What is Ascent of Sap?
Starting from the roots to aerial parts of the plant the upward transport of water is called ascent of sap. In the plants the process occurs through xylem tissue. The water absorbed by hairs passes through the cortex, passage cells and pericycle enter the tracheary elements of xylem. This takes place either actively through DPD gradient or passively through transpiration pull. Water is lifted up to leaves as a continuous column in the tracheary elements of xylem by virtue one or more forces like root pressure, cohesion and adhesion of water and transpiration pull acting concurrently. This continuous and unbroken transportation of water along with dissolved inorganic mineral salts is called the ascent of sap.
Pathway of water
By immersing the cut end of a branch in a water solution of a dye such as eosin or basic fuchsine it is very easy to demonstrate the upward conduction of water through xylem. If a section of the branch is cut after some time, the xylem elements appear to be stained. This proves that the patch of water is through xylem. Ringing of a plant, i.e. removal of a ring of all the tissue outside the xylem does not result in flaccid thereby further proving that water is conducted through xylem. Cutting through the xylem tissue of the stem results in almost immediate wilting of leaves attached to the stem above the cut. It can also easily proved by Isotopic technique also confirmed the ascent of sap occurs through xylem cells.
Ascent of Sap Mechanism
The opinions are piercingly divided as to what brings about upward movement of water in plants. A number of theories have been proposed to explain the mechanism of ascent of sap from time to time. We are going to discuss briefly some of the important theories as follow:
Theory of Vital Force
This theory suggests that the metabolic activity of living cells is very much responsible for ascent of sap. A number of vital force theories have been proposed by different scientist. In the year 1883-84 Westermaier had the opinion that the force for upward conduction of water is provided by xylem parenchyma cells, and that tracheids and vessels simply act as water reservoirs.
In the year 1923 J.C. Bose was also an ardent supporter of vital theories and his views were on experimental evidence. He devised a special apparatus for this purpose, consisting of an electric circuit, an electric probe and a galvanometer. With the help of this apparatus he demonstrated that the innermost layer of cortex is in a state of active pulsation consisting of alternate contraction and expansion. Since no such pulls sensation activity could be seen in the wood, he concluded that xylem acts only as a reservoir and that it is the pulls sensation activity of the living cells which is responsible for ascent of sap. But, the vital force theories received severe criticism, as it could not be establish definitely that pulsations are responsible for upward movement of water. Now it has been established that ascent of sap is independent of living cells and could take place even after the cells are killed.
Root Pressure Theory
The Root Pressure Theory to define the Ascent of Sap suggests that the sap in the xylem is forced upwards under hydrostatic pressure i.e., root pressure developed in roots. The theory seems to be applicable to plants like grape vines which generate considerable amount of root pressure. But in many plants, root pressure has not been observed at all and in still others, it is so low that it cannot account for upward translocation even up to a short distance. So, the root pressure failed properly define the ascent of sap.
Theory of Physical Force
According to the Physical Force theories, the ascent of sap for most part is purely d to physical forces. The living cells in the stem play only a minor role in the process. Here are some physical force theories stated below:
Capillary force theory
Christian Wolf proposed this theory in 1873 and he suggests that water rises up in the narrow tubes of xylem vessels by surface tension. The ascent of sap in tall trees, however, is not possible by capillary force.
Atmospheric pressure theory
According to this theory, atmospheric pressure is responsible for as<
of sap. This theory, however, is not applicable to tall trees as due to atmospheric pressure water can rise only up to 10.4 meters and not more. So this theory is not quite acceptable.
Transpiration pull-cohesive force theory
Ascent of sap and Transpiration pull
This is the most widely accepted theory proposed by Henry H. Dixon and John H. Jolly in 1895. It is based upon the following considerations:
i) There is a continuous water column right from the vein endings of leaves down to root hairs.
ii) There is great mutual attraction among the water molecules which is called cohesive force.
iii) There is a great adhesive force between the water column and the walls of xylem vessels.
iv) The energy for the upward movement of water column is provided by transpiration from leaf cells. This creates a more negative water potential in the leaf cells, causing water to move from xylem ducts to these cells.
According to this theory, the ascent of sap takes place in a well co-ordinated manner. As a result of transpiration, a water saturation deficit results in the walls of leaf mesophyll cells. This water deficit is carried from cell to cell until it reaches the leaf xylem, from there to the stem xylem and finally to root hairs through the root xylem. This stress or tension in the water column is called transpiration pull. The transpiration pull-cohesive force theory provides the most satisfactory explanation of ascent of sap. It can explain lifting of water even to the tops of the tallest trees. So this theory is most acceptable.