an ion with the same charge, is called antiport. The terms symport and antiport were proposed by Mitchell.

Nitrogen entry occurs significantly more easily. The cation NH₄⁺ in aqueous medium is in equilibrium NH₄⁺ ↔ NH₃ + H⁺. The NH₃ molecule penetrates the cell a thousand times faster than other electroneutral molecules, except water. When passing through the membrane, ammonia takes a hydrogen ion H⁺ from water, forming ammonium ion NH₄⁺ and hydroxide ion OH^-:

Alkalization of the cytoplasm occurs, which interferes with proton pumping and operation of the proton pump, while at the same time alkalization promotes phosphorus entry.

Anions NO₃^-, CN^-, I^- pass through the membrane 100–1000 times more difficultly than K⁺ and NH₄⁺. These anions at high concentrations can destroy the membrane structure and are called chaotropic.

Proof of nitrate passage through the membrane is the following experiment. If KNO₃ and valinomycin are added to a medium containing a liposome — a vesicle bounded by an artificial bilayer membrane — a potassium “hole” is formed and K⁺ and NO₃^- ions enter the liposome, which quickly swells. If instead of KNO₃, KCl is used, then the membrane is impermeable to Cl^-. A small amount of potassium will enter the liposome due to valinomycin and the process will stop as a result of the diffusion potential; the liposome will not swell.

Water molecules have the highest rate of passage through membranes.

Pinocytosis

Many cells can absorb solid particles and droplets from the environment. In the case of absorption of solid particles, the phenomenon is called phagocytosis, and in the absorption of liquid droplets — pinocytosis.

Absorption of substances into plants can occur through pinocytosis. At the beginning of the process, absorbed particles are adsorbed on the cell membrane, then the membrane is drawn inward; its edges close at the site of invagination. Thus, a pinocytic vesicle is formed, which detaches from the outer membrane and migrates into the cell.

Two mechanisms of pinocytosis are possible.

Two possible mechanisms of pinocytosis:

A — first mechanism; B–G — second mechanism.

In the case of the first mechanism (A), the membrane is drawn into the cell and forms a narrow channel. Vesicles with captured substance are pinched off from the end of the channel.

In the case of the second mechanism (B–G), the membrane region on which micromolecules have been adsorbed (C) is drawn inward (D). At the site of invagination, the membrane edges close (E), and the formed pinocytic vesicle detaches from the cell membrane (F). In the thickness of the cell membrane, the vesicle is destroyed by enzymes (G), and the captured particles enter the cytoplasm. The process of vesicle formation and its detachment from the outer membrane is associated with energy expenditures provided by ATP.

The pinocytic vesicle is destroyed upon contact with a lysosome, which contains hydrolytic enzymes that split macromolecules.

The phenomenon of pinocytosis is initiated by certain membrane regions by corresponding substances. Reverse pinocytosis is possible, that is, the process of carrying substances out of the cell.

Molecules or ions entering from the external solution, regardless of the method of transfer through the plasmalemma, practically do not enter exchange reactions at the plasmalemma level. After entering the internal space of the cell, they can:

• be included in the cycle of metabolic transformations, with subsequent inclusion in the composition of organic compounds of structural elements of the cell;
• concentrate in vacuoles of root cells, creating a reserve of ions;
• be transmitted through xylem vessels to aboveground parts of plants;
• be removed from the organism into the environment.

Impact of the plant root system on soil

Plant roots affect the soil by releasing organic and mineral substances into the external environment, for example, sugars, organic acids, nitrogen-containing organic compounds, vitamins, enzymes, etc. These exudates serve as a nutrient medium for soil microorganisms. In turn, in the process of vital activity, microorganisms promote mobilization of soil nutrients, increasing their availability, and supply plants with physiologically active substances (auxins, vitamins, antibiotics). This form of impact of the root system on soil is indirect.

Direct impact consists in the ability of plants to influence hard-to-access forms of nutrients (phosphorus). Even D. N. Pryanishnikov proved the ability of buckwheat, lupine, mustard to use phosphorus of trisubstituted phosphates or natural phosphorites, which is connected with the acidity of root exudates. Thus, the acidity of the solution surrounding root hairs of lupine is 4–5, clover — 7–8.

