Water Absorption

Water Absorption and types of Water Absorption 

Deep in the soil and above impermeable stratum, water occurs freely in the previous rocky matter. It is called ground water. Upper layer of ground water is known as water table.

Water is present in the soil in five forms. The ultimate source of water is rain or irrigation. A part of rain water does not enter the soil but is drained away from soil surface along the slope. It is called run away water or run off water.

• Capillary water : This water is present in narrow spaces of soil particles. This water is held against force of gravity in the soil. Only this water is available to plants for absorption.
• Gravitational H₂O : Gravitation water occurs only after good rain or irrigation. Excess water is drained downwardly under the influence of gravity and joins ground water. This water is not available to the plants
•  Hygroscopic water : The water imbibed by soil particles is known as hygroscopic water. Non available to the plants.
• Combined H₂O : It is the water combined with chemicals and is therefore not available.
• Water vapours : They occur in soil air spaces (macropores).

The total amount of water present in the soil is called holard.

Available water is called chesard.

Unavailable water is called echard.

Field Capacity : The amount of  water retained by soil against forces of gravity is called field capacity. Water is absorbed from soil by root system and specially  younger parts (i.e., root tips).

In  water absorption, there is an interaction between plant root with soil and soil water. The root of a plant is distinguishable into five regions namely region of root cap, meristematic region, region of elongation, region of root hair and mature region. The absorption of water takes place from the subterminal portion of the root but the maximum absorption takes place through the region of the root hair because they provide greater exposed surface. This region extends from 1 to 10 cms, behind the root tip. In some plants like some conifers and Lilies, the root hair are poorly developed or absent. Internally a root is distinguishable into epidermis followed by thin walled cortex delimited by the endodermis. The endodermal cells possess a characteristic thickening, the casparian strip. However, there are unthickened cells lying opposite to protoxylem groups, called passage cells. The stele is delimited by a thin walled pericycle enclosing xylem and phloem elements of different radii.

Maximum absorption of water occurs by root hair zone.

Root hairs are more developed in herbs as compared to shrubs and trees.

Root hair is having permeable cell wall, with outer layer of pectic substances (for attachment of soil particles) and inner layer of cellulose. Inner to cell wall is selectively permeable cell membrane and layer of protoplasm with single nucleus near the tip. In the centre is vacuole with cell sap, having osmotic pressure 3—5 atmospheres (O.P. of outer soil water is generally less than 1 atmosphere).

Mechanism of water absorption 

There are two independent mechanisms of water absorption in plants:

• Active water absorption 
• Passive water absorption 

Active water absorption

Here water is absorbed by the activity of root itself or roots play active role or water is absorbed ‘by the roots’. It is common in plants with low transpiration rate.

(a)Osmotic theory of active water absorption : First step of water absorption is imbibition by cell wall of root hair as cell wall is permeable. Water as well as solutes enter through it and reach the surface of cell membrane. Now outer soil water is separated from cell sap of root hair by means of a selectively permeable membrane (cell membrane + cytoplasm).

Due to high O.P. and low T.P., the D.P.D. or S.P. of root hair is increased and thus water is absorbed by endosmosis; After absorption of water by root hair, its T.P. is increased and thus D.P.D. or S.P. is decreased. Water from root hair moves to inner cells of root along conc. gradient and finally reaches into the xylem. As xylem is continuous from roots to leaves, so this goes upwards for consumption.

(b) Non-osmotic theory of active water absorption : Generally it has been seen that absorption of water still occurs when conc. of outer soil water is more than root hair cell sap. Under such conditions, exosmosis should occur, but it has been seen that water is still absorbed against conc. gradient. Osmotic theory does not explain this but is explained by non-osmotic mechanism utilizing respiratory energy. Thus absorption of water against conc. gradient utilizing respiratory energy is non-osmotic theory of active water absorption. 

Passive water absorption

Here roots play passive role in water absorption, as forces responsible for absorption of water are not generated in roots, but in upper parts of the plant, i.e., ‘transpiration pull’.

It is observed in plants with high transpiration passive water absorption is responsible for 98% of total water absorption in plants

Fig.  Different pathways of water absorption

Factors affecting water absorption 

• Availability of water : Capillary water is the only water which can be absorbed by the roots of plant.
• Temperature : Optimum temperature for water absorption is 20°C - 35ºC.
• No water is absorbed in frozen soils. Rate of water absorption decreases below 20°C.
• Concentration of soil water : Water is generally absorbed when conc. of root hair cell sap is more than outer soil water. If conc. of soil water is high due to dissolved salts (saline soil) there are chances of exosmosis. Such soils are called physiologically dry soils
•  Aeration : In poorly aerated soil, rate of water absorption is low. In poorly aerated soil, lack of O2 and increase in CO2 occurs. Due to these factors viscosity of water is increased and permeability of membranes is decreased and there is poor root growth. All these factors together decrease water absorption.