Ion Absorption: Selectivity, Accumulation, Genotype, and, Mechanisms of Ion Absorption

Ion Absorption: Selectivity, Accumulation, Genotype, and, Mechanisms of Ion Absorption


The soil solution contains the ions of the important plant mineral nutrients that are taken up by plant roots. Even if the concentration may be low, the anions and cations that are freely dissolved in the soil solution are the most easily assimilated by the roots. There are three ways in which these nutrients reach near roots:

(1) Ions diffusing through the soil solution

(2) The passively charged ions carried by the bulk flow of water into the roots and

(3) Roots’ development is extended toward the dissolved ions. Ion uptake is characterized by three principles in both higher and lower plants.

1. Selectivity: Plants preferentially absorb some minerals while discriminating against or nearly excluding others. Some minerals may be present in larger concentrations in soil solutions, but they are not necessary for plant growth. Therefore, the plant has a mechanism to only take up or exclude the elements that it needs for growth.

2. Accumulation: Mineral element concentration in plant cell sap may be significantly higher than in external solution due to accumulation. It has been observed that the root cell sap’s ion concentration is typically substantially higher than the nutritional solution’s. This is demonstrated by the greater concentrations of potassium, nitrate, and phosphate that are collected in the root cells.

3. Genotype: The ion uptake of the various plant species varies significantly. This variation is based on the state of the growing medium. For instance, in the case of alga species, Nitella grown in pond water had higher concentrations of potassium, sodium, calcium, and chloride in the cell sap, whereas, in the case of Valonia grown in highly salinized sea water, only potassium remained at higher concentration in the cell sap, while the sodium and calcium concentrations remained at a lower level than in seawater.

Ions accumulate in plant tissues in greater amounts than in the surrounding ambient solution, which suggests that they diffuse into the cell against a gradient of concentration. This kind of ion uptake necessitates the use of metabolic energy, hence it is dependent on the metabolic health of the tissues or cells that are absorbing the ions in plants. Active absorption is the term for the ion or salt absorption and accumulation phenomena, which is most prevalent in rapidly growing tissues like meristematic cells and declines as cells mature.

Some characteristics indicate metabolic energy is needed for the active absorption of solutes:

Salt accumulation inside the cell is influenced by three factors:

(1) A higher rate of respiration

(2) Respiratory inhibitors

(3) A decrease in the amount of oxygen in the medium. These data indicate a direct relationship between salt intake and plant energy levels and respiration rates.

The flow of Solute into Cells from External Solution

There are three possible routes by which the water and dissolved ions in the external solution enter the xylem cells of roots:

(1) Through the apoplast of epidermal and cortical cells

(2) Through the cytoplasmic or symplast system, moving from cell to cell

(3) from the vacuole to the vacuole of the living root cells where the cytosol of each cell forms the pathway. The next sentences go into great detail on this pathway.

Mineral salts that have been dissolved in water are taken up by roots through the xylem vessel, where they are disseminated to all parts of the plant body. Water containing dissolved cations and anions travels from cell to cell through apoplastic pathways, or spaces between cell walls, by diffusion. On the other hand, ions that enter the epidermis’ cell wall go via the cortex’s cell wall, the endodermis’ cytoplasm, and the pericycle’s cell walls before arriving at the stele. The term “symplastic pathway” refers to the pathway that passes through the portion of the cell that is alive (cytoplasm). Through the use of plasmodesmata, nutrients entering the cytoplasm of the epidermis go through the cortex and endodermis of the pericycle to eventually reach the xylem vessels.

The transpiration flow flowing through the xylem transports the nutrients to other plant components after they have been taken up by the roots. A transpirational pull develops as water is repeatedly lost by the plant’s aerial portions through transpiration. This force propels the water and mineral salts in the xylem vessel upward.

This transpirational pull is so strong that it causes the tallest trees growing naturally to transport water and nutrients many feet upward. Mineral nutrients received by the roots are transported through the xylem vessel, as demonstrated by Stout and Hoagland.

The process of nutrient uptake by roots is precise, selective, and well-regulated. Also influenced by several interconnected processes is the subsequent translocation of ions across the cortex to reach the stele (xylem loading). Because of the anatomy of roots, there are certain unique restrictions on the path that ions or solutes can take throughout the root. Ions can travel via channels in the cell wall gaps without ever entering a living cell since all plant cells are divided by cell walls. The term “free space” or “apoplast” refers to the continuous portion of the cell wall that permits extracellular ion transport. The cytoplasms of nearby cells also connect and form a continuum, collectively known as the symplast, in addition to the cell wall constituting a continuous phase. Plasmodesmata are the cytoplasmic bridges that connect the adjacent cytoplasm. These are spherical pores with a diameter of 20–60 nm. Each plasmodesmata (singular) has a short tubule termed a desmotubule, which is the continuation of the endoplasmic reticulum and is bordered by a plasma membrane. The majority of nutrient absorption takes place in the cells close to the root tips, which have a high density of plasmodesmata.

