MECHANISM OF ION UPTAKE AND TRANSLOCATION

ion is an atom or molecule which has lost or gained one or more electrons, making it positively or negatively charged

Anion: A negatively charged ion, which has more electrons in its electron shells than it has protons in its nuclei

Cation: a positively-charged ion, which has fewer electrons than protons

superscript indicating the sign of the net electric charge and the number of electrons lost or gained H+ So4

2−

The soil is a complex physical, chemical, and biological substrate.heterogeneous material - solid, liquid, and gaseous phases

solid phase

Inorganic & organic particles of the provide a reservoir of potassium, calcium, magnesium, and iron and nitrogen, phosphorus, and sulfur.

liquid phase

soil solution- contains dissolved mineral ions and serves as the medium for ion movement to the root surface

Gases

oxygen, carbon dioxide, and nitrogen are dissolved in the soil solution, but in roots, gases are exchanged predominantly through the air gaps between soil particles.

Soil serves as a main source of mineral salts of ionic forms in clay crystals have a

central nuclues - micelle

The micelles are negatively charged and maintain a balance, they attract and hold positively charged ions on the surface

cationic formsK, Mg, Ca, Fe, Mn, Cu, Zn and Co

anionic forms N,P,B,S and Cl

These ions are either in the form of loosely absorbed ions or firmly absorbed ions on the colloidal particles

micelle

The loosely absorbed ions can be easily displaced by decreasing their own concentration in the soil solution

firmly absorbed ions can be replaced by other ions which have more affinity for the colloid or ion exchange

First step in the absorption of mineral salts is the process of Ion exchange which does not require metabolic energy

The ion exchange may be either cation exchange or anion exchange

There are two theories - mechanism of ion exchange

1. Cation Exchange hypothesis (or) Contact Exchange Theory

2. CO2 hypothesis (or) Carbonic Acid Exchange Theory

1. Cation Exchange hypothesis (or) Contact Exchange Theory

oscillation volume of ions on root surface may over by the

oscillation volume of ions on clay particles,

exchanged directly without being dissolved in soil solution.

CO2 released during decomposition of organic matter combines with water to form carbonic acid (H2CO3)

Carbonic acid dissociates into H+ and an anion HCO3 in soil solution

These H+ ions may be exchanged for cations K+, Mg2+, Ca2+, and Mn2+ adsorbed on the clay particles

2. CO2 hypothesis (or) Carbonic Acid Exchange Theory

Mineral salts are absorbed from the soil solution in the form of ions

They are chiefly absorbed through the meristematic regions of the roots near the tips

First step in the absorption of mineral salts is by Ion Exchange

further process of the absorption of mineral salts may be of two types

1. Passive Absorption2. Active Absorption

Different Areas of the Root Absorb Different Mineral Ions

1. Passive Absorption of Mineral Salts

Higher concentration of mineral salts in the outer solution diffuse in to cell sap of the root cells, according to the concentration gradient by simple process of diffusion with out expenditure of metabolic energy

i. Mass Flow Theory (Bulk Flow)

ii. Diffusion theory - Donnan’s equilibrium

Within the soil, nutrients can move to the root surface both by bulk flow and by diffusion In bulk flow, nutrients are carried by water moving through the soil toward the root

Mass Flow Theory (Bulk Flow)

ions are taken up by the roots along with mass flow of water under the influence of transpiration

The amount of nutrient provided to the root by bulk flow

depends on the rate of water flow through the soil toward the plant, which depends on transpiration rates and on nutrient levels in the soil solution

When both the rate of water flow and the concentrations of

nutrients in the soil solution are high, bulk flow can play an important role in nutrient supply

ii. Diffusion theory,

In diffusion,

mineral nutrients move from a region of higher concentration to a region of lower concentration

Nutrient uptake by the roots lowers the concentration of nutrients at the root surface, generating concentration gradients in the soil solution surrounding the root

Diffusion of nutrients down their concentration gradient and bulk flow resulting from transpiration can increase nutrient availability at the root surface

certain pre-existing ions inside the cell, which cannot diffuse outside through membrane. indiffusible or fixed ions

fixied anions in the cell which is in contact with the outer solution containing anions and cations

to balance each other, more cations/ anions will diffuse into the cell and equilibrium is maintained

Donnan’s Equilibrium

When absorption of nutrients by the roots is high and the nutrient concentration in the soil is low,

bulk flow can supply only a small fraction of the total nutrient requirement

Under these conditions,

diffusion rates limit the movement of nutrients to the root surface.

a nutrient depletion zone forms adjacent to the root surface

This zone extends from about 0.2 to 2.0 mm from the root surface,

depending on the mobility of the nutrient in the soil.

