Mechanism of Water Absorption in Plants

There are two types of water absorption in plants:-

i) Passive Water Absorption
ii) Active Water Absorption

i) Passive Water Absorption:- This type of water absorption requires very less amount of energy and  takes place because of loss of water from the exposed surface of the plant mainly due to transpiration as a result this creates a tension of low water potential in the xylem cells. Under the influence of this tension water will start getting absorbed from the soil. Water is absorbed due to tension has been proved by various evidences like
a) The absorption of water is almost equal to the rate of transpiration. The pressure of water in the root hairs is -3 to -8 bars and in the soil it is -.1 to -.3 bars. As a result the water of the soil passes into the root hairs.
b) The can be also absorbed by the shoot even in the absence of roots.
c) In tracheary elements water continues to go up due to the tension created by transpiration pull.


ii) Active Water Absorption:- In this type of absorption energy is utilized by the plant and this energy comes from ATP not from the sunlight. Active metabolically living cells are required. There are following possibilities for the active absorption:-
a) Accumulation of the salts and sugars in the trecheary elements of the xylem. Hence DPD of these cells become high results in the absorption of water from the adjacent cells.
b) The cells adjoining the trecheary elements have pumping activities and force the water into trecheary elements.