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Mineral (root) nutrition of plants

Mineral (root) nutrition of plants is the process of absorption of water and nutrients from the soil by the root system of the plant.

History

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

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

Sachs used the concept of “accumulative diffusion” because he believed that chemical processes occurring in the cell upset the equilibrium of concentrations of substances inside the cells and in the surrounding solution.

Proponents of the diffusion-osmotic theory include Pfeffer, De Fries, Meyer, and other scientists. According to this theory, nutrients are sucked into plants through the root system along with water, and water constantly evaporates. Consequently, the supply of nutrients is in direct correlation with the intensity of water evaporation by plants. However, the available data on the patterns of nutrient inflow did not fit into the framework of the ideas of the diffusion-osmosis theory.

K.A. Timiryazev also noted the lack of certain dependence between water and nutrients inflow to plants. He wrote that plants do not need those enormous amounts of water that they evaporate in order to nourish themselves. 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 paste.

At the end of the 19th century Overton put forward 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 Ruland proposed the ultrafiltration theory according to which penetration of nutrients through the cytoplasmic membrane is determined by the size of pores and molecules. Dravet established the dependence of the penetration of 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 molecule sizes could not be explained by the proposed theory.

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

Many studies by D.A. Sabinin and other scientists trace the connection of substances absorption with activity of cells, the role of root system in the process of substances absorption is shown. Concentration of substances in passococcus 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 absorption of substances. Stuard, Lundegaard, Bjurström and other scientists established the connection of respiratory processes in tissues with the process of absorption of mineral salt ions. Hoagland and Breuer have shown that increase in absorption of substances by plant cells and tissues is observed under good aeration of nutrient solution, introduction of glucose, increase in temperature and other conditions contributing to activation of respiratory processes.

D.A. Sabinin proved the relationship between plant nutrition and the formation and development of individual organs.

The above-mentioned theories had their significance in the development of views on the process of substance entry into plants. They, in essence, simplistically reflect the different sides of the influx of elements of nutrition. The modern views on the mineral nutrient inputs include a number of basic concepts from the previously proposed theories.

Root

The root is a specialized part of the plant that performs the functions of fixation of the plant in the soil and absorption of nutrients: primary assimilation, inclusion in metabolism; distribution and transport of water and minerals. Numerous biosynthetic processes take place in the root and some special functions are performed.

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

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

root
Root structure: 1 - phloem; 2 - xylem; 3 - root hairs area; 4 - extension area; 5 - division area; 6 - root sheath; 7 - apical meristem; 8 - primary bark; 9 - epiblum with root hairs

The root system of field crops has a huge absorptive surface, which reaches its maximum, including the active, as a rule, during the flowering period.

Table. Changes in the surface area of roots of spring wheat[1] Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Development phase
Root surface of one plant, m2
Ratio of active surface to inactive surface
общая
деятельная
Tillering
9,60
4,91
1,05
Booting
29,39
10,81
0,59
Beginning of blooming
36,73
17,07
0,86
End of blooming
40,09
24,76
1,30
Waxy ripeness
30,86
14,80
0,92

Root hairs greatly increase the absorptive surface of the root. It is generally accepted that the root hairs zone is the absorption zone. However, nutrient uptake 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 phosphate movement in barley from an area further than 40 cm from the root tip remains the same as in the root hairs area.

In Clarkson’s experiment, it was shown that root hairs have no special absorption properties. Barley grown in an aqueous culture with strong aeration of the solution does not form root hairs on the roots, but the absorption of ions is quite intense. Probably the main role of root hairs is to maximize the root surface to supply plants primarily with phosphorus.

The movement of phosphorus in the soil is very slow, but the rate at which it is absorbed by the plant is high. In a short period of time, the roots absorb the phosphorus around them and increase their surface area to absorb the next amounts of this element. Other ions are more mobile and root hairs are less important for their absorption. In the soil, due to chemotropism, the root grows and advances toward a higher concentration of nutrients.

Depending on the biological characteristics of the crop and growing conditions, the root system develops different capacities. On poor soils and in arid conditions, plants produce a relatively large mass of roots in search of food and water. Application of fertilizers reduces the ratio of the mass of the roots and the above-ground part of the plant, but increases the total mass and depth of root system spreading.

Root cell membrane

The membrane determines the ability of the root cell to selectively absorb ions. Reactions of metabolism and energy flow on its surface. The contact of a cell with the soil environment is carried out by means of cytoplasmic membrane, or plasmalemma.

