How water moves through plants:

The importance of water in plant functioning:

I. We know from previous discussions why water is so important to plants:

  • Structurally water makes up 80-90% of the fresh weight of herbaceous plants, and more than 50% of woody species. In it proteins and fats form the protoplasm of the cell, and fill the vacuole which it permits it various functions to occur.
  • Cell expansion: it is the influx of water which causes the vacuole to expand creating turgor pressure which then pushes against the pliant walls for cell expansion.
  • Water acts as a solvent for organic and mineral nutrients as well as gases and it aids in the transport of these molecules
  • Photosynthesis and other hydrolysis reaction are dependent on water. Water supplies the electrons and protons needed for the light reaction. Although the amount involved is small, it is critical
  • Water helps to cool plants - transpiration is critical for the loss of heat especially in exposed leaves.

The energy balance of a leaf can be represented as follows: 

    Rn = H + E + hn 

    where Rn = net radiation 
    H= sensible heat exchange with atmosphere (+ve or -ve) 
    E= latent heat of evaporation (-ve) or condensation (+ve) 
    hn= energy used in photosynthesis (-ve) or released in other metabolic processes (+ve), usually not more than 2-3% of Rn

Normally about 80% of the net radiation is dissipated by evaporation of water from the leaf surface (transpiration). If water not available for transpiration, leaf temperature rises and heat is transferred to air.

2. For plants to flourish, they require large amounts of water. For example 500 kg of water is required by tomatoes to produce 1 kg of new organic matter. Most of what a plant absorbs is lost to transpiration. As consistent flow rates need to be maintained, a relatively inexpensive way to procure water was a necessary requirement for evolution on land.

3. Before we attempt to understand the mechanisms for water transport, we first need to ponder the concept of water pressure and potential.

  • Water flows passively because of differences in water potential, which is the ability of water to do work.
  • Water flows from areas of high water potential to areas of low water potential. Thus a plant's leaf must have a lower or negative water potential relative to the root for water to move up the stem.
  • Water potential is measured in terms of pressure [ energy/volume] and the units can be presented as megapascal or MPa or in terms of bars of pressure, where 1 bar of pressure = 0.1 MPa. As a reference, an inflated car tire is usually pushed to 0.2 MPa.

. Water potential can be broken down into its 4 components:

Ys  = potential due to solutes (-ve): When sugar is added to a beaker, the solute potential or energy drops as water molecules 'bond' to the sugar. Thus to decrease  in leaf relative to the root, sugars or other molecules accumulate in the leaf dropping it potential -1 to -2 MPa.

Yp  = pressure potential (+ve or -ve) : as water is transpired from the leaves it creates a negative pressure or tension. Imagine sucking on a straw rapidly down to the 'last drop'...The solution 'hangs' in the straw due to this negative pressure. This pressure generally is on the order of less than -2 M Pa ( megapascals)

Pressure can be positive however as with guttation, when there are so many solutes in a root that water accumulates and pushes up. Ions continue to be pumped into the xylem even though transpiration has ceased at night . The increased concentration results in a lower osmotic potential in the xylem, and a gradient is developed across the root. Water moves into the xylem in response. 

Ym  = matric potential (-ve) : As water adheres to cell walls it lowers it potential. The walls of xylem are made up of carbohydrates which attract water molecules. In a sense, matric potential is equivalent to solute, but occurs when the molecules are bound rather than in solution.

Yg   = potential due to gravity  this forces in not critical at a cellular level but may be important in tall trees.

Now on to how water does move up the plant.....