Short notes on transpiration pull theory

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This theory which has been widely accepted in spite of some short comings was first proposed by Dixon and Joly in 1894 and Askenasy (1895). The theory was proposed again (to answer the objections raised against it) with experimental detail with the support of several plant physiologists like Renner (1911), Curtis and Clark (1951), Bonner and Galston (1952), Kramer and Kozlowski (1960), Milburn and Johnson (1966), Hammel (1967) and Levitt (1969). There are three main aspects in this theory. These are:

a) Cohesion of water molecules:

Attraction and adhesion to one another is called cohesion. Water molecules have a strong cohesive force due to which they are held tightly to one another and cannot be easily separated. In addi­tion to this, water molecules have an adhesive property also which makes them have a strong attraction to the inner wall of xylem. Hence water mol­ecules cannot be easily torn away from the wall of the xylem.

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Some calculations have been made about the strength of cohesion and many physiolo­gists agree that it is in the range of 350 atm far in excess of what is required for ascent of sap. However there are certain forces within the plant itself that work against the cohesive force of water these are – a) the weight of the column.b) resistance encountered by water column.

All these add up to about 50 atm. But as has been pointed out already, the observed cohesive force far exceeds the forces that can break it. In order to answer some objec­tions raised against the maintainability of water column, Dixon (1914) mea­sured the cohesive force of water in the xylem sap of Ilex aquifolium He took a shaped glass tube and partially filled with xylem sap.

The sap was heated until it almost filled the long arm. The tube was then sealed and gently inclined, so that the long arm was completely filled by the sap. After­wards the tube was inverted to see, whether the water column would break in the long arm.

The water column did not break due to the cohesive and adhesive force among its molecules. The water column would break, on cooling. Dixon tested several samples of the sap and observed the extent of cooling required to break the column. From this he calculated, that the sap had a cohesive strength of 133-207 atm.

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The works of Gremidge (1957), Loomis (1960) etc., have supported the existence of cohesive force among water mol­ecules, but not as high are reported by Dixon. They opine that the maximum cohesive strength is about 30 atm. Considering the fact that even a force of one atm can maintain a height of 30 feet, 30 atms can ac­count for about 900 feet. The water column can be streched also without breaking. According to Fischer, a force of 13,000 bars is required to break the water col­umn.

b) Continuity of water column within the plant:

According to Dixon and Joly, the entire water column in the plant, starting from root hair cells, corti­cal cells, xylem, to mesophyll cells of the leaf around a stoma is a continu­ous chain, so that any influence in the upper part or the lower part is felt by the entire column. The water columns present in different xylem vessels are not independent. They are laterally connected. In other words, the whole of the water present in the plant from root tip to stem tip is a single continuous system.

c) Transpiration pulls or suction force:

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The water network has two termi­nals, one at the root tip near the absorbing regions and the other at the sub stomatal cavities in the leaf mesophyll. These two menisci of water repre­sent the two extremities. Water evaporates from the mesophyll cells due to transpiration. This increases their DPD and as a result water flows into them from the adjacent mesophyll cells.

The DPD of these cells increases and they attract water from the mesophyll cells abutting the vasculature, which (mesophyll cells) draw water from the xylem elements. The DPD of water in xylem increases as a result of continued drawal of water by the meso­phyll cells.

Continued transpiration in the leaf creates a tension (negative pressure) in the xylem elements in the upper parts. Since the water column is continuous and unbroken, this tension is transported down into the root up to the area of absorption and water moves up due to this force. In other words water is pulled up due to the suction force generated by transpiration. This may be likened to the pulling up of a rope (water column) through a tube (xylem).

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