The cell membrane consists of two layers of phospholipids with hydrophobic ends. Molecules of carrier proteins are embedded in certain areas of the phospholipid layer. The mosaic structure of the cytoplasmic membrane consists of positively and negatively charged sites through which cations and anions from the external environment are adsorbed (Bergelson, 1975).

cell membrane
Mosaic model of the cell membrane:
a - lipids; б - proteins

Phospholipids form several types of liquid-crystal structures.

Phospholipid molecules consist of polar “heads” – hydrophilic groups, and non-polar “tails” – long hydrocarbon hydrophobic chains. Phospholipids are poorly soluble in polar solvents (water) due to non-polar “tails” and in non-polar media (oil) due to polar “heads”. The monomolecular layer at the interface creates a limitation of permeability of substances (Bergelson, 1975).

Phospholipids
Structure of lipid aggregates:
a - gel-like; б - lamellar (layered); в - lipid cylinders in water; г - water cylinders in the lipid phase

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 the lipid and water, so that positive ions are attracted to the membrane surface and negative ions are repelled.

Phosphatidylcholine
Phosphatidylcholine molecule

Membrane proteins are used to build pores and channel walls in the membrane. In the cytoplasmic membrane, absorbed ions are involved in metabolic reactions. Some of the membrane proteins have enzymatic activity. Membrane proteins in the form of protein globules have a hydrophobic anchor, due to which molecules are able to interact with lipids. Proteins can be bound to polar membrane lipid heads via bridges formed by divalent metal cations.

Monomolecular layer of lipids
Monomolecular layer of lipids and micelles in oil and water

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

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

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

The way of attachment of negatively charged proteins
A way of attaching negatively charged proteins and polar heads of membrane lipids through bridges formed by divalent metal ions
Diffusion
Distribution of solute molecules as a result of diffusion through the permeable membrane. [Sa] and [Sb] are concentrations of dissolved substance in chambers A and B. Black circles are molecules of dissolved substance, light circles are water molecules
Osmosis
Movement of water molecules through a semipermeable membrane separating two chambers originally filled with dissolved substance (black circles) at different concentrations

Passage of nutrients through the membranes of root cells by the mechanism of passive transport is possible in the presence of hydrophilic openings (pores) in the membranes. Non-specific passive transport of ions and molecules occurs by diffusion through the 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 pore area 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 concentration difference and the electric potential.

Root nutrition of plants

At present, the composition, quantity, and forms of compounds in the form of which minerals enter the plants are well enough defined. This information has been obtained by numerous experiments in growing plants in aquatic and sandy crops and in hydroponics, where nutrient mixtures of mineral salts are used to produce record high yields.

Plants can also use organic compounds for nutrition, such as amino acids, organic acids, sugars, and sucrose phosphates. Nitrogen from amino acids in plants undergoes deamination, and the released ammonia is included in the processes of nitrogen nutrition.

Table. The main forms of consumption of nutrients by plants (Kidin, 2008)[2]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Element
Main ions
Element
Main ions
Nitrogen
NH4+, NO3-, NO2-
Manganese
Mn2+, MnO4-, Mn4+
Aluminum
Al3+, Al(H2O)63+
Copper
Cu2+, Cu+
Bor
H2BO3-, H4BO72-
Molybdenum
MoO42-
Iron
Fe2+, Fe3+, Fe(H2O)63+
Sodium
Na+
Potassium
K+
Selenium
SeO42-
Calcium
Ca2+
Sulfur
SO42-, SO2
Cobalt
Co2+, Co3+
Phosphorus
H2PO4-, HPO42-
Silicon
H3SiO4-, HSiO3-
Zinc
Zn2+
Magnesium
Mg2+

By the end of the XX century, the study of the process of mineral nutrition began to be studied not only under conditions of deficiency of a particular nutrient, but also under increased supply. This is due to the need to determine the conditions of mineral nutrition, under which the potential productivity of the plant is achieved.

Physiology of plant mineral nutrition
The main issues and sections of the physiology of mineral nutrition of plants (by A.N. Pavlov)

A number of characteristics of agronomic practices depend on the function of different parts of the root: the depth of tillage and fertilization. Thus, the root of cereal crops has three main zones:

  • zone of growth and extension with a length of 1.5 mm, due to the division of cells in the apical meristem the growth of the root occurs;
  • root hairs zone, or suction zone, which has root hairs up to 1 mm long, the length of the zone itself is 1-2 cm;
  • lateral root zone.

In the field, the root hairs zone, also called the absorption zone, is of primary importance in plant nutrition.

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

The rate of nutrient uptake varies with root age. For example, as corn plants age from 20 to 80 days, the rate of absorption of nitrogen, phosphorus, potassium, calcium, and magnesium decreases tenfold.

