Ascent of Sap: Do you like to drink coconut water? Have you ever thought about how the water is transported to trees that are several metres high? This upward movement of water along with dissolved minerals from roots to the aerial parts of the plant is called the Ascent of Sap, which takes place through the xylem vessels. The ascent of sap is the flow of water and minerals from the roots to all other parts of the plant. The xylem vessels of plants are in charge of transporting water and minerals throughout the plant.

Water transfer is, however, a pressure-driven process. Water and minerals travel from a high concentration point (the root) to a low concentration point (the surface) (xylem vessels). Root pressure rises as a result of this movement in the xylem. Root pressure is a positive force that propels water and ions through the plant.

Ascent of Sap Definition

Ascent of Sap is defined as the translocation of water, usually upwards from the roots towards the top of the plant through the stem, i.e., to the leaves and growing points or apical meristems and other aerial plant parts.

Theories of Ascent of Sap

Many attempts were made by various biologists to explain the mechanism of the ascent of sap. Few theories were put forward, which are as follows:

1. Vital Force Theory
2. Root-pressure Theory
3. Physical Force Theory

1. Vital Force Theory

Many biologists thought that the ascent of sap was brought about by the living cells of the stem, although there was no direct evidence. The various vital theories given is as follows:

a) Relay-Pump Hypothesis

1. It was proposed by Godlewski \((1884).\)
2. According to this theory, the upward movement of water is due to the pumping activity of the living cells of the xylem parenchyma and medullary rays.
3. It is brought about by periodic changes in their osmotic pressure or water potential.
4. With a decrease in the water potential of these living cells, water diffuses into them from the adjacent tracheids and vessels that act as the reservoir of water.
5. Due to the increase in the water potential of the living cells, the water absorbed is now pumped into the vessel elements situated above, and the process is repeated again and again.
6. But this theory was soon discarded by Strasburger when he showed that water translocation continued in plants even after killing the living cells by high temperature or poison treatments.

Fig: Godlewski’s Relay-Pump Hypothesis

b) Pulsatory Movement Theory

1. This theory was proposed by Sir J.C. Bose \((1923).\)
2. This theory is also known as pulsation theory.
3. According to this theory, the upward conduction of water is provided by xylem parenchyma cells.
4. The innermost living cortical cells of the root were in a state of pulsatory motion, i.e., alternate expansion and contraction, which helped in the absorption of water from the soil and pump it into the xylem vessels.
5. The tracheids and vessels act as the reservoir of water.
6. It was not supported by enough evidence as living cells do not seem to be involved in the ascent of sap. So, this theory was discarded.

Fig: Experiment by J.C. Bose on Pulsatory Movement Theory

2. Root-Pressure Theory

1. This theory was put forward by Priestley \((1916).\)
2. All plants absorb excess water by an active process and tend to build up a positive hydrostatic pressure within the root system called root pressure.
3. Due to this activity, the water is pushed upwards along the length of the stem to a small height.
4. The pressure inside the xylem is caused due to the diffusion gradient and is maintained by the activity of living cells.
5. Root pressure thus provides a great contribution to the transport of water.

 

Fig: Experiment to demonstrate Root Pressure in plants

Limitations

1. It cannot lift sufficient amounts of water upward to fulfil the requirement of water.
2. It fails to play a role in the movement of water in tall trees like gymnosperms.
3. The amount of fluid transported by the root pressure is not enough in measuring the movement of water in the xylem in many trees.
4. Root pressure seems to be absent in summer when the water requirement is high.
5. It is generally observed at night when evapotranspiration is low.

3. Physical Force Theory

As per this theory, the ascent of sap takes place through the dead xylem vessels. The mechanism of the ascent of sap is entirely physical in which living cells are not involved. The following theories are included under the physical force theory:

a) Capillary Force Theory

1. This theory was proposed by Boehm \((1809),\) who compared xylem vessels to the capillary tubes.
2. He believed that the normal atmospheric pressure and the capillarity of the xylem vessels are responsible for the ascent of sap.
3. Water rises up in the walls of xylem channels having small diameters due to adhesion.
4. The cohesive force that is present among the water molecules pulls the water upwards through the xylem vessels.
5. The upward movement of the water continues till the forces of adhesion and cohesion are balanced by the downward pull of gravity.

