Passive water absorption

Passive water absorption

This theory explains that the forces responsible for absorption of water into the roots are governed by other cells.  In actively transpiring plants absorption of water takes place due to the forces developed at the transpiring surface of the plant. In passive absorption water is just pulled through the roots and not by the roots.

Mechanism of passive water absorption

Water is continuously lost by the plants through the process called transpiration.  Loss of water from the transpiring surface results in increased diffusion pressure deficit in the mesophyll cells.  As a result of which water from the xylem elements diffuse into mesophyll cells.

The xylem elements inside the plants form a continuous column which remains filled with water.  Therefore, diffusion of water from the xylem elements at the top creates a tension on the water column present in the entire series of xylem vessels.  This tension along with diffusion pressure deficit pulls the water.

In this process, the force created at the upper part of plant plays important role and the root plays passive role.  The process does not depend upon supply of any energy, and therefore, the term passive absorption is used.

The rate of passive absorption is dependent on the rate of transpiration by leaves rather than any activities of root.  Passive absorption accounts for 98% of total water uptake.  This is the most rapid method of water absorption.

This theory involves apoplastic movement of water, i.e., water moves through intercellular space and cell wall in the cortex and simplest movement through endodermis and pericycle.

Passive water absorption

Fig: Passive water absorption

Pathway of water across the root cells

Water is first absorbed by root hair cells.  As a result the root hair cells become turgid.  The turgor pressure of root hair increases, which decreases the diffusion pressure deficit.  The adjacent cells of cortex have greater diffusion pressure deficit and less turgor pressure as compared to root hair cells.  As a result, water moves from root hairs to adjacent cortical cells.

Once the water enters into cortical cells, it decreases the diffusion pressure deficit of cell.  The diffusion pressure deficit of next cortical cell is comparatively greater.  This results in movement of water from cell to cell in the cortex.

Water finally reaches up to the endodermis.  The endodermis cells are characterised to possess Casparian thickenings in their radial and transverse walls.  They are, therefore, impermeable to water.

However, there are certain cells in endodermis which are devoid of thickening, called passage cells.  These passage cells are usually situated opposite to protoxylem elements.  These passage cells allow movement of water which finally enters into pericycle cells.  The cells of pericycle now become turgid and their diffusion pressure deficit is decreased.

Next to pericycle are situated  the xylem elements. The xylem elements lack turgor pressure and they always take water from adjacent pericycle.  Water which enters into xylem pericycle moves upward along the ascent of sap.

Factors affecting the rate of water absorption

Most of the factors which affect the rate of water absorption are soil factors, although atmospheric condition also affects the process.  Some of the important factors are as follows:

Available soil water:

Capillary water is the most effective soil water which is readily available to plants.  It is filled in the spaces between non colloidal soil particles.  Water is absorbed uniformly between the field capacity and permanent wilting percentage.  If water in soil is present below permanent wilting percentage or beyond field capacity, then the water absorption is affected.

Temperature of soil:

The absorption of water is maximum generally between 20-30 centigrade of soil temperature.  The low temperature of soil that is less than 20 degree centigrade reduces the rate of absorption of water.

At low temperature, the soil solution is more viscous, protoplasm is less permeable.  As a result the growth of root is inhibited.  These factors inhibit the absorption of water.  But increase in temperature above 30 degree centigrade causes decrease in the rate of absorption.

Concentration of soil solution:

If the soil water is in form of concentrated solution (due to dissolved salt and mineral) its osmotic pressure will be greater than that of cell sap.  The water will tend to move out due to exosmosis rather than entering the root hair cell.  Thus, the concentration of soil inhibits the absorption of water.

Aeration of soil:

Poorly aerated soil, particularly when it is deficient in oxygen retards absorption of water.  This is brought about by retarding growth and development of fresh root and also disturbing their metabolic activities.

Accumulation of carbon dioxide also inhibits absorption of water.  Increase in the amount of carbon dioxide in soil air causes increase in the viscosity of protoplasm and reduces the membrane permeability. As a result the rate of absorption slows down .

In water-logged soil, all the pore spaces of soil get filled with water and air is driven out, hence the water absorption is slow.

Rate of transpiration: 

Transpiration pull causes passive absorption of water .  Hence the rate of transpiration has got direct relationship with the rate of absorption.  With the increase in the rate of transpiration the rate of absorption increases.  During day time transpiration rate is more than that of the rate of water absorption.  At night the reverse takes place.  At night absorption continues due to active absorption.


Differences between active and passive water absorption


Active water absorption Passive water absorption
1.  It cannot take place in the absence of roots. 1.  Passive water absorption can occur even in the absence of roots.
2.  Root cells play an active role in this type of water absorption. 2.  Root cells have no active role in passive water absorption.
3.  Living root cells either pump water into xylem or deposit solutes in the xylem. 3.  A tension is created in xylem due to transpiration.
4.  Force of active water absorption lies in the roots. 4.  Force of passive water absorption develops in shoot.
5.  Active water absorption takes place in some plants only . 5.  Passive water absorption occurs in all the plants.
6.  Active water absorption requires metabolic energy. 6.  Passive water absorption requires radiation energy.
7.  Transpiration does not influence active water absorption. 7.  It is dependent upon transpiration.
8.  Active water absorption immediately influenced by metabolic poisons. 8.  Metabolic poisons do not have any immediate effect on passive water absorption.
9.  Root pressure causes active water absorption. 9.  Transpiration pull causes passive water absorption.
10.  It creates a positive pressure in the xylem channels. 10.  Passive water absorption produces a negative pressure in the xylem channels