A study of the function of germinal and knot roots made it possible to determine the important role of the tiller node in the distribution of water and minerals. The tiller node consists of loose, porous parenchyma tissue that facilitates the easy movement of nutrients. The roots of the tillering node are characterized by high absorption capacity and play a significant role in plant nutrition than the primary (germinal) roots. Their role especially increases during tillering, accompanied by increased branching of knot roots.

Depending on the nature of the energy expended, the absorption of nutrients can be active and passive.

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

In all movements and transformations of nutrients, the protein plasma of the cell, which has a dual nature, basoid and acidoid, plays a role. In protein plasma, acidoid and bazoid parts are arranged mosaically, so that during the movement of nutrients from cell to cell, there is a gradual exchange and interaction of cations and anions with positively charged (bazoid) and negatively charged (acidoid) parts of protein plasma molecules.

Active uptake and movement of ions occurs through a system involving protoplasts of cells connected by plasmodesms. In the case of passive absorption, ions reaching the root surface by mass overflow or diffusion get into the free space of the root and move around the plant with the transpiration current. Plant cells have loose cellulose membranes connected with each other and forming a continuous system – apoplast. Through this system, the transpiration of water by the leaves causes the movement of substances dissolved in it.

The metabolic role of roots
The cycle of substances and the metabolic role of roots

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

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

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

The exchange process of adsorption consists in the ionic exchange of nutrients in the form of ions such as K+, Ca2+, Mg2+, NH4+, NO3, H2PO4, SO42-, for ions H+, HCO3. The latter are excreted by the root surface during respiration. Root system is capable of releasing large amounts of carbon dioxide, so mustard plants in 85 days of vegetation emit 2.25 tons of CO2 from 1 hectare.

Emerging carbon dioxide reacts with water to form carbonic acid:

CO2 + H2O → H+ + HCO3.

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 NO3, H2PO4, SO42- anions are exchanged and displaced into solution by HCO3 anion.

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 transfer process in the plant.

The 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 excreted by the roots. For example, plant roots can secrete citric acid, malic acid, oxalic acid, and other organic acids that dissociate into H+ and organic anions.

Closer contact of the root system with the soil absorbing complex contributes to the intensification of absorption processes. The best feeding conditions are created when there are enough ions in the soil solution and in the adsorption-bound state.

Adsorption absorption of nutrients is characterized by the absorption capacity of cations and anions of root hairs as well as of soil colloids. Root absorption capacity depends on plant species, feeding conditions and other factors. Thus, the capacity of cationic absorption per 100 g of root dry matter for legumes is 40-60 mmol, for potatoes and tomatoes, 35-38 mmol, and for cereals, 9-29 mmol.

Root cationic uptake capacity increases with nitrogen nutrition, which seems to be related to protein synthesis. Hydrogen ions (H+) account for the major part of cationic absorption capacity.

Anionic absorption is similar to cationic absorption. Hydrocarbonate-ion HCO3 plays the main role. In many crops, anion exchange quantitatively exceeds cation exchange, which indicates the presence of more colloids of active part of roots with positive charge.

The main barrier for 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 the membranes.

Attempts to act on root membranes to regulate the supply of ions to the roots have some importance in controlling plant nutrition. Membrane-active compounds of interest in this regard are those that affect ion transport in plants. These include valinomycin, gramicidin (antibiotics), 2,4-dinitrophenol, and dimethyl sulfoxide. The latter is the most promising because it has a milder effect, increasing the conductivity of the sulfolipid layer of membranes.

The treatment of sugar beet plants with 2.5-5% aqueous dimethylsulfoxide solution under field conditions resulted in increased absorption and movement of nitrates and phosphates in the plants, promoted the use of soil nutrients and fertilizers, activated growth and increased the root crop by 3-6 t/ha, increased sugar yield by 0.5-1.0 t/ha.

Mechanisms of nutrient transport to plant roots

Nutrient absorption is a complex physicochemical and metabolic process involving:

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

Diffusion is important in the movement of soluble nutrients in the soil to the roots. Exchange adsorption is when nutrients enter the plant through the root system. The incoming nutrients interact with the cell protoplasm in a metabolic or non-metabolic way.

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

Non-metabolic, or passive, absorption may be unrelated to plant activity, so it depends little on temperature and other plant conditions. An example of non-metabolic absorption is pinocytosis, i.e., the capture of a part of a nutrient solution, in which young root cells absorb ions and solution droplets.

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

  • root interception,
  • mass flow,
  • diffusion.

