Nabgha nosheen

1375

Ion traffic into roots

What are essential elements?

There are 113 or so different elements in this

planet, of which fourteen or fifteen are

absolutely required for the life processes

without which plants exhibit diseased

symptoms and ultimately die.

eight elements are required in sufficient

quantities----macronutrients

other in small quantities -----

micronutrients

The macronutrients are carbon, hydrogen, nitrogen,

phosphorus, potassium, calcium, sulphur, magnesium

and iron.

The micronutrients are manganese, zinc, boron, copper, molybdenum and cobalt.

plants also contain elements other than

the macro and microelements ----- non

essential elements.

These are sodium, aluminum, silicon,

chlorine, gallium, etc.,

Sources of nutrients

Atmosphere water soil

Most of the above said

elements are found in soil

solution either in the form of

inorganic or organic salts or

ions; which may exist in either

in free state or bound to clay

particles

Whenever there is depletion of any

free ions from the soil solution,

respective ions are released from clay

particles into the soil solution to

maintain the equilibrium. This is

achieved by a process called

ion exchange process

Ion exchange

Process

1. contact ion exchange

mechanism

2. carbonic acid ion exchange mechanism

CONTACT ION EXCHANGE MECHANISM

Plant roots are in contact with soil clay

particulates which have colloidal dimensions,

Root cells which are living, secrets hydrogen

ions which are positively charged and have

greater affinity so it can easily displace cations

like K+, Na+ ions that are bound to clay

particulates. Thus the cations are made

available for the root system to absorb the

required ion.

Catio

n exc

hang

e

CARBONIC ACID ION EXCHANGE MECHANISM

҉ Roots continuously respire irrespective of day or night, and liberate significant quantities of CO2, which when dissolved in soil water produces carbonic acids. Immediately they ionize into H+ and bicarbonate ions (HCO3).

҉similarly organic matter also has negative charge – phenolic group ionize—carboxyl group ionize into coo- and H+ , and hydroxyl group(-OH)

into O- and H+.

ION EXCHANGE MECHANISM

Cation exchange

Anion exchange

҉ Clay mineral fraction is abundant in soil.

҉ Clay largely consist of Si4+ & Al3+

҉ Si4+ and Al3+ Mg2+ and Fe2+

҉ Such replacement results in availability of negative sites on clay particles so that dissolved cations adsorbed on their surfaces.

Cation exchange

Anion exchange ҉ Mineral anions are usually repelled by –tv charge of clay.

҉ anion exchange capacity is usually smaller than the cations.

҉ common anions are phosphate(PO4),nitrate(NO3-),

sulfate (SO32-) and chloride (Cl-).

҉ because of this repulsion they leached out from soil.

Soil pH҉ pH can effect the growth of plant root and microorganism ----- favoured by slightly acidic pH values.҉ pH determines availability of plant nutrients-----pH favors weathering of rocks—release K+,Mg+,Ca2+ and Mn2+.

҉ Low pH increase solubility of the salts and their absorption by roots------salts such as CO3

2-, SO42-, PO4

3-

etc.

Three ways dissolved minerals moves through root hairs

1) Apoplast: dissolved minerals moves through cell walls and never enter cells

2) Symplast: it moves from one cell to another through the cytoplasm

3) Transmembrane: repeatedly moves back and forth from cell wall, crossing the membrane, and through the cytoplasm(vacuolar)

apoplasticsymplastic

Pathway of dissolved minerals Movement

1) Uptake via root hairs 2) travels via apoplast or symplast

through the cortex until it reaches the endodermis that lines the vascular cylinder(stele).

3) Endodermis has a “casparianstrip”, a selectively waxy layer, which BLOCKS the apoplastpathway, so water MUST move into the stele via symplast (toregulate what minerals in the water can enter the stele)

4) Xylem within the stele transports water to shoots

Mechanism of transport