Given the basics of water potential, we can now explore the dynamics of water flow.

Many potential explanations have been devised to explain water flow or flux in a plant:

? Could capillarity explain flux?

Adhesion to the walls lifts the water column but gravity we know pulls it down.

We expect the thinner the column, the greater the height, but even with a column as thin as a xylem vessel, the maximum height would be 1 meter.

Thus capillarity does not appear to be viable on its own....

? Could atmospheric pressure explain flux?

Air pressure on the water 'pushes' the water column up - however gravity pulls it down, thus even with a thin column, the maximum height the column can reach is 10 meters, not enough for to explain water movement in trees....

? Could root pressure explain flux? as we learned earlier, the build of minerals in the root xylem can generate a potential of +0.2 MPa. However, this condition ( guttation) usually only occurs on warm wet nights

 

? Could active transport be responsible?: this proposal has never been taken to seriously when you consider the prohibitive energetic cost that would be involved to move the quantities of water that move through a plant on a daily basis.

Also, we know that xylem is dead, and as most of water transport occurs in xylem, active transport is not a viable consideration as it would require energy that dead cells don't have.

? Could evaporation explain flux?

The Cohesion Theory first proposed by Henry Dixon ( 1893) has been accepted by the majority of plant physiologists for the last century, although there is some speculation that it is not sufficient in itself.

The complete explanation involves the forces of TACT : transpiration, adhesion, cohesion and tension.

  • Basically the theory states that the force that pulls water through a plant originates at the top, where the water evaporates to form water vapor. Water evaporates from the wet walls of the mesophyll. The water vapor diffuses through the stomata to the drier air outside
  • However before the water reaches the mesophyll it must be obtained from the soil and move through the length of the plant......as long as the water potential is less in one area relative to another it will move to the area of lower potential...

Soil potential (-.05 ) ----> root potential ( -0.2)

Lower water potential in the the root relative to the soil draws water inward. The flow pattern however is not simple. Apoplastic flow of water through cells walls and spaces is energetically inexpensive and rapid... however to prevent toxic levels of substances from moving through into the xylem, there must be regulation of entering molecules. The endodermis forces the water to move through the cell membrane and protoplasm which slows down the process and may even require active transport at times if osmotic forces are not sufficient.....

Cohesion holds water molecules together forming a continuous column This is due to the dipolar nature of the water molecule (-O/+H) Consider, when you turn on the water, a continuous stream of water flows forth, not a cloud of gas molecules.

Although Matric potential decreases the water potential as the water molecules adhere to the walls, this bound water wall actually causes a resistance, as free flowing water must skirt the bound layer.

The wider the column as with a vessel, the lower the resistance due to matric forces and the faster the flow; the narrower the column as with tracheids, the greater the resistance and the slower the flow rate.

The primary driving force is transpiration; the lower water potential of air (-100 MPa) relative to that of the leaf ( -1.5 MPa) moves the water outward.

A typical value of water potential in a transpiring leaf is -1 MPa 

The driving force is the difference in vapour pressure between the evaporating surface in the leaf and that in the air above the leaf : eL - eair

yL - yair 

Which factors cause resistance to the transport of water ? 
 
  • Stomatal resistance (rst) : 0.4 - 16
  • Cuticular resistance (rc) - large compared with the other leaf resistances : 40 -480
  • resistances in the mesophyll (rm
  • the boundary layer resistance of the air (ra) : 0.6-0.8
  • rst - varies according to the stomatal aperture. 
    ra - the boundary layer resistance is determined by wind velocity, which makes the layer thinner as velocity increases. 
    If instead of laminar flow, the velocity is such that turbulence is created, this breaks-up the boundary layer. In reality, the increased flow will cool the leaf, and reduce transpiration. The structure of the leaf surface also affects the thickness of the boundary layer. 

    Over the past 10-15 years it has been established that roots sense a drying soil and send a signal to the shoot causing reduced leaf expansion or stomatal closure before there is any detectable change in leaf water status.

But is this explanation sufficient to explain water flow?.......Lets continue on....