Not only the outer walls of rhizoderm cells, but also the mucilage layer covering the root surface is important in the feeding process. Mucilage is the layer of mucus around the root hairs and young parts of the root, which participates in the process of contact exchange.

Different nutrient ions are involved in the transport mechanisms in different ways. For example, phosphorus and potassium are delivered to the roots mainly by diffusion, while calcium and magnesium are delivered by mass flow.

Mass flux becomes important at high nitrate concentrations in the soil solution (about 10 mol/L). At low nitrate concentrations, the diffusion process predominates.

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

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

Root interception

Root interception is the process of taking up nutrients by the root as it grows in the soil and comes into contact with nutrients. The share of root interception in nutrition with the volume of root system in the soil at a depth of 15 cm does not exceed 0.5-2% of the total soil volume.

The role of root interception increases when the soil contains nutrients in large quantities compared with the needs of the plant.

Mass flow

Mass flow is the process of transferring nutrients in the soil with the soil solution. As plant roots absorb water, it creates a movement of soil solution in the soil column to the roots and with it nutrients.

Depending on the type of plant and weather conditions, the intensity of mass flow can vary greatly, but as a rule it is in the range from 1 to 10 cm3 of water per 1 cm2 of the root surface per second.

Diffusion

Diffusion is the absorption of a nutrient by the root, 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 the type of soil and the nature of the absorption of ions 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 be as high as 1. For strongly absorbed soil ions, such as phosphate, this ratio can be 10-4.

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

Nutrient intake theory

The exchange of substances between the plant and the environment occurs through the surface cells of the root system and above-ground organs. Plant cell walls are easily permeable, since the radius of mineral 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 outer nutrient solution, there would be an equalization of the concentration of substances by diffusion. But in plant organisms, nutrients are in higher concentrations than in the outer nutrient solution. The entry of individual substances and their concentration are different and do not correspond to the ratio of concentrations in the nutrient solution. This is due to the presence of a plasmalemma which prevents the loss of substances and at the same time ensures the penetration of water and nutrients.

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

There are a number of theories of the entry of substances into the cell. The transport of mineral salts into the cell is ensured by two mechanisms – the passive flow of substances along the electrochemical gradient and their active transport in the direction against the electrochemical gradient.

The electrochemical gradient is the distribution of mineral salt ions between the cell and the external environment under the action of the difference in electrical potentials and the difference in concentrations. The movement of ions along the electrochemical gradient is passive and against it is active.

The ratio of mineral salt transport mechanisms varies in plant ontogenesis and depends on a number of conditions. For example, passive transport of ions prevails at high concentrations of salts in the external environment, which is formed under conditions of salinization or local fertilization.

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

Diffusion is the movement of molecules of gases, liquids or dissolved substance in the direction of the concentration gradient and depends on the concentration difference and the area through which the substance and ions pass.

The free space is the part of the total volume of root system tissues into which ions enter and from which they are released due to diffusion. Free space accounts for about 4-6% of the total root volume. Free space is localized in the loose primary sheath of cell walls outside the protoplast outside the plasmalemma.

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

Exchange or donnan space is located closer to cell walls. Concentration of cations occurs here due to negative charge of walls.

The process of substances entering the free space occurs rapidly with an increase in concentration according to the saturation curve. The process is characterized by weak selectivity, its magnitude remains constant under the action of inhibitors or lower temperatures. The process is reversible.

The negative charge of the membrane is connected with negatively charged carboxyl groups of protopectin and plasmalemma embedded in cell walls. Exchange adsorption is important at this stage of absorption. For example, a root that has been incubated in a solution of calcium ions and transferred to a solution containing potassium ions loses adsorbed calcium through exchange adsorption.

The network of intercellular cells present in plant tissue provides additional transport. The loose cellulose covers of cells create a mass of micro-parts for adsorption and passive flow of substances through the tissue.

Substances entering the free space due to diffusion can undergo primary exchange adsorption. In this case they can be displaced from the free space into the outer solution or into the inner part of the cell. The process of adsorption consists in binding of mineral salt ions by cell membranes and their retention by electrostatic attraction forces.

Absorption of ions by the root
Course of ion absorption (with radioactive label) after transfer of roots from pure water to salt solution: a - normal course (nonmetabolic + metabolic absorption); в - at 0 °С or in the presence of respiration inhibitors (only nonmetabolic respiration); a' - metabolic respiration

Consequently, the free space is an area of the nutrient medium and simultaneously the absorption zone. Further, depending on the intensity and type of metabolism, selective absorption occurs. It is more correct to consider the entry of nutrients into the free space as the first preparatory stage of absorption.

