Mechanism of ascent of sap

Mechanism of ascent of sap
    A number of theories have been put forward to explain the mechanism of ascent of sap as follows.
    1. Vital theories
    2. Root pressure theory
    3. Physical force theories

    1. Vital theories

    According these theories, the living cells of the plants are responsible for the ascent of sap. Vital theories include two theories:

    Relay pump theory (Godlewski, 1884)

    According to this theory, the living cells of xylem pump the water upwards and the xylem tracheids and vessels act as reservoirs.

    Pulsation theory (J.C. Bose, 1923

    According to this theory, ascent of sap takes place due to pulsatory activity of the living cells of inner most cortical layer, just outside the epidermis. Here, the cells absorb water from outside and pump the same to the vessels. The objection to this theory was put forward by Strasburger that the ascent of sap can take place even if the living cells are killed by treating with poisons like picric acid.

    2. Root Pressure theory (Priestley Stocking, 1956)
    According to this theory, the ascent of sap is due to a hydrostatic pressure developed in the roots by the accumulation of absorbed water. The pressure developing in the tracheary elements of the xylem as a result of the metabolic activities of root is referred as root pressure. If a plant system is cut a few inches above its base, the xylem sap is seen flowing out through the cut end. This phenomenon is called exudation or bleeding.

    Some scientists believe that root pressure is responsible for ascent of sap. But, it does not seem to be an effective force in the ascent of sap due to following reasons.

    1. Root pressure is not observed in plants grown in cold, drought or less aerated soil, where ascent of sap is normal.
    2. Strasburger observed ascent of sap in plants in which the roots are removed.
    3. The magnitude of root pressure is very low (about 2 atm.)
    4. In many tall plants, there is no root pressure. In gymnosperms, root pressure has been rarely observed.

    3.Physical Forces Theories

    According to this theory, the dead cells of the xylem are responsible for the ascent of sap. There are four theories to support this mechanism

    a. Atmospheric pressure theory (Boehm, 1800) The theory says that ascent of sap takes place due to atmospheric pressure. But, it is not accepted because the atmospheric pressure cannot raise water beyond 34 feet.
    b. Imbibition theory (Unger, 1868; Sachs, 1978) According to this theory, ascent of sap takes place by imbibition through the walls of xylem but, it was observed later that ascent of sap take place through the lumen of the xylem and not through walls.
    c. Capillary theory (Boehm, 1868) This theory states that water rises in narrow tubes due to surface tension. In plants, the xylem vessels are placed one above the other forming a sort of continuous channel. This can be compared with long capillary tubes and water rises in the capillary tube due to capillary force. However, the following objections are observed.

    1. For capillarity, a free surface is required.

    2. The magnitude of capillary force is very low.

    3. In gymnosperms, the vessels are usually absent. Other xylem elements do not form continuous channels

    4. Transpiration Pull theory/ Cohesion hypothesis/Cohesion- Tension theory (Dixon and Jolly, 1834)

    According to this theory, the ascent of sap is due to transpiration and the cohesion and adhesion of water. This theory is very convincing and supported by many scientists as it involves two important aspects.

    1. Cohesive and adhesive properties of water

    Animation showing “Cohesion – tension model of xylem transport” :


    • The attraction between water molecules is called cohesion.
    • The cohesive force developed between the water molecules make the water remain in the form of continuous water column in the xylem and the magnitude of cohesive force is very high (350 atm.)
    • The adhesive properties of the water molecules and the walls of the xylem further ensure the continuity of water column in the xylem.
    • The xylem vessels are tubular structures extending from roots to the top of the plants. Cells are place one above the other, with their end walls perforated forming a continuous tube.
    • One end of the xylem tube is connected with the root hairs via pericycle, endodermis and cortex and another end is connected with the sub stomatal cavity in the leaves via mesophyll cells. This tube is filled with water and due to the cohesion and adhesion properties, it forms continuous water column.


    2. Transpiration pull Animation on transpiration pull:

       
    • The pulling force developed in the water column of xylem due to transpiration is called transpiration pull.
    • When transpiration takes place in the upper part of the plant, water evaporates from the inter cellular spaces of the leaves to the outer atmosphere through the stomata.
    • More water is released into the intercellular spaces from the mesophyll cells and in turn, mesophyll cells draw water from the xylem of the leaf. Due to this, a tension is created in water in the xylem elements of the leaves and this tension is transmitted downward to water in the xylem of root through xylem of stem.
    • Now, the water is pulled upward in the form of a continuous unbroken water column to reach the transpiring surface up to the top of the plants.
    • This theory was also objected by Milburn and Johnson (1966) and other scientists that the presence of air bubbles in the conducting channels due to variation in the day and night temperature will break the continuity of the water column.

Last modified: Monday, 25 June 2012, 8:03 AM