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The following points highlight the three important theories of water translocation. The theories are: 1. Capillarity 2. Root Pressure 3. Cohesion Theory.
Water Translocation: Theory # 1. Capillarity:
Question arises, can the water be raised to the top of such trees by capillarity?
Such a rise of water in small tubes comes about because of the adhesion between water molecules and the wall of the tube in which the process is taking place. This adhesion forms a meniscus in which the water tends to climb up the side of the container.
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The smaller the diameter of the tube, the greater is the lifting force of the meniscus compared to the amount of water to be lifted. Hence the smaller the tube, the higher the column of capillary water will rise. Force of gravity also affects water uptake by capillarity.
According to this theory, water is first driven in due to force of adhesion between water and the walls of thin xylem vessels, i.e., tracheids. As the water flows upward along the wall, strong cohesive forces between water molecules begin to function and pull the water upward. This upward pull of water continues until the forces of adhesion and cohesion are balanced by downward force of gravity.
However, capillarity alone in the usual sense does not operate in plants. This seems to be an insufficient force to lift water in tall trees. Water is lifted less than one meter in glass or cellulose tubes the size of xylem elements.
Water Translocation: Theory # 2. Root Pressure:
Such pressures from below, produced by the roots have been observed in several species. Grapevines, for example, are capable of showing fairly high root pressure (about 5 or 6 bars), yet no one considers root pressure as a mechanism for the translocation of water in tall trees.
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If a stem of plant is cut transversely above the soil surface, xylem sap will exude in the form of a drop from the cut surface. This exhibits the presence of a positive pressure in the xylem. This pressure is called root pressure.
Although, root pressure which is developed in the xylem of the roots can raise water to a certain height but it does not seem to be an effective force in ascent of sap due to following reasons:
(i) Magnitude of root pressure is quite low (about 2 bars).
(ii) Even in the absence of root pressure, ascent of sap continues.
(iii) Root pressures are not observed in conifers.
(iv) Mineral ions from the soil are taken up by roots and get deposited in the xylem vessels. Accumulation of ions in vessels leads to the lowering of osmotic potential.
(v) Root pressures are usually negligible, in temperate regions, where during summer transpiration is rapid.
Water Translocation: Theory # 3. Cohesion Theory:
This theory was given by Henry Dixon in 1914. This theory is quite convincing and has been widely accepted. As the name suggests, the cohesion theory is based on the force of cohesion between water molecules. This forms a continuous water column throughout the xylem vessels of the plant, from the top to the root.
Water molecules remain joined to each other due to presence of H-bonds between them. Although H-bond is very weak, but when they are present in enormous numbers as in case of water, a very strong mutual force of attraction or cohesive force develops between water molecules and hence they remain in the form of a continuous water column in the xylem.
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The magnitude of this force is very high (about 350 atm or bars), therefore, continuous water column in the xylem cannot be broken easily due to force of gravity or any other means of resistance.
According to this theory, evaporation of water from the leaf to the atmosphere results in a decrease in the water potential of the epidermal cells, which are in direct contact with the atmosphere.
The water lost from epidermal cells is replaced by the water moving from the adjacent cells along a water potential gradient. The water lost from leaf cells is replaced by the water moving from the xylem elements in leaf veins (i.e., xylem elements). Due to this pattern of water movement, a tention is created in water in the xylem elements of the leaves.
This tention is transmitted downward to water in xylem elements of the roots through the xylem of petiole and stem and the water is pulled upward in the form of continuous unbroken water column to reach the transpiring surface up-to the top of plants.
Also the walls of xylem vessels are constituted of ligno-cellulose have strong affinity for water molecules. The water potential of root cells is lower than the water potential of soil, and therefore, movement of water occurs from soil to the roots. Uplift of water, is also referred to as transpiration pull.