Cellulose membranes of plant cells are united with each other by a system of vessels called apoplast. As a result of leaf transpiration and water flow, water and mineral salts are sucked into the intercellular spaces in the roots. To reach the xylem vessels for transport to the terrestrial organs, nutrients must travel a considerable distance in a radial direction across the root. This pathway of ions movement along the apoplast is interrupted by Caspari belts, watertight areas in which intercellular walls contain suberin, a substance that does not allow water solutions to pass through. However, some of the ions enter the conducting vessels through extracellular pathways – “passer cells”.

In the growing endoderm root tip, the Caspari bands do not have time to form, so ions can pass through the extracellular pathway. It should be noted that only a small number of ions enter the plant via the apoplastic pathway, and its role in balancing nutrients under normal life conditions is probably insignificant.

After ions cross the cell membrane (plasmalemma), their further movement occurs along a single cell system, the symplast, from cell to cell due to protoplasts linked by bridges through plasmodesmata, which ensure contact of cell plasmalemma. The mechanism of the plasmodesmata allows regulation of the rate of transport of substances through the symplast pathway, which is 2-4 cm/h.

Stages of entry:

  1. Saturation with ions of the free space of the apoplast by exchange adsorption, diffusion, and passive adsorption.
  2. Overcoming of membrane barrier by ions and their transfer to symplast.
  3. Radial movement of ions in root tissues and vascular-fiber bundles.
  4. Incorporation of ions into metabolism.
  5. Vertical movement of ions along the above-ground parts of the plant.
  6. Inflow into photosynthetic cells, utilization and reutilization, outflow.
  7. Transport of assimilates and ions down 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 transfer can be passive, i.e. along the electrochemical gradient, and active – against the electrochemical gradient. The latter, the mechanism of active transport of ions through the phospholipid membrane, is the most important.

According to the transporter theory, the ion crosses the membrane in the form of a complex with a transporter molecule. On the inside of the membrane, the complex dissociates and releases the ion inside the cell. The transport of ions is carried out with the help of different types of transporters.

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

In facilitated diffusion, substances move along the concentration gradient at a high rate.

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

The ionic permeability of phospholipid membranes is increased by some antibiotics produced by bacteria and fungi, such as Gramicidin and Nystatin.

Transport of substances inside 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.

Two systems of ion transport have been established. The first system is characterized by a higher selectivity and is activated in vivo at low external ion concentrations of less than 1 mmol. Increasing the concentration of ions in the external solution leads to saturation of the first system and engages the second, less selective system.

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

Dependence of ion absorption by roots on solution concentration
Dependence of K+ uptake by barley roots on solution concentration. The course of the curve indicates two different transfer mechanisms (according to Epstein)

Protein globules with a diameter greater than the thickness of the cell membrane can be used as carriers. In this case, the movements of the globule around its axis ensure the transfer of ions from the outside to the inside. An example of such transport is the ion pump represented by the transport potassium-sodium ATPase. ATPases are so called because they can break down adenosine triphosphoric acid (ATP) molecules.

The energy released is used for transport and the transport ATPase is reversibly phosphorylated. The molecular weight of the transport ATPase is 200,000-700,000. Simultaneously with phosphorylation and dephosphorylation of the transport ATPase, ion binding and release and simultaneous conformational changes of the ATPase molecule occur, allowing the ion to be transported inside the cell.

The active site of the ATPase molecule consumes metabolic energy, and the M+ cation is attached to the exchange site. In the activated pump, the exchange site changes its orientation and faces the external environment. Under these conditions, its electric field strength changes and the C+ cation binds to it preferentially. Relaxation is the transition from the activated state to the non-activated state. Then the original orientation of the pump is restored: the exchange site with C+ cation turns to the internal environment, where M+ is preferentially bound to the exchange site, while C+ is released.

electroneutral ion exchange pump
Hypothetical mechanism of the electroneutral ion-exchange pump that allows the transfer of ions against the electrochemical gradient: A - activation; B - reorientation; C - relaxation; o - external environment; i - internal environment

Ion pump is an active transport of ions into cells by ATPases. Its task is to maintain a constant ionic composition inside the cell.

Transport ATPases in animal cells were discovered in 1957, in plant cells – in 1964. In addition to the potassium-sodium pump, there is also a proton pump that pumps out H+ ions, which creates a negative charge in cells.

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

The rate of transport of ions due to transporters is determined by the rate of their turnover, which depends in turn on temperature, oxygen content, the presence of an inhibitor, the number of binding sites of the transporter, and the occupancy of active sites.

