Mechanism of Water Absorption

Mechanism of Water Absorption

In plants, water is absorbed through root hairs, which are in contact with soil water. The wall of the root hairs are permeable and consists of pectic and cellulose substances which are strongly hydrophilic (water loving) in nature. There are two types of absorption viz.,

(a) Active absorption, and

(b) Passive absorption.

(a) Active absorption – Here the process of osmosis plays an important role. The soil plant water movement can be effected due to forces of imbibition, diffusion and osmosis.

Significance of Osmosis

• Large quantities of water are absorbed by roots from soil by osmosis.

• Cell to cell movement of water and other substances takes place through this process.

• Opening and closing of stomata depends upon the turgor pressure of guard cells.

• Due to osmosis the turgidity is maintained and give a shape to the plants.

(b) Passive absorption – It is mainly due to transpiration and the root cells do not play active role. Passive absorption takes place when rate of transpiration is very high. Rapid evaporation from the leaves during transpiration creates a tension in water in the xylem of the leaves. These tension is transmitted to the water in xylem of roots through the xylem of stem.

Due to this, water rises upward to reach the transpiring surface. As a result, soil water enters into the cortical cells through the root hairs to reach xylem of the roots to maintain the supply of water. The force for this entry of water is created in leaves due to rapid transpiration and hence the root cells remain passive during this process. It is otherwise known as transpiration pull.

Factors Affecting Absorption of Water

(i) Available soil water – Capillary water is available to plants. Hygroscopic water and gravitational water are not available to plants. The capillary water is absorbed by the plants, which in turn reduces the soil water potential. Hence, the water from higher potential area tends to move to lower potential area and root will absorb this water. This is the chain of process involved in water uptake.

(ii) Concentration of soil solutions – High concentration affects the process of osmosis.

(iii) Soil air – Sufficient amount of O2 should be there and excess amount of CO2 affects the availability of water by root suffocation.

(iv) Soil temperature – Up to 30°C favours absorption. Very low and very high temperature affects absorption.

(v) Soil texture

Clay – Neither good nor bad

Sand – Not good for absorption

Loamy – Good for absorption