ASCENT OF SAP

ASCENT OF SAP

2.     ASCENT OF SAP

Water is mainly absorbed by roots, which goes upwards to replace water loss due to in transpiration and to be used in photosynthesis. This upward movement of water from roots to leaves through stem is against the force of gravity. This movement is called as the ascent of sap. The exact mechanism of the ascent of sap is not clear but one thing is very clear that the path of the ascent of sap is through xylem (lumen of xylem and not the walls). The conducting cells in the xylem are typically non-living and include two types of tracheary elements – tracheids and vessel elements. Vessel elements are present only in angiosperm and the small group of gymnosperms. In addition to tracheary elements, the xylem tissue also contains parenchyma cells (storage function) and fibers (mechanical function).
2.1.    Evidence:-
When a shoot of the plant is immersed in the solution of eosine or basic fuchsine, its flowers turn red and when the transverse section of stem is cut, the only xylem is stained red.
2.2.    Ringing or curdling experiment:- the Ringing experiment was first of all performed by Hartig (1837). In the first set of experiment, when ring girdle of bark is removed, water continued to rise up through central xylem. In the second set of the experiment when central portion (xylem) of dicot stem is removed leaves in the upper part with as water is unable to rise up.
Theories:-
2.2.1.    Root pressure theory:- 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 the rate of water absorption is more than the rate of transpiration and as a result of which water is pushed up in tracheary elements of the root. Some workers explained the ascent of sap due to root pressure. This theory has the following drawbacks.
(i)    The magnitude of root pressure is up to two atmospheres whereas about 20 atmospheric pressure is required to raise water to the top of all tall trees.
(ii)    Root pressure is absent in most of the conifers.
(iii)    Root pressure is absent in rapidly transpiring plants.
Further, it was reported by Strasburger that even in the absence of root, water rises up.
2.2.2.    Vital Force theory:-
Main supporters of vital force theory are Wester Maier (1983) Berzelius (1883, 84), Sir Jagadish Chandra Bose (1923) Westermaier consider that only cells of xylem parenchyma provide the force for the ascent of sap. Godlewski gave 'relay pump theory'. According to him, vital activities of cells of xylem parenchyma and medullary rays are responsible for the ascent of sap. Sir J.C Bose gave 'pulsatory force theory' according to it to which innermost layer of the cortex is responsible for the ascent of sap.
2.2.3.    Physical force theories:-
(i)    Imbibition Force Theory:- First given by Vnger (1868) and later supported by Sachs. According to this theory, the ascent of sap is due to imbibition by thick walls of xylem vessels. 
(ii)    Capillary Force Theory:- This theory was given by Pachm. According to this theory, xylem vessels act as minute capillaries and water rises in these capillaries due to capillary action.
(iii)    Transpiration pulls theory or Cohesion-tension theory – This theory was given by Dixon and Jolly (1894).
It is the most accepted theory of ascent of sap.
(a)    Water forms a continuous column from root to leaf through xylem ducts.
(b)    Loss of water from mesophyll cells of leaves due to transpiration decreases their T.P. and hence increases their DPD (as DPD = OP – TP). As a result of increased DPD, they absorb water from adjacent xylem vessels of leaves and thus xylem sap is under pull or tension. This pull or tension is transmitted towards and is relieved when water is absorbed through roots.
(c)    The column of xylem resists breaking due to forces of cohesion (between water and water) and Adhesion (between wall and water). Thus water is pulled from roots as a result of transpiration pull.


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