In this article we will discuss about:- 1. Meaning of Solute 2. Passive Absorption of Solute 3. Active Absorption 4. Ion Movement into the Root.
Meaning of Solute:
Besides water, the plant absorbs from the environment considerable quantities of mineral salts, gases and various other salts. All these are absorbed in the form of aqueous solutions.
The mineral salts or solutes are absorbed from the external solutions by the roots. It is believed that they are not absorbed as whole molecules but as ions. This is evident from the unequal accumulation of the anions and cations of the same salt in the cell sap.
For many years it was supposed that various solutes entered the roots along with soil water in the process of osmosis; in other words the salts swept in with the water current. This view is erroneous because the diffusion of solute is quite different from the diffusion of water.
Bonner (1953) has shown that salts can be absorbed by a cell even if there is higher concentration of a particular salt. Hoagland (1944) also found that solutes can be absorbed from a culture solution either more less rapidly than water is absorbed depending on the kind of solutes.
Passive Absorption of Solute:
In most cases, the movement of mineral ions into the root occurs by diffusion. Molecules or ions diffused from a region of their higher concentration to a region of their lower concentration. As these substances diffuse they exert a pressure called diffusion pressure. The movement of mineral ions into root cells as a result of diffusion is called passive absorption.
Ion Exchange Theory:
According to the theory of ion exchange ions from the external solution (in which tissue is immersed) are exchanged with the ions of similar charge absorbed on the surface of the cell wall or membrane of the tissue. The colloidal fraction of the soil has an important role in ion exchange.
These colloidal particles are known as micelles, possess negative charge and attract cations such as calcium, magnesium, potassium, ammonium, sodium, as well as hydrogen ions arising from biological activity.
In the similar way, the root surface, which has a negative charge, carries many cations on its surface. These cations can be exchanged with other cations present in the soil. Thus a new cation can be absorbed on the root surface. Further transport of cations inside the root cell takes place by diffusion. Like cations, anions can also be exchanged with free ‘OH−‘ ions.
This theory explains the passive accumulation of ions, that are non-diffusible, which may be present on one side of the membrane. Unlike diffusible ions, the membrane is not permeable to non-diffusible ions. Such ions are called fixed ions. They may be anions or cations.
In a system, in which there are no fixed ions, there are equal number of anions and cations on both sides of the membrane at equilibrium. But in Donnan equilibrium, in order to balance, the charge of the fixed ions (anions), more ions of the other charge (cations) would be required.
For example, there is a membrane that separates a cell from the external medium and allows exchange of some ions and not others. To the inner side of this membrane there are anions, which are fixed and non-diffusible, and therefore, the membrane becomes impermeable to these anions.
In such a situation, for equilibrium to be reached, additional cations are needed to balance the negative charges of the anions that are structurally formed to the inner side of the above membrane.
According to the theory, Donnan equilibrium is attained if the product of anions− and cations+ in the internal solution becomes equal to the product of anions and cations+ in the external solution, depicted by the equation as follows:
For example, a membrane which is permeable to Mg+ and CI− ions and to X− ions (i.e., fixed ions) present inside the cell. Here, membrane has 6X fixed ions on the inner side. CI− ions make across the membrane by diffusion along the concentration gradient. The concentration of anions− on the inner side is now more than that of cations+.
In order to balance electrochemical equilibrium within the cell sap, Mg++ ions move across the membrane against the concentration gradient. Similarly, if there are fixed cations the anions shall move against the concentration gradient to bring about equilibrium.
Mass flow hypothesis. According to this hypothesis, roots also absorb a large quantity of ions along with absorption of water, due to transpiration. The ion absorption increases with increase in 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 occurs due to passive absorption of ions by free diffusion into the free space of a tissue. Thus, mass flow of ions through root tissue occurs as a result of transpirational pull in the absence of metabolic energy. This theory was supported by Krimer (1956), Russel and Barber (1960).
Active Absorption of Solute:
Generally, the lipid-protein membrane of a cell is largely permeable to free ions. The energy is considered to be involved in the transport of such free ions across the membrane. Thus the absorption of ions, involving use of metabolic energy is called active absorption.
The absorption of solutes takes place only in some cases by simple diffusion. Only limited quantities of minerals can pass into the cells by this method. Hoagland and Davies (1923) and many scientists have found that there is also a movement of salts against the concentration gradient, i.e., from lower concentration to higher concentration for each ion.
This cannot be explained on the basis of osmosis and most scientists now accept the theory of “active absorption of solutes” put forward by Hoagland (1923).
Active Absorption of Solutes or Theory of Salt Accumulation:
According to this theory Hoagland (1923) suggested that absorption of solutes takes place against higher concentration of salts. The cells near the tips of roots also have the capacity of accumulating ions. If the initial salt content of the root cells is low and if other conditions are favourable, the concentration of ions in the absorbing cells may greatly be increased than that present in soil solution.
This involves an expenditure of energy. This energy is supplied by the respiratory activity of the absorbing cells. The rate of accumulation is often influenced, therefore, by the previous metabolic status of the absorbing cells.
The phenomenon of salt accumulation seems confined largely to cells which have the capacity for cell division and growth. Meristematic cells and cells in the early stages of enlargement are particularly active in absorbing ions.
As cells lose their capacity for growth, they also lose their capacity of mineral salt accumulation. As already referred earlier, that accumulation of salt requires expenditure of energy which is supplied by respiratory activity of cells.
Hoagland and Broyer (1936) found that if excised roots (young roots) are immersed in dilute solutions of mineral salts through which N2 is bubbled, little or no accumulation of salts occurs in the root cells. If on the other hand, O2 is bubbled through the solution, a rapid accumulation of salts within root cells takes place.
Lack of O2 checks aerobic respiration and prevents absorption of ions. The accumulation of ions of root cells and their retention within these cells in a free condition requires an expenditure of energy which is supplied by the process of respiration.
Salt accumulation is also affected by the rate of photosynthesis because on this process depends the supply of carbohydrates which are the respiratory substrates for efficient respiration. So any factor, which reduces photo synthesis, also reduces salt accumulation.
The fact that salt accumulation by root cells is dependent on respiration, suggests that temperature may have also marked effect on salt accumulation process.
Ions, which are accumulated in cells, may move into the inner space against concentration and for this movement additional energy is required. This additional energy is derived directly or indirectly through metabolism. This theory of active absorption has been supported by various evidences which show that active ion uptake is carried out by carrier mechanism for both influx and efflux of ions.
Unlike ion channels, the carrier proteins do not have pores. The membrane does not allow the ions to pass through as it is. The activated ions combine with carrier proteins and form ion-carrier complex, which is capable of moving across the membrane.
This complex moves across the membrane and reaches the inner surface. Here, the complex breaks and releases ions into the cytoplasm of the cell. Carriers are specific, and combine with a particular type of ion.
Ion Movement into the Root:
Mineral nutrients absorbed by the root are carried to the xylem.
This absorption takes place by two pathways, they are:
(i) Apoplast pathway, and
(ii) Symplast pathway.
(i) Apoplastic Pathway:
This pathway, essentially involves diffusion, and mass flow of water from cell to cell through specs between cell wall polysaccharides.
The ions that enter the cell wall of epidermis move across cell wall of cortex, cytoplasm of endodermis, cell wall of pericycle, and finally accumulate in xylem vessels.
(ii) Symplastic Pathway:
In this pathway, ions that enter the cytoplasm of epidermis move across the cytoplasm, cortex, endodermis and pericycle through plasmodesmata, and finally reach to xylem vessels.