The upward movement of sap (water with dissolved ingredients) from the root towards the top of the plant is known as ascent of sap. Roots are organs concerned with the absorption of water. If the plant is to survive and grow, this water absorbed by roots must reach the top of plants to replace the water lost in transpiration and to be used in photosynthesis as a raw material.

The upward movement of water occurs through the tracheary elements (tracheids and vessels) of xylem. Sap is lifted from near the root tip to the shoot tip against the force of gravity, sometimes to great heights. Some trees are much taller, often attaining a height upto 110-130 m, e.g., Sequoia sempervirens (height 110 m), Picea sitchensis (height 95-100 m), Pseudotsuga menziesii (height 100 m), Eucalyptus (height above 130 m), etc. The force required to move water to such heights are substantive. The rate of translocation is 25-75 cm/min (15-45 m/hr).

Several theories have been put forward to explain the mechanism of ascent of sap. The four main theories are: vital force theory, relay pump theory, root pressure theory, capillary force theory and transpiration pull theory.

Vital force theory
A common vital force theory about the ascent of sap was put forward by Sir J.C. Bose in 1923. It is called pulsation theory and it believes that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels.

Relay pump theory
Relay pump theory was put forward by Godlewski (1884). According to this, upward conduction of water is due to the pumping activity of xylem parenchyma cells and the cells of medullary rays.

Root pressure theory
Root pressure theory was put forward by Priestley (1916). Root pressure refers to positive hydrostatic pressure which sometimes develops in the xylem sap of roots as a result of metabolic activities of roots. It is a manifestation of active water absorption. Root pressure is observed in certain seasons which favour optimum metabolic activity and reduce transpiration.

Root pressure is maximum during rainy season (in tropical regions) and spring season (in temperate habitats). It is retarded or absent under conditions of starvation, low temperature, drought and reduced availability of oxygen. Root pressures are usually not more than + 1 to + 2 bars. Therefore, it could account for the ascent of sap only to a height of about 20 m.

Hence, this theory can account for the ascent of sap only in the herbaceous plants. The magnitude of pressure developed is too small to push the water to the apical regions, in the tall trees. Besides, root pressure is not found in all plants. No or little root pressure is found in gymnosperms which have some of the tallest trees in the world. In rapidly transpiring plants, no root pressure is found, instead a negative pressure is found under such conditions.

Capillary force theory
Capillary force theory was given by Boehm (1809). According to this theory, water rises up in narrow tubes of xylem vessels by surface tension or adhesion and cohesion. The upward movement of water will continue till the forces of adhesion and cohesion are balanced by the downward pull of gravity.

The value of capillarity is very small which can raise water to a height of about 1 metre in vessels of normal diameter (0.03 mm). Therefore, if operational, it will be useful to only small sized plants and cannot operate in plants having tracheids due to the presence of end walls.

Transpiration pull or cohesion-tension theory
The most widely accepted theory for ascent of sap is transpiration pull theory or cohesion-tension theory. This theory was proposed by Dixon and Jolly in 1894.

This theory is based on the following points:
– Transpirational pull exerted on the water column.
– Cohesive and adhesive properties of water molecules so as to form an unbroken continuous column of water in the xylem.

Transpiration pull
Water is lost from mesophyll cells to the intercellular spaces of leaves as a result of transpiration. The water vapours move out of the plant through stomata. As a result of loss of water from mesophyll cells, the diffusion pressure deficit (DPD) increases.

With the increase of diffusion pressure deficit, these cells absorb water from adjoining cells, ultimately the water is absorbed from xylem elements of vascular bundles of leaf.
Since the xylem elements are filled with continuous water column, a tension or pull called transpiration pull develops at the top of the column. This tension or pull is transmitted down from petiole to stem and finally to roots leading to upward movement of water.

Cohesion and adhesion of water in xylem
Xylem tracheids and trachea are long tubular structures filled with water, extending from root to leaf. Thus, one end of xylem (continuous with one another) is in the root and other end is in the leaf. Water molecules remain attached to one another by a strong mutual force of attraction called cohesive force, which is due to the presence of hydrogen bonds amongst adjacent water molecules.

Supplementing the cohesion between water molecules is adhesion between water molecules and the walls of tracheary elements of xylem. Thus, according to this theory, water ascends in the plant because of transpiration pull and this column of water remains continuous because of cohesive and adhesive forces of water molecules.

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