Synthetic capacity of roots

For many years it was believed that the formation of complex organic compounds is possible only in leaves. However, thanks to the labeled atom method it was established that active synthetic processes also occur in roots.

One can say that the very essence of plant life is expressed in the life of the leaf, that the plant is a leaf.

K. A. Timiryazev

The plant absorbs more of those substances in which it has greater need, which is explained by the physiological laws of the living organism. Thus, when sodium nitrate NaNO₃ is applied to the soil, the plant absorbs the NO₃^- anion and to a lesser extent the Na⁺ cation. When ammonium sulfate (NH₄)₂SO₄ is applied, the plant absorbs the NH₄⁺ cation to a greater extent and the SO₄^2- anion to a lesser extent. Substances physiologically necessary for the plant, upon entry through roots, undergo processes of assimilation (uptake), whereas substances that plants do not need remain in mineral, readily soluble ionic form in the same form as before entering the plant.

According to numerous studies, the root system performs not only the functions of absorption, accumulation and transfer of nutrients, but also the function of synthesis of organic substances. In studying the sap of plant roots, amino acids, oligopeptides, sugars and growth substances were found in it. From the lower, dying leaves, assimilates flow into the roots in the form of sucrose, which is used by the root for metabolism, growth and maintenance of mature, functioning cells in a physiologically active state; for synthesis of substances released by the root into the external environment and substances entering aboveground organs.

A great contribution to the study of the physiology of the root system and its properties was made by D. A. Sabinin. He was one of the first to put forward the assumption about the synthetic capacity of the root system. In 1940 he proposed the concept of transformations of substances during their passage through the root. This concept was further developed to the position of the synthetic activity of the root. The main provisions of this concept:

1. The root is capable of absorbing mineral substances, fully or partially processing them and supplying them to aboveground organs in changed form.
2. The synthetic activity of the root is carried out at the expense of assimilates flowing into the roots, that is, it depends on photosynthesis.
3. The root affects aboveground organs by supplying water, mineral substances and products of specific metabolic reactions occurring in the roots — phytohormones of non-auxin nature.

One of the main growth substances found in sap are cytokinins, which promote intensive metabolism of leaves and delay their aging. Cytokinins are synthesized predominantly in the root and partly in leaves.

Other growth substances synthesized by the root are gibberellins, necessary for stem growth. Stopping the growth of aboveground organs when roots are removed is connected not only with deterioration of nutrient entry, but also with cessation of synthesis of cytokinins and gibberellins.

During aging of plants, the concentration of calcium increases in cells, while the concentration of potassium decreases. For a young and actively functioning plant, high concentrations of K⁺ ions are characteristic. Treatment of plants with the growth substance kinetin leads to outflow of Ca²⁺ ions from cells and an increase in concentration of K⁺ ions. Yellowed leaves become green, destruction of subcellular structures stops and protein biosynthesis intensifies.

F. V. Turchin in 1965, using the stable nitrogen isotope ¹⁵N, established that practically all ammonium nitrogen (¹⁵NH₄⁺) absorbed by the root system and a significant part of nitrate (¹⁵NO₃^-) nitrogen is found in root sap in the form of nitrogen-containing organic compounds. In the roots of various agricultural crops, about 18 amino acids are synthesized.

Studies using labeled atoms of phosphorus (³²P), sulfur and other elements showed that in plant roots the phosphoric acid anion (H₂PO₄^-) is included in the composition of organic compounds by means of ester bonds, and absorbed sulfate ion (SO₄^2-) is reduced and included in sulfur-containing amino acids: cystine, cysteine and methionine.

The synthetic capacity of the root system was proved by Academician A. A. Shmuk in 1941 together with co-workers. They established that nicotine in tobacco plants is synthesized in the root system, and not in leaves. If tobacco is grafted onto tomato, then nicotine is practically not found in leaves of tobacco, and, conversely, in leaves of tomato grafted onto tobacco, up to 3–4% nicotine accumulates.

Plants use relatively small amounts of assimilates for root exudates. Thus, from bean roots 0.5–0.7% of carbon entering leaves in the form of carbon dioxide is released.

The synthetic activity of leaf and root of plants is closely interconnected.

The discovery of the synthetic capacity of the root system is one of the major achievements of science in the 20th century in the field of physiology of root nutrition of plants.