2. Active Absorption of Mineral Salts

involved the metabolic energy for the transport of ions from soil solution to the plants cells

Based on the nature of participation of metabolic energy,

Theories

i. Carrier Concept Theory (Honert,1973) (for movement of both cation & anion)

ii. Protein-lecithin as Carrier (Bennet-Clark,1956) (for uptake of both cation & anion)

iii. Cytochrome-pump Theory (For the movement of anions only)

iv. ATP Theories

plasma membrane is impermeable to free ions

The carrier combines with ions to form carrier-ion complex, which can move across the membrane

On the inner surface of the membrane,

this complex breaks releasing ions into the cell while the carrier goes back to the outer surface to pick fresh ions

Here, the metabolic energy is required in the process of formation of carrier-ion complex, its transport, breakdown of complex, regeneration of carrier and movement of carrier molecules back

i. Carrier Concept Theory (Honert,1973)

ii. Protein-lecithin as Carrier (Bennet-Clark,1956)

Phosphatide

choline group

decomposition

regeneration

iii. Cytochrome-pump Theory –( Lundegardh and Burstrom (1933) (For the movement of anions only)

When a plant is transferred from water to salt solution, the rate of respiration increases

Salt Respiration

The actual transport of anions occurs through a cytochrome system

reduced outside

Oxidised inner

Red. cyto.oxidised

Fe++

Fe+++

iv. ATP Theories

According to this theory, ion uptake into the cell is energized by ATP molecules.

role of ATP in this theory is of two kinds. i.e., by removal or addition of phosphate group. Case I

organic compound is first phosphorylated which on dephosphorylation makes the organic compound capable to combine with cation The cation is released when phosphorylation occurs again.

Case II

phosphorylated organic compound combines with cation and the cations are released on hydrolysis of the complex (dephosphorylation).

ION TRANSPORT IN ROOT TO SHOOT

Mineral nutrients absorbed by the root are carried to the shoot by the transpiration stream moving through the xylem

Both the initial uptake of nutrients and the subsequent movement of mineral ions from the root surface across the cortex and into the xylem are highlyspecific, well-regulated processes.

pathways and mechanisms involved in the radial movement of ions from the root surface to the tracheary elements of the xylem

Ion absorption by the roots is more pronounced in the root hair zone than in the meristem and elongation zones.

The root hairs are simply extensions of specific epidermal cells that greatly increase the surface area availablefor ion absorption.

Ions Move through Both Apoplast and Symplast

Plant cells are interconnected by cytoplasmic bridges called plasmodesmata cylindrical pores 20 to 60 nm in diameter

Each plasmodesma is lined with a plasma membrane and contains a narrow tubule, the desmotubule, that is a continuation of the endoplasmic reticulum

Ions, on the other hand, appear to move from cell to cell through the entire plant by simple diffusion through the symplast

From the apoplast of the cortex, an ion may either cross the plasma membrane of a cortical cell, thus entering the symplast, or diffuse radially all the way to the endodermis via the apoplast

An ion that enters a root may immediately enter the symplast by crossing the plasma membrane of an epidermal cell, or it may enter the apoplast and diffuse between the epidermal cells through the cell walls

Once an ion has entered the stele through the symplastic, connections across the endodermis, it continues to diffuse from cell to cell into the xylem. Finally, the ion re enters the apoplast as it diffuses into a xylem tracheid or vessel element.

Xylem Parenchyma Cells Participate in Xylem LoadingOnce ions have been taken up into the symplast of the root at the epidermis or cortex, they must be loaded into the tracheids or vessel elements of the stele to be translocated to the shoot. xylem loading

Plant nutrientdemand

Nutrient-useefficiency

Shootgrowth

CO2

Carbohydrates orphotoassimilates

CH2O

Photosynthesis&transpiration

Rootcompensatory adjustments· Symbiotic associatione.g. mycorrhizas and N2 fixation· Uptake kinetics· Turnover & life span

Water flux

Nutrient availability at theroot surface· Decomposition & mineralization· Mass flow· Diffusion

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nNutrient uptake by Roots