Ascent Of Sap

Ascent Of Sap and Mechanism of Ascent Of Sap

Water is mainly absorbed by roots, which  goes upwards so as to replace water loss in transpiration  and to be used in photosynthesis. This upward movement of water from roots to  leaves through stem against force of gravity is called ascent of sap.

The exact mechanism of ascent of sap is not clear but one thing is very clear that path of ascent of sap is xylem (lumen of xylem and not the walls).

Evidences

• When a cut shoot of balsam plant is immersed in solution of eosine or basic füchsine, its flowers turn red and when T.S, of stem is cut, only xylem is stained red.
• Ringing or Girdling experiment : Ringing experiment was first of all performed by Hartig (1837). In first set of experiment, when ring girdle of bark is removed water continue to rise up through central xylem. In second set of experiment when central portion (xylem) of dicot stem is removed leaves in upper part wilt as water is unable to rise up.

Mechanisms Of Ascent Of Sap

Upward movement sap (from the top of the plant) is known as ascent of sap.

Different theories have been given from time to time and all of these are grouped into three heads

• Root pressure theory
• Vital force theories
• Physical force theories.

Root Pressure Theory (Priestley) :

It has been defined as a pressure developed in the xylem vessels as a result of the metabolic activity of roots.

Root pressure is developed when rate of water absorption is more than the rate of transpiration and as a result of which water is pushed up in trachery elements of root. Some workers  explained ascent of sap due to root pressure. This theory has following drawbacks.

• Magnitude of root pressure is upto 2 atmosphere whereas about 20 atmosphere pressure is required to raise water to top of tall trees.
• Root pressure is absent in most of the conifers.
• Root pressure is absent in rapidly transpiring plants.

Further it was reported by Strasburger that even in absence of roots, water rises up.

Vital Force Theories :

The advocates of vital force theory say that ascent of sap is due to activities of living cells.

Main supporters of vital force theory are Westermaier (1983) Godlewski (1883, 84),  Sir J.C.Bose (1923).

Westermaier considered that only cells of xylem parenchyma provide the force for ascent of sap.

Godlewski (1884) gave ‘relay force theory’. According to him, vital activities of cells of xylem parenchyma and medullary rays are responsible for ascent of sap.

Sir J.C. Bose (1923) gave pulsatory-force theory, according to which innermost layer of cortex (just outside endodermis ) is responsible for ascent of  sap.

Physical Force Theories

Supporters of these theories are of opinion that forces responsible for ascent of sap are purely physical in nature and even dead cells can raise water.

Main physical force theories are :

• Imbibitional Force Theory : First given by Unger (1868) and later supported by Sachs (1878). According to this theory, ascent of sap is due to imbibition by thick-walls of xylem vessels.
• Capillary Force Theory : Given by Boehm According to this theory, xylem vessels act as minute capillaries and water rises in these capillaries due to capillary action.
• Transpiration Pull Theory or Cohesion Tension Theory : Given by Dixon and Jolly (1894).

It is  the most accepted theory of ascent of sap.

Main Points of this Theory are :

• Water forms a continuous column from root to leaf through xylem ducts.
• Loss of water from mesophyll cells of leaves due to transpiration decreases their T.P. and hence increases their D.P.D (as D.P.D. = O.P. – T.P.). As a result of increased S.P., they absorb or suck water from adjacent xylem vessels of leaves and thus xylem sap is under pull or tension.
• This pull or tension is transmitted downward and is relieved when water is absorbed through roots.
• The column of xylem resists breaking due to forces of cohesion  (between water-water) and adhesion (between wall and water). Thus water is pulled from roots as a result of transpiration pull.