Mechanism of Water Absorption

American physiologists, A. S. Crafts and T. C. Broyer proposed a mechanism of absorption of water by roots based on the apoplast – symplast concept. They suggested that soil solution, containing ions of salts dissolved in water, diffuses into the cortical apoplast. The ions are absorbed into the symplast by active absorption. They are then actively transported through the symplast from one cell to the other via plasmodesmata into the stele, where they are released into the stelar apoplast. The concentration of ions in the stelar apoplast becomes higher than that in the cortical apoplast. This sets up an osmotic gradient. Since water potential is higher in the cortex than in the stele, water diffuses across the endodermis into the stele. As a result of flow of water into the stele the hydrostatic pressure in the stele increases. This is the main cause of root pressure.

The active absorption of ions by the symplast of the cortex form the cortical apoplast takes place by using energy released in respiration of the cortical symplast. The energy required for transport of ions form the cortical region into the stele is also provided by cortical symplast. The symplast of the stele then releases ions into the apoplast of the stele, because the energy required by stelar symplast to retain the ions against potential gradient is not sufficient.

From the above discussion it becomes clear that there is more supply of energy in the cortical symplast than in the stelar symplast. It is on account of the fact that cortex is nearer to the soil than the stele, so that it receives more supply of oxygen required for respiration which releases energy.

Antagonism

Presence of one ion in a solution reduces the uptake of another ion by the cell. This phenomenon is known as antagonism. A plant tissue placed in a dilute solution of potassium chloride will rapidly accumulate potassium ions, which may reach a high level, that is toxic to the cells. If, however, trace amounts of calcium are present in the solution, the absorption of potassium is much reduced and its poisonous effects are avoided. Calcium is thus said to antagonize the uptake of potassium. It also antagonizes sodium. Similarly antagonizes sodium or potassium will antagonize calcium absorption. Potassium or calcium also antagonize magnesium uptake. It appears from these facts that ions which antagonize one another effectively must be unrelated (not in the same group in the periodic table). Sodium will not antagonize potassium uptake, because both are included in the same group. Like wise barium will not antagonize calcium. Sodium or potassium will antagonize barium or calcium. Antagonizing ions are required in very small quantities to show their effects.

How antagonism occurs is not yet known. One possible explanation is that antagonizing ions compete for the same carrier system on the cell membrane, hence the presence of one ion hinders the uptake of the other. It has also been suggested that antagonizing ions may change the colloidal structure of absorbing surface, thus reducing the uptake of other ions, but for such a change in permeability of the absorbing surface large quantities of ions are required which are not needed in antagonism.

Antagonism has advantages and disadvantages for the field plants. For instance, many soils have surplus ions of potassium or calcium, which may produce toxic effects, had there been no antagonism. Excess of certain ions in the soil solutions may prevent the uptake of other ions that are essential for the plants and hence may produce deficiency symptoms in the plants, although the required ions are present in sufficient quantities in the soil. Excess of sodium ions in the soil, for example, may produce calcium deficiency by antagonizing the uptake of calcium, although calcium may be present in sufficient amounts in the soil.

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