There was a time when people believed that the mineral absorption in plants was carried out together with the absorption of water. They were inclined to think that the solutes simply enter the plant tissue along with water. Later it was found that mineral salt absorption and water absorption are two different and independent processes.
- From the soil solution, mineral salts are absorbed in the form of ions.
- They are chiefly absorbed through the meristematic region of the root, near the root tip.
- The region of active solute absorption in the root does not coincide with the region of active absorption of water.
This again shows the fallacy of former belief.
Moreover, we know that the absorption of solutes is an independent process but the actual mechanism involved in the process is still unknown. However various theories have been put forward to explain this mechanism.
Among them, the theory of diffusion along a gradient, active solute absorption, ionic exchange, unequal absorption, Donnan equilibrium etc, are worthy of mention.
Ion Exchange
Ion exchange is the primary way of absorption of solutes by plants. This process does not require any energy consumption. Here, the ions that are present on the external surface of the root tips are exchanged with ions of other minerals inside the cells, having the same sign (+ or -). Thus this process is called ion exchange.
This process has been experimentally proved in barley plants in which radioactive K+ ions have been allowed to accumulate from a solution and are transferred to distilled water. Almost none of the radioactive ions are lost from the roots into the water.
The cytoplasmic membrane behaves as if it is completely impermeable to those ions which have entered their cells, readily before they are transferred to distilled water.
When similar roots are transferred to a dilute solution of non-radioactive potassium bromide (KBr), some of the radioactive K+ ions move out of root cells into the solution and non-radioactive K+ ions are moved into the cell from the solution.
Under this condition, the cytoplasmic membrane is permeable to K+ ions moving in both directions. Detailed studies made it evident that radioactive K+ ions have, in effect, exchanged places with non-radioactive K+ ions in the solution.
The ions absorbed on the surface of walls or membranes of root cells may be exchanged with ions of the same sign from the external soil solution. There are two theories regarding this mechanism of ion exchange.
Contact Exchange Theory
According to Contact Exchange theory, the ions adsorbed on the surface of root cells and clay particles are not held tightly but oscillate within a small volume of space.
The orbits of ions adsorbed on the root and clay particles overlap each other when they are in close contact. When this occurs, the adsorbed ions may exchange places directly without first being dissolved in the soil solution.
Carbonic Acid Exchange Theory
According to this theory, carbon dioxide released during respiration combines with water to form carbonic acid H2CO3. This carbonic acid dissociates into H+ and an anion HCO3- in soil solution. The H+ ions may be exchanged for cations adsorbed on clay particles. The cations thus released into soil solution from clay particles may be adsorbed by root cells in exchange for H+ ions. Thus soil solution plays a significant important role, as per the carbonic acid exchange theory.
In some cases, cations of one type can be displaced from clay particles and replaced by cations of another kind. This can be proved by treating acidic soil with a solution of calcium chloride. Ca++ ions will replace H+ ions and an equivalent amount of H+ ions will be displaced in the solution.
Diffusion or Physical Absorption or Passive Absorption of Mineral Salts
The simple process of diffusion along a gradient is the first mechanism involved in the consideration of solute or mineral absorption. Diffusion may be defined as the absorption of solutes by cells according to the ordinary law of diffusion.
It has been observed that the protoplasmic membrane is permeable to all inorganic solutes that are present in soil solution. It is implied that mineral salts enter through root cells by simple diffusion from regions of higher concentration to regions of lower concentration.
If some or all the solutes were removed from the cells combined, precipitated or otherwise, the concentration of solute continues to lower in cell sap than in the surrounding medium so that it will continue to enter the cell.
Also, the minerals are free to move to the adjoining cell. This type of absorption does not require much energy and the mechanism is too simple to account for most mineral absorption. In submerged water plants, this is a common method of salt absorption.
Active Absorption or Physiological Absorption or Primary Salt Absorption
Active absorption of mineral salts takes place against the concentration gradient, from a region of lower concentration to a region of higher concentration, by consuming energy. This was first observed by Hoagland and Davis in Nitella.
They found that the total concentration of soluble ions K+, Ca++ and phosphate ions in Nitella sap was greater than the concentration of these ions in the surrounding medium. Still, these materials accumulate in the cell.
In plants such as Laminaria, Fucus, etc, the concentration of iodine is greater. Yet iodine passes into and accumulates inside the cell. Both anions and cations can accumulate in plants by the mechanism of salt accumulation. This is done against the concentration gradient at the expense of metabolic energy, supplied by respiration.
If young excised barley roots are immersed in a dilute solution of certain salts, accumulation of salts occurs readily, if air is bubbled through the system. But little or no accumulation occurs if nitrogen is bubbled through the solution.
In the absence of oxygen, aerobic respiration is checked and the accumulation of salts is seized. In addition to aeration, conditions like a sufficient supply of respiratory substrate, a favourable temperature, etc should prevail.
Any condition which markedly reduces the rate of photosynthesis in the leaf or the rate of downward translocation may therefore bring about a decrease in salt accumulation. The absorption of salts and their accumulation is confined largely to growing and dividing cells where the rate of respiration is relatively high.
