Two different mechanisms have been proposed to explain absorption of mineral salts from soil to cells of roots. The mechanism of uptake of salts or ions is of two types, such as non-mediated or passive uptake and mediated or active uptake.
I. Non-mediated or passive uptake of ions:
If ion transport in and out of cells occurs spontaneously down a gradient of electrochemical potential energy, it is called passive uptake. Metabolic energy is not necessary for passive absorption. Passive absorption may take place by the following means:
i) Mass flow:
Karmer (1956), Russel (i960) believe that ions arc absorbed by the root along with the mass flow of water taking place under the influence of transpiration. Lopushinsky (1964) working with detopped plants supported the concept that an increase in transpiration rate could increase the absorption of salts.
It has been observed that an initial passive or nonmetabolic uptake of ions take place when a plant cell or tissue is kept in a medium of relatively high salt concentration. This free diffusion of solutes to the plant cell obeys the simple law of diffusion.
iii) Ion exchange:
In ionic exchange mechanism, anions or cations from within cells are exchanged with anions or cations of equivalent charge of the external solution. The process of ionic exchange can be explanied by two theories.
a) The contact exchange theory:
According to the contact exchange theory an ion may be absorbed by the plant root without being first dissolved in the soil solution. An ion adsorbed electrostatically to a solid particle such as
a plant root or clay micelle, is not held too tightly, but oscillaties within a certain small volume of space. Exchanges of ions take place when the oscillation volume of one ions overlaps the oscillation volume of another ion.
(b) The carbonic acid exchange theory
According to this theory, the CO2 released during respiration of root cells combines with water to form carbonic acid. Carbonic acid dissociates into II+ and IICO3 in soil solution. These 11+ ions may be exchanged for cations adsorbed on clay particles.
The cation thus released into the soil solution from the clay particles, may be adsorbed on root cells in exchange for II+ ions or as ion pairs with bicarbonate. Thus soil solution plays an important role in carbonic acid exchange theory.
Ions exchange theory allows faster rate of absorption of ions from the external medium by the cells of root.
II. Mediated or Active uptake of mineral salts:
Most of the experimental findings suggest the concept of mediated transport or active uptake of mineral salt in plants. The uptake of ions in plants is affected due to the presence of carriers in the cell membrane and expenditure of energy in the form of ATP released as a result of respiration.
Evidence in support of active uptake of mineral salts:
(i) The actual rate of absorption of minerals is too rapid in plants than that can be explained by passive absorption.
(ii) Sometimes the absorbed mineral is at a higher concentration within the cell and is at a lower concentration outside (soil)
(iii) The factors like low temperature, deficiency of oxygen, metabolic inhibitors etc. which inhibit metabolic activities like respiration in plants also inhibit absorption of ions.
(iv) The rate and amount of absorption of minerals is directly related to the expenditure of metabolic energy.
It has now been accepted that active absorption of mineral salts involves the operation of carrier conpounds in the plasma membrane of the cells.