In this article, we will discuss about the role of transpiration pull in ascent of SAP.
Among the various theories put forward to explain the mechanism of ascent of sap, the theory by Dixon and Jolly based on transpiration pull is most satisfactory and most widely accepted. It is popularly known as theory of cohesive force.
Theory of Cohesive Force:
Of all the theories given for the ascent of sap (vital and physical force theories), this theory is most widely accepted & advanced because:
(i) It gives most satisfactory explanation of the process as per the need of the plant.
(ii) It is based on known facts.
(iii) It explains the mechanism on the basis of such physical forces, which actually exist in plants. Therefore, the theory has a sound base.
It is explained on the basis of following facts and physical forces:
(a) Water is conducted through the lumen of xylem vessels and tracheid’s,
(b) It is based on known facts.
(c) It explains the mechanism on the basis of such physical forces, which actually exist in plants.
Physical Forces:
(i) Force of Cohesion:
Molecules of water in the xylem are held together because of great cohesive force (up to 350 atmosphere) of attraction between them. This helps to maintain continuity of water column.
(ii) Force of Adhesion:
There is attraction (adhesion) between the water column and walls of xylem. This prevents accumulation of air blocks in xylem and also prevents break in water column.
(iii) Suction Force (Transpiration Pull):
The water deficit or suction force developed in leaf cells due to transpiration is responsible for pulling the water column in the upward direction (ascent of sap) . Hence, it is called transpiration pull. The magnitude of suction force is sufficient to lift the water column to any heights, even in tallest trees.
However, it is not enough to break the continuity of water column. Thus, greater the transpiration, greater is the transpiration pull (suction force) and hence higher the rate of ascent of sap. Similarly, when transpiration is low, the suction force (transpiration pull) is also low and hence the rate of ascent of sap is also lower.
Significance and Role of Transpiration Pull:
(i) It helps in ascent of sap.
(ii) It is responsible for absorption of major quantity of water by roots by way of passive mechanism without requiring energy in the form of ATP.
(iii) Transpiration pull initiates the large scale movement of water form soil into root xylem and then upwards to all aerial parts (ascent of sap).
Evidences in Support of Theory:
(i) The amount of water transpired is equal to the amount of water absorbed.
(ii) Upward movement of water continues even when cut shoots (from below) are placed in water.
(iii) Cohesive force that holds the water molecules in a column as narrow as xylem vessels is very strong and they withstand the opposing transpiration and gravitational pulls.
(iv) Because of the opposing forces, tension develops in the water column. As a result, xylem elements become slightly narrow. This results in the contraction and expansion of the stem which has been demonstrated by using dendrometer. The diurnal behavior of rhythmic contraction and expansion is a good evidence for the water column to be in tension which the transpiration is rapid or not.
(v) The forces that operate cohesion transpiration pull are just passive forces and no metabolic energy is involved in this phenomenon, because present the movement of water upwards.
Objections against Cohesion Theory:
(i) A serious objection generally raised against this theory is that the air dissolved in the sap shall form bubbles under tension and high temperature. The introduction of such air bubbles in xylem would break the continuity of water column and thereby stop ascent of sap due to transpiration pull. This objection was, however, dismissed by Scholander et. al. (1957) who proposed that the continuity of water column is maintained because of the presence of pits in the lateral walls of xylem vessels.
(ii) Nobody has yet devised a method of measuring the tension in the xylem of an intact plant. Until the existence of substantial tension has been confirmed by direct measurement, the cohesion theory would remain just a theory.