The root surface through the cortex is all covered in the apoplast, which forms a continuous phase. At the junction of the cortex and the vascular cylinder, a layer of specialized cells known as endodermis exists. As a barrier to the entry of water and mineral ions into the stele through the apoplast, the endodermis comprises a layer or strip of suberised cells known as the Casparian strip. Each endodermis cell is surrounded by this Casparian strip, which has hydrophobic qualities. To reach the symplast, a solute must first pass the plasma membrane of an epidermal cell, or it must diffuse through the cell walls of the epidermal cells. Ions and solvents from the apoplast of the cortex can either diffuse radially to the endodermis via the apoplastic route or they can cross the plasma membrane of the cortical cell, adopting the symplastic path. In either scenario, the Casparian strip means that the ion/solute must first enter the symplast before entering the stele.

Ions are absorbed into the tracheid or vascular element of the stele and transported to the shoot after entering the symplast of the root at the epidermis or cortex. Since the xylem treachery components are dead cells, the ions must traverse the plasma membrane twice to leave the symplast. The act of moving ions from the symplast into the conducting cells of the xylem is known as “xylem loading.” Once more, the Casparian strip stops ions from back diffusion through the apoplast. The fundamental benefit of this strip is that it aids the plant in maintaining an ionic concentration in the xylem that is greater than the soil water surrounding the roots, although it acts as a barrier to the transport of water and solutes.

Ions may enter the xylem’s tracheid and vessel components by passive diffusion. However, in this instance, there would only be a single step that required metabolic energy to transfer the ions from the root surface to the xylem. The root epidermal, cortical, and endodermal cells’ plasma membrane surfaces are where this single energy-dependent ion uptake phase takes place. According to the passive diffusion model, ions passively flow into the stele via the symplast through a concentration gradient and subsequently leak out of the living cells of the stele into the nonliving conducting cells of the xylem (perhaps because there is less oxygen available inside the root). Using ion-specific microelectrodes, the electrochemical potential of different ions across the roots could be monitored. Studies have shown that as compared to the external medium, epidermal and cortical cells actively take up K+, Cl-, Na+, SO4 2- and NO3- and keep them in the xylem against an electrochemical potential gradient. None of these ions, however, are electrochemically more potential in the xylem than they are in the cortex or living part of the stele. Therefore, passive diffusion may have contributed to the final migration of ions into the xylem. However, additional research has indicated that the stele may also be actively involved in this latter stage of xylem loading. Researchers have demonstrated that ion loading in the xylem and ion uptake by the cortex function separately by employing inhibitor treatments and other plant hormones. Treatments with cytokinin (benzyladenine), or with a protein synthesis inhibitor like cycloheximide, reduce xylem loading without altering cortical uptake. This finding demonstrated that uptake by cortical cells and efflux from stelar cells are controlled differently.

The xylem parenchyma cells are thought to play a part in xylem loading, according to recent biochemical investigations. H+ pumps, water channels, and a variety of ion channels with specialized functions for influx or efflux can all be found in the plasma membranes of xylem parenchyma cells. In contrast to meristem or elongation zones, root hair exhibits a more dramatic level of ion absorption by roots. The surface area available for ion absorption is considerably increased by the presence of root hairs.

Mechanisms of Ion Absorption

Fick’s law states that molecules or ions always diffuse freely down a concentration or chemical gradient until equilibrium is attained, going from a high concentration to a low concentration. Passive transport refers to this diffusion-based movement of molecules without the need for metabolic energy. Once equilibrium has been attained, there will be no more net solute movement without the use of a driving force. On the other hand, active transport refers to the movement of molecules against a concentration gradient or after equilibrium has been attained while using metabolic energy.

Frequently Asked Questions

Question: How ions are absorbed by plant roots?

Ans:  By Active transport, against the concentration gradient.

Question: How minerals are absorbed by plants?

Ans: Through their roots, plants take up minerals.

Question: How do ions enter plant cells?

Ans: Active transport or diffusion allows ions to enter the plant.

Question: How are potassium ions absorbed in plants?

Ans:  Through soil-derived roots.

Question: How do mineral ions move in a plant?

Ans: Carrier proteins are present in the cell membranes of root hair cells. These absorb mineral ions.

Question: How do plants absorb nutrients?

Ans: Near the very tip of the roots, root hairs are responsible for the majority of nutrition absorption.

Question: How are nitrate ions absorbed by plants?

Ans: Through their roots, plants take up nitrates from water.

Question: How are magnesium ions taken up in plants?

Ans: Through the roots of the plant.

Question: Where do plants get their ions from?

Ans: soil

Question: What is the process of plants absorbing water?

Ans: Diffusion, Active transport, Osmosis,

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