 |
| Theories for Ascent of Sap |
|
| The various theories to explain ascent of sap are: |
| |
 Vital theories |
|
| |
|
 Root pressure theory |
|
| |
|
 Physical theories |
|
| |
|
|
| According to vital theories upward movement of water takes place due to activity of living cells of plant bordering xylem. It is explained with the help of two theories: |
| |
 Godlewsk's relay pump theory |
| |
 Pulsation theory |
| |
|
| Stocking (1956) has defined root pressure as: a pressure developed in the tracheary elements of xylem as a result of metabolic activities of the root. |
| |
| The root pressure is said to be an active process which is confirmed by the following facts: |
| |
 Living cells are essential in root for the root pressure to develop. |
|
| |
 Oxygen supply and some metabolic inhibitors affect the root pressure without affecting the semi permeability of membrane systems. |
|
| |
 Minerals accumulated against the concentration gradient by active absorption utilising metabolically generated energy lowers the water potential of surrounding cells, leading to entry of water into the cells. |
|
| |
|
| Set up the experiment as shown in the figure. |
| |
 |
| |
 |
| |
| |
| Limitations |
| |
 Taller plants
require about 20 atm. pressure but root pressure has value upto 5 atm. |
|
|
| |
 Plants growing in cold, drought or less aerated soil, root pressure fails to appear. |
|
| |
|
| Transpiration pull and cohesion theory (cohesion tension theory) |
| |
| The theory was originally proposed by Dixon and Jolly (1894) and supported by Renner (1911 to 1915), Curtis and Clark (1951), Benner and Golston (1952) Kramer and Kozlowski (1960). The features on which the theory is based are: |
| |
 |
| |
| Water is lost from the surface of the mesophyll cells of the leaves. The water escapes as water vapour into the intercellular spaces and finally into the atmosphere through stomata. The sequence of events are as given below: |
| |
 Water vapours diffuse out from the intercellular space to the atmosphere. |
|
| |
 Evaporation of water vapours from thin walled mesophyll cells (leaf) into the intercellular space. |
|
| |
 Reduced water content in the cell |
|
| |
 Increased osmotic potential and low y in cell sap |
|
| |
 Movement of water from adjacent cell by osmosis |
|
| |
 Movement of water from xylem to adjacent cell |
|
| |
 |
| |
| Diagrammatic Representation of the Path of Water Through the Plant and the Various Physical Forces That are Concerned in the Movement of Water |
| |
| A kind of water potential gradient exists from the leaf to the root and creates a transpiration pull. The water molecules have a strong force of cohesion (a tendency to stick together). The force of cohesion is even more in narrow tubes such as xylem vessels. |
| |
| In addition, there is also a force of adhesion between the walls of xylem vessels and water. The ligno-cellulose walls not only provide strength but have a strong affinity for water molecules. Both the forces, adhesion and cohesion together, help to form thin, unbroken columns of water in the capillaries of xylem vessels. The transpiration pull lifts the water column up by creating a tension. It is almost similar to the liquid being taken up from a straw. |
| |
 |
| |
| Absorption of Water and Conduction Through the Xylem Vessels a, b, c, d, e Refer to the Sequence of Events During Transpiration |
| |
| The transpiration pull creates a negative pressure in the xylem vessel called tension which is transmitted down to the roots. Tension causes a decrease in the water potential and hence in the uptake of water by the roots. |
| |
| This phenomenon provides a possible explanation of how water is lifted to the tallest trees that occur on earth, though it is said that any break in the water column which may be due to air or water vapour should stop the flow. So far it is the most widely accepted theory. The water potential as low as - 30 bars has been recorded in the leaves of higher plants which is sufficient to overcome the resistance offered by the narrow capillaries of the xylem and the gravitational pull. |
| |
