Mineral Salt Absorption
Most of the mineral elements required by the plants are absorbed from the soil by the roots and are then translocated to various parts. The minerals are absorbed as ions. Many theories have been put forward to explain the mechanism of mineral salt absorption. These theories can be grouped into following two categories:
- Passive mineral absorption.
- Active mineral absorption.
Passive Mineral Absorption
When the absorption of solute or salt takes place without the expenditure of metabolic energy and salt enters into the plant cell by free diffusion. It is called passive absorption. It can be explained by the following theories:
Transport of ions across the membrane occurs passively by simple diffusion and facilitated diffusion.
It is also known as non-mediated diffusion. In this type of diffusion, ions while moving across the membranes do not associate themselves with the constituents of the membranes.
Single ion channels are known to occur in the membranes. They are transmembrane proteins which function as selective pores for the diffusion of ions. Similarly there are aquaporins for the diffusion water.
The channels known as aquaporins permit diffusion of substrate in both directions through the membranes. The substances move only in the direction of lower concentration.
No energy is required for keeping the pores open or maintaining solubility of matrix. Passive diffusion operates through physical forces like chemical potential, electrochemical gradient, hydrostatic pressure, and diffusion pressure gradient.
Water, oxygen, carbon dioxide, and sodium are known to follow passive diffusion.
It is movement of solute molecules or ions from one side of the membranes to the other through the membrane protein. Plasma membranes of both prokaryotic and eukaryotic cells as well as membranes of subcellular organelles contain proteins. These membrane proteins function as transport proteins. These transport proteins translocate the molecules or ions across the membranes. This rapid protein-mediated diffusion is called facilitated diffusion. The direction of transport in facilitated diffusion is from higher concentration to its lower concentration.
The rate of transport through channels is much higher than by transport proteins.
Ion Exchange Theory
According to this theory, ions from the external solution are exchanged with the ions of similar charge adsorbed on the surface of the cell wall or membranes of the tissue. The colloidal fraction of the soil has an important role in ion exchange. These colloidal particles called micelles, possess negative charge and attract cations such as calcium, magnesium, potassium, ammonium, sodium etc. Similarly, the root surface which assumes a negative charge, carries many cations on its surface. These cations can be exchanged with other cations present in the soil solution. Thus, a new cation can be absorbed on the root surface. The process of exchange between adsorbed ions in the solution is known as ion exchange. The ion exchange mechanism can be explained by two theories.
- Contact exchange theory.
- Carbonic acid exchange theory.
Contact Exchange Theory
An ion which is adsorbed electrostatically to a solid particle is not tightly bound but oscillates within a small volume of space. This volume is called oscillation volume. Therefore, the cations and anions adsorbed to the surface of root cell membranes or clay particles oscillate in limited area. Suppose, H+ ion is adsorbed on root cell surface and K+ ion on the clay particles and both oscillate. They oscillate in such a way that the oscillation volume of H+ ion overlaps that of K+ ion. This results in transfer of H+ ion to clay particles and K+ ion to root surface. This phenomenon is called as contact exchange theory.
Carbonic Acid Exchange Theory
Respiration occurring in a root cell results in the production of carbon dioxide, which forms H2 CO3 when dissolved in water. The carbonic acid dissociates into H+ ion and HCO3- ion. These ions then may be exchanged for similar charged ions of the soil solution.
This theory describes the effect of fixed or non-diffusible ions, which mostly accumulate on the inner surface of the outer membrane. Outer membrane is impermeable to fixed anions. However, the cell membrane is impermeable to cations and anions present in the external medium. Normally, both cations and anions diffuse into cells in equal numbers until an equilibrium between the cell sap and external medium is attained. Now, the equilibrium has to be electrically balanced, therefore, more cations from the external medium will be needed to electrically balance fixed anions present in the cell. Thus, the cations will be greater in the internal solution than in the external medium. This electrical balance or equilibrium is known as Donnan Equilibrium.
According to Hylmo (1953-1955), the absorption of ion increases with increasing transpiration. The ions have been considered to move in a mass flow with water from the soil solution through the root and eventually to the shoot. This uptake depends on transpiration pool. An increase in transpiration will cause an increase in the absorption of ions. An increase in water flow due to transpiration pull also increases the total uptake of ions by the roots. This is a passive process and occurs as a result of transpiration pull. Metabolic energy is not required for the process.
Lopushinsky (1960) demonstrated using stomata plant that an increase in transpiration can increase salt absorption. It has been demonstrated by many other workers that the rate of transpiration affects the salt uptake. The theory was supported by Kramer (1956), Russel and Barber (1960).