Some problems with this explanation as it stands...
When you consider that a very tall tree ( 300 ft; redwoods) may require a water potential of -20 atmospheres for flow to occur, many questions arise...
- Can water withstand tensions of -20 to -30 atmospheres? If not, embolisms ( breakage of water column with formation of gas bubble) would occur
- Is adhesion enough to support such a long column and prevent gravity from draining the column? Or in fact is the resistance induced by matric potential actually slowing water flow?
These and many other questions have caused some to speculate on a modification of the TACT explanation.
Since the 1990's, the compensating-pressure theory has arisen to deal with some of these concerns. U. Zimmerman of Germany developed a microcapillary manometer which measures pressure directly. He found:
a. water potential values only in the range of +0.2 - -0.2 MPa, not the -1-2 MPa required for flow in tall plants.
b. he found cavitation to occur at less than -0.2 MPa on a regular basis.. This runs counter to the idea that embolisms only occur rarely, generally at the end of the growing season. If this is true then how do plants 'fix' the column break?.
A number of physiologists disagree with his methods and interpretation, yet they warrant a closer look...
The new theory stipulates the following:
1. living cells generate positive pressure: we know this as turgor pressure - when vacuoles fill with water they press against the rigid wall and generate a + pressure... This pressure can be quite strong, as when a seedling pushes through soil or when a closed bottle is filled with bean seeds and water... as the seeds absorb the water and expand they can generate enough pressure to crack the glass or lift the lid.
If living cells line the dead xylem they could generate enough pressure to cause embolisms to refill by squeezing water out of the cells and into the xylem by reverse osmosis. There must be a reservoir of water available to these cells
2. there needs to be 'valves' at both ends to prevent the dissipation of the + pressure; pressure in the xylem has no influence on these valves. In the leaf As water loss in the leaf is due to gas diffusion, not pressure in the xylem, the mesophyll is a sufficient valve. The roots however need to produce a positive pressure, as in the case of guttation, to reestablish a column of water in the xylem which no longer exhibits negative pressure. This is possible.
3. As with the cohesion theory, evaporation pulls water up the xylem. Living cells however prevent the breaking of the water column by tension by raising the working pressure in the water column to 0 atmospheres and secondly it pushes water from reservoir cells to repair embolisms when they occur.
The close proximity of phloem or parenchyma cells to the xylem could act as the water reservoir, and the conversion of starch to sugar which we know exists in parenchyma (from our review of slides of root and stem tissues weeks ago) would allow a flux of water into these cells to generate a high + pressure.
For further explanation see the paper: Transporting water in plants by M.J. Canny ( 1998) American Scientist Vol 86' 152-159 from which the above discussion heavily borrows from.