Mechanism of nutrient absorption

Mechanism of nutrient absorption

1. Mechanism of Nutrient A bsorption Krishna Aryal Asst. Professor (Soil Science) [email protected] 1

2. 2 Movement of nutrient from soil to root Roots do not intentionally grow towards a nutrient source. For nutrient uptake to occur, the individual nutrient ion must be in position adjacent to the root. There are three basic methods in which nutrients comes in contact with the root surface for plant uptake; they are; i . Root interception ii . Mass flow iii . Diffusion

3. 3 Root interception: • Root interception occurs when a nutrient comes into physical contact with the root surface. • As a general rule, the occurrence of root interception increases as the root surface area & mass increases, thus enabling the plant to explore a greater amount of soil. • Root interception may be enhanced by mycorrhizal fungi, which colonize roots & increases root exploration into the soil . • Root interception is responsible for an appreciable amount of Ca and P uptake, & some amounts of Mg, Zn & Mn .

4. 4 ii. Mass flow : Mass flow occurs when nutrients are transported to the surface of roots by the movement of water in the soil (i.e. percolation, evaporation etc .). The rate of water flow governs the amount of nutrients that are transported to the root surface . Therefore , mass flow decreases as soil water decreases . Most of the nitrogen, calcium, magnesium, sulphur , copper, boron, manganese & molybdenum move to the root by mass flow.

5. 5 iii. Diffusion : • Diffusion is the movement of a particular nutrient along a concentration gradient . • When there is a difference in concentration of a particular nutrient within the soil solution, the nutrient will move from an area of higher concentration to an area of lower concentration . • Diffusion delivers appreciable amounts of phosphorus, potassium, zinc, & iron to the root surface. • Diffusion is a relatively slow process compared to the mass flow of nutrients with water movement toward the root.

6. 6 Nutrient absorption by plant roots Plant absorption of ions from the soil solutions/root space can be described by; i . Passive absorption( Apoplastic ) ii. Active absorption ( Symplastic )

7. 7 Passive Absorption Rapid movement of ions into the free spaces (space between cell wall and plasma membrane called apoplast ) Ions moves along the concentration gradient( higher to lower) Do not require energy(ATP) thus called passive. The major hypothesis (theories) that explain the mechanism of passive absorption of ions are: 1. Mass flow theory. 2 . Contact Exchange theory . 3.Carbonic Acid Exchange Theory 4. Donnan Equilibrium

8. 8 Mass flow theory According to this theory ions are absorbed by the root along with mass flow of water under the effect of transpiration. An increase in transpiration pull increases the uptake of ions by the roots, (the uptake of ions by free diffusion). Thus, mass flow of ions through the root tissues occurs due to transpiration pull in the absence of metabolic energy.

9. 9 Contact Exchange Theory • According to this theory, the ions adsorbed on the clay micelles get adsorbed to the root in exchange for hydrogen ions, previously, adsorbed on the root. • Ions adsorbed on solid particles oscillate within a small space. When two particles are close enough, the oscillation space of an ion adsorbed to one particle overlaps the oscillation space of an ion adsorbed to another particle. Thus exchange of ions may take place.

10. 10 Carbonic Acid Exchange Theory • The soil solution provides medium for exchange of ions between the root and clay micelles. • Carbon dioxide released in respiration of root forms carbonic acid by reacting with water of the soil solution . • Carbonic acid then dissociates in the soil solution to form H⁺ and HCO₃⁻ ions shown above expression . • H⁺ ions are adsorbed to the clay micelles in exchange for cations, such as K⁺ , which are released into the soil solution . • From here they may diffuse to the root, where they may be adsorbed in exchange for H⁺ ions.

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12. 12 Donnan Equilibrium As cell membrane is composed of macromolecules of proteins and lipids that have many carboxyl groups (-COOH) and phosphate (HPO₃⁻) groups, from which positively charged particles like protons of hydrogen (H⁺) can dissociate, leaving the macromolecules with negative charge called non diffusible anions. To counterpart the negative charge positively charged anions moves inside the cell membrane and thus equilibrium exists known as Donnan Equilibrium. C+ ( ouside ). A-(outside) = C+ (inside ). A- (inside )

13. 13 ii. Active Absorption Movement of ions through inner space (called S ymplast ) Ions moves against the concentration gradient Requires energy(ATP) thus called active. Slow movement of ions The major hypothesis that explain the mechanism of active transport of ions is : Carrier Concept – transport by a carrier protein.

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