The theory of carriers made it possible to explain selectivity of absorption, interaction of ions and inhibition of the process. The theory suggests the presence of a control mechanism regulating the entry of substances into the cell according to the feedback principle. According to Pitman’s data, the maximum content of ions in the root cells is 80-90 mg-eq/g raw weight. For various plants this value does not depend on the concentration of ions in the solution, and is reached by 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.

Some substances can be transported across the membrane by low-molecular-weight fat-soluble transporters. The carrier captures the ion, penetrates through the phospholipid membrane, and releases the ion inside the cell. The transport of substances inside the cell through membranes can occur through pores in the plasma membrane. Such pores can exist either temporarily or permanently. Their diameter is supposedly 0.5-0.8 nm. For comparison, the diameter of hydrated potassium ion is 0.34 nm.

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

The transfer of substances through hydrophilic channels or pores can proceed by channel selectivity. The relay mechanism is the passage of substances through the membrane, in which an ion or molecule is sequentially transferred from one carrier to another. In this process, the molecules of the carrier are embedded in the membrane in sequence and transfer ions or molecules on a relay. If the carrier moves with the ions being transferred, the transfer mechanism is called a shuttle mechanism.

Symporter and antiporter

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

In doing so, alkalinization 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. This transfer mechanism is called symporter. The opposite process – pumping the H+ proton out of the cell and delivering it inside the cell to keep the ion with the same charge electroneutrality – is called antiporter. The terms symporter and antiporter were suggested by Mitchell.

Nitrogen entry is much easier. The NH4+ cation in aqueous medium is in equilibrium NH4+ ↔ NH3 + H+. The NH3 molecule enters the cell a thousand times faster than other electroneutral molecules except water. As it passes through the membrane, ammonia takes the hydrogen ion H+ from water, forming the ammonium ion NH4+ and the hydroxide ion OH:

The cytoplasm is alkalized, which prevents proton pumping and proton pump operation, while at the same time alkalization promotes phosphorus influx.

NO3, CN, I. anions pass through the membrane 100-1000 times harder than K+ and NH4+. These anions in high concentrations can destroy the membrane structure and are called chaotropic.

Proof of the passage of nitrate through the membrane is the following experience. If in the medium where the liposome – a ball bounded by a two-layer artificial membrane – is placed, to put KNO3 and valinomycin, a potassium “hole” is formed and K+ and NO3 ions enter the liposome, it quickly swells up. If KCl is used instead of KNO3, the membrane is impermeable to KCl. A small amount of potassium will enter the liposome due to valinomycin and the process will stop due to the diffusion potential, the liposome will not swell.

Water molecules have the highest rate of passage through membranes.

Pinocytosis

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

By pinocytosis, substances can enter plants. At the beginning of the process, the absorbed particles are adsorbed on the cell membrane, then the membrane is drawn inside; its edges close together at the place of retraction. Thus, a pinocytic vesicle is formed, which detaches from the outer membrane and migrates inside the cell.

Two mechanisms of pinocytosis are possible.

In the case of the first mechanism (I), the membrane is retracted inside the cell and forms a narrow channel. From the end of the channel the vesicles with the captured substance are laced off.

In the case of the second mechanism (II-VII), the section of the membrane on which the micromolecules were adsorbed (III) is retracted inside (IV). At the place of retraction, the membrane edges close (V), and the resulting pinocytic vesicle is detached from the cell membrane (VI). In the thickness of the cell membrane, the vesicle is destroyed by enzymes (VII), and the trapped particles enter the cytoplasm. The process of vesicle formation and its detachment from the outer membrane is associated with energy expenditure, which is provided by ATP.

pinocytosis
Two possible mechanisms of pinocytosis: I - first mechanism; II-VII - second mechanism

The pinocytic vesicle is destroyed by contact with the lysosome, which contains hydrolytic enzymes that break down macromolecules.

The phenomenon of pinocytosis is initiated by certain areas of the membrane with the appropriate substances. Reverse pinocytosis, i.e., the process of carrying the substance out of the cell to the outside, is possible.

Molecules or ions coming from the external solution, regardless of the method of transport through the plasma membrane, practically do not enter into reactions of exchange at the level of the plasma membrane. After entering the inner space of the cell they may:

  • be included in the cycle of metabolic transformations, with subsequent incorporation into the organic compounds of the structural elements of the cell;
  • concentrate in the vacuoles of root cells, creating a stock of ions;
  • to be transmitted by xylem vessels to the above-ground parts of plants;
  • to be excreted from the organism into the environment.