 Fig: Capillary Force Theory

Limitations

1. The value of capillarity is very small.
2. It can raise water up to a height of about 1metre or so, which is useful to only small-sized plants.
3. Xylem vessels are not directly connected with the soil water.

b) Imbibition Theory

1. This theory was proposed by Sachs \(\left( {1878} \right).\)
2. According to this theory, water is imbibed through the cell wall materials and translocated upwards.
3. But this theory was discarded since water is translocated through the lumen of the vessels.

c) Atmospheric Pressure Theory

1. According to this theory, atmospheric pressure is responsible for the ascent of sap.
2. Due to active transpiration, a vacuum is created in the xylem vessels, and to fill this vacuum, and water moves upward from below so as to equalise the pressure.
3. But atmospheric pressure can lift the water up to a height of \(10\) meters, and the operation of atmospheric pressure depends on the surface at the base, which does not exist in the plants.
4.  

d) Transpiration Pull or Cohesion-Tension Theory

1. This theory was put forward by Dixon and Joly in \(1894\).
2. But it was further improved by Dixon in \(1914\), so this theory is also known as Dixon’s theory of the ascent of sap.
3. This is the most accepted theory for the study of the ascent of sap.
4. In the water column, the water molecules remain attracted by the cohesive force and cannot be separated easily from one another.
5. There is an attraction between water molecules and the inner wall of xylem ducts, due to which the water column cannot be pulled away from the walls of the xylem ducts.
6. Hence, due to strong cohesive and adhesive forces, the continuity of the water column from roots to leaves is maintained.
7. Water lost from the mesophyll cells of the leaves creates a strong negative water potential that leads to a negative pressure in the water column. This pressure exerts an upward pull over the water column, which is known as transpiration pull.
8. This tension or pull is transmitted up to the roots in search of more water.
9. The water column (formed in the xylem elements of roots) now moves upwards under the influence of transpiration pull.
10. Thus, the cohesive, adhesive forces and transpiration pull all together help in lifting up water through xylem elements.
11. Because of the critical role of cohesion, the transpiration pull is also called the cohesion-tension transpiration pull model of water transport.
12. The water inside the xylem vessels forms a continuous column from top to bottom.
13. The tension may cause a break in the water column but due to the adhesive and cohesive property of water, the continuous column of water does not break.

Fig: Model of Transpiration Pull Theory

Fig: Transpiration Pull or Cohesion-Tension Theory

Importance of Ascent of Sap

The ascent of sap serves the following functions:
1. It helps in the upward movement of the water through the xylem vessels in the trees that are several meters high.
2. It helps in the development of a suction force called transpiration pull that helps the root to absorb more and more water from the soil.

Summary of Ascent of Sap Theory

Plants themselves have a continuous water column in their xylem vessel that starts at the base (i.e., roots) and continues up to leaves from where water is lost through the process of transpiration. Thus, despite the absence of a circulatory system in plants, the flow of water upward through the xylem in the plants is maintained by three forces, i.e., cohesive and adhesive forces, and transpiration pull.

These three forces help in the upward movement of water molecules, and this process is called the ascent of sap.

Frequently Asked Questions (FAQs) on Ascent of Sap

Q.1. What is the ascent of sap?
Ans: Ascent of Sap is defined as the translocation of water, usually upwards from the roots towards the top of the plant through the stem, i.e., to the leaves and growing points or apical meristems and other serial plant parts.

Q.2. What causes the ascent of sap?
Ans: Ascent of sap is caused due to the three kinds of forces. They are cohesive forces between the water molecules, the adhesive force between the water molecule and the walls of xylem vessels, and the transpiration pull.

Q.3. Which theory best explains the ascent of sap?
Ans: The transpiration pull or cohesion-tension theory best explains the ascent of sap.

Q.4. What are the functions of the ascent of sap?
Ans: The ascent of sap serves the following functions:
(a) It helps in the upward movement of the water through the xylem vessels up to tall heights.
(b) It helps in the development of a suction force called transpiration pull that helps the root to absorb more and more water from the soil.

Q.5. Who discovered the vital force theory of the ascent of sap?
Ans: Sir J.C. Bose discovered the ascent of sap in \(1923\).