In the March 2003 issue of Discover magazine there was an article titled "The Minus Touch". Since this issue is likely explained to primary or secondary school students and I was recently taught about the subject in a first year university course, I was amazed at lack of knowledge and comprehension from the university professors consulted for this article. Although I did not read the original September 2002 article, due to an apparent lack of understanding amongst scientists in the subsequent March 2003 article I thought that I would address the issue of negative pressure within plant xylem.

First of all, an explanation of surface tension and capillary action. The hydrophilic walls attract water and cause a concave meniscus to form because the attraction between water and the walls of the xylem is greater than between the water molecules. This causes the water to creep up the walls and the surface tension of the water keeps the surface area of the meniscus small, adding to the upward pull. Cohesion: hydrogen bonding between water molecules, and adhesion: hydrogen bonding between water and the xylem walls, are imporant because they act together to keep the water molcules strung together in a chain. The real pull is due to negative pressure.

It is not really negative pressure, just pressure negative relative to atmospheric pressure. Transpiration of water through the stomata due to drier air outside the leaves than on the inside causes the water to be sucked out and creates a pull on the water in the xylem. As I mentioned, the hydrogen bonding of water maintains its cohesion while it is pulled up the xylem. There can also be positve pressure from the roots pushing water upward, but its effect is comparatively minor.

On a warm day, the rate of transpiration can be great enough to decrease the diameter of both the xylem and the entire tree trunk. This has a positive effect because the surface tension of the meniscus is inversly proportional to the radius of the water surface. If you understand capillary action, the effect of the smaller diameter should be obvious.

The main issue of disagreement in the Discover magazine seems to be terminology. A biologist who mentions negative pressure is speaking about relative pressure. A physicist considers negative pressure as an absolute and something they would claim that does not exist for liquids. In fact it is the gaseous water which causes the negative pressure as it exits the stomata.

The basic explanation for this entire concept can be explained by the Water Potential Gradient. The Water Potential Gradient is the tendency of water to move from a high gradient to a low gradient, to move from liquid in the xylem to vapour in the leaf to the hotter, drier air of the atmospere.

Perhaps the most amazing part about the ability of water to move up a tree of any height is that the only energy required is solar energy.