The effect of the plant root system on the soil

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

The direct impact lies in the ability of plants to influence the hard-to-reach forms of nutrients (phosphorus). Even D.N. Pryanishnikov proved the ability of buckwheat, lupine, mustard to use phosphorus of tri-substituted phosphates or natural phosphorites, which is related to the acidity of root secretions. Thus, the acidity of the solution surrounding the root hairs of lupine is 4-5, that of clover is 7-8.

Synthetic capacity of roots

For many years, it was believed that only in leaves the formation of complex organic compounds was possible. However, thanks to the method of labeled atoms, it was found that active synthetic processes also take place in the roots.

“We can say that in the life of the leaf the very essence of plant life is expressed, that the plant is the leaf.”

K.A. Timiryazev

The plant absorbs more of those substances which it needs more, which is explained by the physiological laws of the living organism. For example, when sodium nitrate NaNO3 is applied to the soil, the plant absorbs the anion NO3 and to a lesser extent the cation Na+. When adding ammonium sulfate (NH4)2SO4, the plant absorbs more NH4+ cation and less SO42- anion. Substances that are physiologically necessary for the plant are assimilated (assimilated) through the roots, while substances that are not needed remain in the mineral, easily soluble ionic form in the same form as they were 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 a study of plant root resin extraction (pasoka) the following were found in it: amino acids, oligopeptides, sugars and growth substances. From the lower, dying leaves, assimilants flow into the roots in the form of sucrose, which is used by the root for metabolism, growth and maintaining mature, functioning cells in a physiologically active state; for the synthesis of substances released by the root into the external environment and substances that enter the above-ground organs.

D.A. Sabinin made a great contribution to the study of the physiology of the root system and its properties. He was one of the first to put forward the assumption of synthetic ability of the root system. In 1940, he proposed the concept of transformation 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 feeding them to the above-ground organs in a modified 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 influences the above-ground organs by providing water, minerals and products of specific metabolic reactions occurring in the roots – phytohormones of non-auxin nature.

One of the main growth substances found in Pasok are cytokinins, which contribute to intensive metabolism of leaves and delay their aging. Cytokinins are synthesized mainly in the root and partially in the leaves.

Other growth substances synthesized by the root are gibberellins, necessary for stem growth. Stopping the growth of above-ground organs when the roots are removed is associated not only with the deterioration of nutrient supply, but also with the cessation of cytokinin and gibberellin synthesis. 

As plants age, the concentration of calcium increases in their cells, while the concentration of potassium decreases. Young and actively functioning plants are characterized by high concentrations of K+ ions. Treatment of plants with the growth agent kinetin leads to an outflow of Ca2+ ions from the cells and an increase in the concentration of K+ ions. Yellowed leaves become green, the destruction of subcellular structures stops and protein biosynthesis increases.

In 1965, F.V. Turchin, using a stable nitrogen isotope 15N, determined that almost all the ammonium (15NH4+) and a significant portion of nitrate (15NO3) nitrogen absorbed by the root system is located in the root passover as nitrogen-containing organic compounds. About 18 amino acids are synthesized in the roots of various crops.

Studies using labeled atoms of phosphorus (32P), sulfur and other elements have shown that in plant roots the phosphoric acid anion (H2PO4) is incorporated into organic compounds through ether bonds, while the absorbed sulfate ion (SO42-) is reduced and incorporated into sulfur-containing amino acids: cystine, cysteine and methionine.

The synthetic ability of the root system was proved by Academician A.A. Shmuk and his collaborators in 1941. They found that nicotine in tobacco plants is synthesized in the root system, not in the leaves. If tobacco is grafted onto tomato, there is virtually no nicotine in the tobacco leaves, and conversely, up to 3-4% nicotine accumulates in the leaves of tomato grafted onto tobacco.

Root excretions of plants use relatively small amounts of assimilates. For example, 0.5-0.7% of carbon from the roots of beans is excreted into the leaves in the form of carbon dioxide.

The synthetic activities of the leaf and root of plants are closely related.

The discovery of the synthetic ability of the root system is one of the major achievements of science in the XX century in the field of physiology of plant root nutrition.

Excretory function

Root excretions include sugars, amino acids, organic acids, and, in small amounts, vitamins, enzymes, and volatile organic substances such as ethylene. Root excretions vary in quantity and composition and are determined by the species and varieties of crops.

Root excretions are related to the ability of plants to regulate the nutrient regime of soils. For example, lupine, due to the acidifying effect of root excretions, is able to assimilate hard-soluble forms of phosphorus.

Accumulation of root excretions when growing isolated roots under sterile conditions leads to growth suppression due to accumulation of amino acids, the main component of root excretions, to toxic concentrations for plants.