Excretory function

The composition of root exudates includes sugars, amino acids, organic acids, in small amounts — vitamins, enzymes, volatile organic substances, for example, ethylene. Root exudates differ in quantity and composition and are determined by species and varietal characteristics of crops.

The excretory function of the root is connected with the ability of plants to regulate the nutrient regime of soils. For example, lupine, due to the acidifying action of root exudates, is capable of assimilating hard-to-dissolve forms of phosphorus.

Accumulation of root exudates when growing isolated roots under sterile conditions leads to suppression of growth, which is caused by accumulation of amino acids — the main component of root exudates — to concentrations toxic for plants.

Selectivity of nutrient absorption

The plant absorbs more of those substances in which it has greater need, which is explained by the physiological laws of the living organism. Thus, when sodium nitrate NaNO₃ is applied to the soil, the plant absorbs the NO₃^- anion and to a lesser extent the Na⁺ cation. When ammonium sulfate (NH₄)₂SO₄ is applied, the plant absorbs the NH₄⁺ cation to a greater extent and the SO₄^2- anion to a lesser extent. Substances physiologically necessary for the plant, upon entry through roots, undergo processes of assimilation (uptake), whereas substances that plants do not need remain in mineral, readily soluble ionic form in the same form as before entering the plant.

After equilibrium is reached between concentrations in the cell and soil solutions on the surface of the root hair and on the surface of soil colloidal particles, ion entry processes stop. When the concentration of nutrients on the side of the cell solution decreases due to assimilation of these nutrients by the plant and their transfer to other parts of the plant, the absorption process will continue.

The selective property of plants to absorb nutrients is evidenced by the fact that the concentration of salts of a number of nutrients in cell sap is much higher than in the nutrient solution in which the root system is immersed. Thus, the concentration of potassium in maize sap is 20 times, phosphorus — 14 times, and calcium — 4 times higher than in the external nutrient solution.

With the completion of the life cycle, the entry of nutrients into plants stops and equilibrium of their concentration on the surface of the root hair and in the soil solution occurs.

Selectivity of nutrient absorption determines the physiological acidity of fertilizers. If a plant absorbs more cations of a mineral fertilizer, then anions remain and accumulate in the soil solution, which causes acidification of the soil solution, and the fertilizer itself is physiologically acidic. Thus, physiologically acidic fertilizers include ammonium sulfate, ammonium chloride, ammonium nitrate, potassium chloride. And conversely, if a plant absorbs the anion of a mineral fertilizer, and the cation accumulates in the soil solution, then the fertilizer is physiologically alkaline, for example sodium nitrate, calcium nitrate.

In the presence of toxic substances in the soil solution, for example, heavy metals, pesticides, a significant part of them is retained in roots, as well as in stems and leaves, and only an insignificant part enters seeds.

Periods of nutrient consumption

In the life of a plant, two periods of nutrition are distinguished. The first, or critical, period occurs in the initial phases of growth and development of plants. During this period, plants are most sensitive to deficiency or excess of nutrients. During this period, plants are especially demanding of mineral nutrition conditions.

The second period, or period of maximum nutrient consumption, is characterized by consumption in later phases of development and is determined by the biological characteristics of plants. For example, the entry of nutrients into cereal plants, with the exception of maize, ends by the completion of the heading phase, although by this time only 50–60% of plant mass of the full yield is formed.

Periodicity of mineral nutrition of plants allows regulating the formation of yield and its quality. In particular, fractional application of fertilizers is based on this property of plants.

Application of fertilizers in one dose and in one soil layer does not always allow achieving full use of potential yield. For example, doses of readily soluble mineral fertilizers sufficient in the critical period of nutrition will be small for the period of maximum consumption. And conversely, a large dose in the first critical period is harmful for young roots sensitive to high salt concentrations. For this reason, the fertilizer system of plants provides for a combination of basic, pre-sowing fertilizer and top dressings during plant growth.

List of sources

1. Mineev V. G., Sychev V. G., Gamzikov G. P. et al. (2017). Agrochemistry. Textbook. Ed. by Mineev V. G. Moscow: Publishing House of VNIIA named after D. N. Pryanishnikov.
2. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. (2002). Agrochemistry. Ed. by B. A. Yagodin. Moscow: Kolos.