Absorption of water

Absorption of water

1) Water is absorbed by root hairs. Root hairs are more developed in xerophytes

2) First step in absorption of water is imbibition

* Root cap is formed by calyptreogen 

* Multiple root cap is found in pandanus

(Screw pine)

* Maximum absorption of inorganic salts is through the zone of cell division

Renner first time recognised and classified the water uptake mechanism into ' Active absorption' and 'passive absorption '

Active Absorption:

 1)Root hairs play an active role

2) Water enters into root hair by osmosis

3) Process occurs against the concentration gradient and

4) Metabolic energy is spent

There are two major theories to explain active Absorption:-

a) Osmotic Theory:

1) Atkins (1916) and Priestley were amongst the first to postulate an osmotic theory of active absorption

2) The value of osmotic pressure of the cell sap of the root hairs is generally 2.0 atm. 

3) Higher DPD of the cell sap of the root hairs causes endoosmosis of soil water

4) But a problem is how a sufficient conc. of solutes is maintained in the root xylem to maintain a higher DPD in the xylem sap

b) Non - Osmotic Theory:

1) Water is absorbed against a concentration gradient

2) Absorption requires an expenditure of energy released from respiration

3) Renner's active theory is called 'Osmotic active' .

Passive Absorption:

1) Process occurs along the concentration gradient

2) And hence energy is not involved

3) Absorption is controlled by transpiration

T/A Experiment:

T/A means Transpiration/ Absorption ratio let 

• Factors affecting the rate of water absorption:

1) Available Soil water

2) Conc. of the soil solution

3) Soil aeration

Ascent of Sap:

Ascent of Sap is the translocation of water and Inorganic solutes

Ascent of Sap means:

a) Movement of water and Inorganic solutes

b) from root to the leaves 

c) through xylem vessels

d) against the force of gravity

e) and water column remains in a state of tension

* Ringing and Girdling experiment by Malpighi confirmed the ascent of sap through xylem

* Mass Flow or pressure flow theory for the movement of food was given by Munch .

Root pressure Theory:

Root pressure Theory was coined by Stephen Hales( Father of plant physiology)

The hydrostatic pressure developed due to the accumulation of water absorbed by the roots is called root pressure.

Root pressure is measured by manometer

• Cohesion and Adhesion theory or 

Transpiration Pull theory :

This theory was given by Dixon & Jolly 

This theory is most accepted theory .

There are three features of this theory:

a) Strong cohesive force or tensile strength of water

b) Continuity of water column in the plant

Loss of water:

The loss of water from the living tissues of plants takes place either in vapour form (i.e. Transportation) or in a liquid form ( i .e. Guttation, Bleeding)

Transpiration:

 The loss of water in the form of vapour from the living aerial parts of the plant is known as Transpiration.

The principal organ of Transpiration is leaf.

Stomata:

Stomata are specialised epidermal cells which are distributed all over leaf surface 

1) Each stomata has two kidney (or bean ) shaped guard cells

2) Inner wall of guard cell is thick and outer wall is thin 

3) Guard cells are surrounded by epidermal or subsidiary or accessory cells.

Agricos

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