Transpirational pull

Transpirational pull is the main phenomenon driving the flow of water in the xylem tissues of large plants.

Mechanisms

Transpirational pull results ultimately from the evaporation of water from the surfaces of cells in the interior of the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. Inside the pores, the water forms a concave meniscus. The high surface tension of water pulls the outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. Transpirational pull only works because the vessels transporting the water are very small in diameter, otherwise cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble - an embolism forms, which will spread quickly to other adjacent cells, unless "bordered pits" are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading).

Measurement

Until recently, the negative pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a "Scholander bomb" to counteract it. The name comes from the inventor, P.F. Scholander, and from its disconcerting tendency to explode in the experimenter's face. is the first published independent test showing the Scholander bomb actually does measure the tension in the xylem.

* recent update of the classic book on xylem transport by the late Martin Zimmermann

External links

* [http://www.uni-bayreuth.de/departments/planta/research/steudle/steu5a.htm Research reported by E. Steudle]
* [http://www.oeb.harvard.edu/faculty/holbrook/ Research reported by N. Holbrook]
* [http://www.rr.ualberta.ca/People/Index.asp?page=Directory&ID=1588 Research reported by M. Tyree]
* [http://www.biology.utah.edu/faculty2.php?inum=39 Research reported by J. Sperry]