Nutrient absorption selectivity

The plant absorbs more of those substances that it needs more, 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 anion NO3- and to a lesser extent the cation Na+. When adding ammonium sulfate (NH4)2SO4, the plant absorbs more NH4+ cation and less SO42- anion. Substances that are physiologically necessary for the plant are assimilated (assimilated) through the roots, while substances that are not needed remain in the mineral, easily soluble ionic form in the same form as they were before entering the plant.

After the equilibrium between concentrations in the cellular and soil solutions on the surface of the root hair and on the surface of soil colloidal particles is reached, the processes of ion inflow cease. When the concentration of nutrients on the cellular solution side 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 the cell sap is much higher than in the nutrient solution into which the root system is immersed. For example, the concentration of potassium in the corn graze is 20 times higher, phosphorus 14 times higher, and calcium 4 times higher than in the external nutrient solution.

With the end of the life cycle, nutrient uptake into plants stops and an equilibrium of their concentration on the root hair surface and in the soil solution sets in.

The selectivity of nutrient absorption determines the physiological acidity of fertilizers. If the plant absorbs more cations of mineral fertilizer, the 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 fertilizer includes ammonium sulphate, ammonium chloride, ammonium nitrate, potassium chloride. Conversely, if the plant absorbs the anion of the mineral fertilizer and the cation is accumulated in the soil solution, the fertilizer is physiologically alkaline, such as sodium nitrate, calcium nitrate.

If there are toxic substances in the soil solution, e.g. heavy metals, pesticides, much of it is retained in the roots as well as in the stems and leaves, and only a small part goes to the seeds.

Nutrient intake periods

There are two periods of nutrition in the life of a plant. The first, or critical, period is during the initial phases of plant growth and development. During this period, plants are most sensitive to a lack or excess of nutrients. In this period the plants are particularly demanding to the conditions of mineral nutrition.

The second period, or the period of maximum intake of nutrients, is characterized by the intake in later phases of development and is determined by the biological characteristics of plants. For example, nutrient intake in cereal plants, with the exception of corn, ends by the end of the earing phase, although by this time only 50-60% of the plant mass of the full crop is formed.

Table. Dynamics of accumulation of nutrients in plants, % of maximum[3]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Timing and growth phase
Winter wheat
Barley
Oats
N
P2O5
K2O
N
P2O5
K2O
N
P2O5
K2O
Autumn and early spring
47
30
43
-
-
-
-
-
-
Beginning of earing
69
65
68
71
56
73
51
36
54
Blossoms
90
93
95
96
74
100
82
71
100
Full ripeness
100
100
82
100
100
64
100
100
83

Winter wheat with good development in the autumn period assimilates up to 43-47% of nitrogen and potassium, while the dry weight of plants is not more than 10% of the total yield. The same is typical for winter rye, which during the autumn period assimilates 50-60% of nitrogen, phosphorus and potassium. Oats and barley absorb 100% of potassium during the flowering phase, and even lose it afterwards (exoosmosis, or excretion). The accumulation of nutrients by corn plants is slower: by the beginning of flowering 30-40% of nitrogen and potassium and 15% of phosphorus of the content of these substances in corn at its maturity occurs.

Such crops as sugar beets, potatoes, cabbage and other vegetable crops assimilate nitrogen, potassium and phosphorus almost throughout the growing season.

Table. Dynamics of the influx of nutrients in the plants of sugar beet (according to the Research Institute of Sugar Beet), % of the maximum[4]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Date of observation
Dry matter
Nitrogen
Phosphorus
Potassium
root
above-ground part
5/VI
0,3
2,3
2,7
2,4
2,6
15/VI
1,1
8,2
9,6
8,5
9,2
1/VII
6,8
35,7
36,0
39,0
35,0
15/VII
16,7
48,0
51,0
46,0
46,0
1/VIII
33,0
64,0
68,0
68,0
69,0
15/VIII
46,0
63,0
77,0
77,0
73,0
1/IX
60,2
73,0
85,0
87,0
85,0
15/IX
85,0
100
100
100
100
1/X
93,0
77,0
89,0
90,0
91,0
15/X
100
58,0
-
-
-

Periodicity of plant nutrition allows you to regulate the formation of yield and its quality. In particular, fractional fertilizer application is based on this property of plants.

Application of fertilizers at one time and in one layer of soil does not always allow to achieve full use of the potential yield. For example, doses of readily soluble mineral fertilizers that are sufficient during the critical feeding period will be small for the period of maximum consumption. Conversely, a high dose in the first critical period is harmful to the young roots, sensitive to high salt concentrations. For this reason, the plant fertilization system involves a combination of basic, pre-sowing fertilization and top dressing during plant growth.

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.