Absorption of Water by Roots (With Diagram)!
Now for the actual absorption of water by the roots—it can be shown that when a root hair is in contact with a number of cortical cells of the root and finally a trachea or a xylem vessel, water will enter the root hair, pass from there into the cortical cells and finally into the xylem if there be a gradient of water potential from the root hairs to the xylem vessels.
It is the gradient of water potential from the root hair to the xylem vessels that is essential for the absorption of water by the roots.
If the root hair A has a high osmotic pressure and is exposed to the surrounding water in the soil and the cells B to J and the xylem vessel K have progressively lower pressures, water will pass from A→K by diffusion from the soil to the roots following the same osmotic relations we have discussed so far.
Thus A, the root hair will take up water from the surrounding medium as water tends to move by diffusion from regions of higher water potential to regions of lower water potential.
The first essential condition for the diffusion of water to take place from outside to inside root hairs is that the root hair cell sap must have higher salt concentration or in other words lower water potential than the water in the soil.
Thus A, the root hair will take up water from the soil and it will ultimately become turgid and as a result its water potential will fall below B. B will now take up water from A and will in turn become turgid and now its absorbing capacity will fall below B1; B1 will now draw water from B and so the process will continue until the xylem vessel K is reached.
The force with which water will be drawn from the soil will depend entirely upon the difference between the osmotic pressure external to A and the osmotic pressure of the xylem vessels. The greater the difference, the greater will be the force with which water is drawn into the vessels through the cortical cells.
Ordinarily the osmotic pressure of the cell sap of the xylem K can scarcely reach a value as low as the osmotic pressure of the surrounding soil solutions which (vessels) form a continuous pipe line from the roots to the leaves, for once the water from the soil reaches the main transpiration current in the xylem vessels, water is taken upwards to the leaves for utilisation and for ultimate escape of the excess water through stomata.
As a result there generally is a higher osmotic pressure in the sap of the xylem vessels than the water outside the root hair in the soil.
The osmotic pressure of the root hair cells generally varies from 3 to 5 atmospheres. Thus whenever water potential of such soil exceeds that of young root cells and root hairs, water will move from soil into the root and since the osmotic pressure of the soil solution in inert soils is only a fraction of an atmosphere, the absorbing capacity or suction need not be very great before water will enter them.
The absorption process which has been described above undoubtedly accounts for the intake of most of the water which the roots absorb, but it is certainly not the only mechanism of absorption known to operate in plants.
This mechanism of absorption of water is sometimes referred to as passive absorption because the entry of water into the roots is brought about probably by conditions which originate in the top of the plant and the root cells apparently play a passive or subsidiary role.
‘Originate in the top of plant’ means that the osmotic concentration of the root cells is due to the presence of soluble metabolic products which are synthesised in the aerial parts of the plants and translocated downwards to all the tissues including the root cells.
In many plants, an internal pressure known as root pressure often develops in the xylem. This can be demonstrated in some plants which are growing vigorously, particularly in spring.
If the root system of a suddenly decapitated plant is immersed in a potometer, absorption of water can be shown clearly by the movement of the bubble in the horizontal arm of the potometer.
Thus it is seen that water can be absorbed by root cells by forces which develop there. In this type of absorption, the mechanism involved is localised in the root system and is often called active absorption.
This active absorption apparently takes place in plants when transpiration rate is relatively low and the soil contains water in abundance. This active absorption of water can only be in very small amounts since water would tend to leak out so rapidly by diffusion that enormous amount of respiratory energy would be required to maintain the gradient.