Transpiration

Transpiration in a Tall Tree

The movement of water through a plant is called the transpiration stream. This pathway begins at the root epidermis and continues symplastically and apoplastically to the endodermis. The path continues in the vascular cylinder of the root. The water and mineral solution accumulated by the endodermal transport proteins is conducted up the xylem of the root, the stem, and the leaf. In the leaf the solution coats the mesophyll cells apoplastically, evaporates into the gas spaces between the cells, and escapes the leaf through the stomata and into the atmosphere. Thus water passes through (trans-) the plant and exits by small pores (-spira-) in this process (-tion). Transpiration is supported by four critical factors.

Root pressure pushes water up the xylem

To be honest, the influx of water into roots is not uniform along the length of a root. The figure below shows this quite nicely. As you can see the increasing suberization in older parts of the root prevent water uptake through the endodermis. Indeed water intake is limited to the zone just behind the elongation area at the root tips.

However, the important point here is that the endodermis has transport proteins that allow the root to accumulate ions by active transport. This burns much sugar (note drop in ψs as you compare across the endodermis above) but importantly produces a more concentrated solution of mineral inside the vascular cylinder of the root. Water follows the flow of minerals osmotically and this can develop pressure inside the vascular cylinder if the transpiration rate is slow (weather is humid, cloudy, cool). The pressure that develops at the bottom of the column of xylem can push water up that column. The photo below shows this nicely; a severed shoot often exudes a drop of liquid from the xylem. The total pressure that roots can provide this way is about 0.05 to 0.5 MPa depending on weather conditions. This amount of pressure in a xylem tracheid might permit a push to a few meters up a shoot...but no more.

Root pressure is also sometimes visible on leaves. Under conditions of high humidity, cool temperature, and low light exposure root pressure can push xylem fluids through leaf mesophyll and out some larger pores in the leaves called hydathodes. Thus on a cool morning as you walk across the grass you notice a drop of liquid on the tip of each blade. You may have thought this was dew, but because it is on the upward pointing tip, you realize that this cannot be so. A test of solutes would demonstrate that this is xylem sap, not condensed humidity! The process by which this exudes is called guttation and it is driven by root pressure. Parking your car under certain species of trees can leave some nasty "water spots" on your wax job. Again, if this were dew, the pure condensation would not leave a mineral spot behind; this is xylem sap that dries, leaving a mineral deposit.

Capillarity allows water to climb up the xylem

We have already discussed this in a previous lecture. Again the amount of climb that is possible in a tracheid of normal diameter is perhaps a meter up the plant. This capillarity is a function of adhesion of the liquid to the cell wall of the xylem, and cohesion of the water molecules to each other.

Cohesion in the column of water prevents cavitation

The column of water in the xylem extends from the root xylem, through the stem xylem, and up into the leaf xylem. This column must be unbroken for water to be able to continue to flow. As water is drawn up a tall tree, the tensile strength of water is needed to keep the column intact. Fortunately the cohesion of water molecules provides this tensile strength. In addition, some trees have evolved an anti-cavitation device: bordered pits. Along the side walls of the cells in the xylem these pits permit water to enter or exit the column based upon pressure and so on. The pit membrane is really a primary cell wall that is reasonably flexible. As pressure changes occur in a tracheid, the membrane can respond by closing the border on the pit or opening the border on the pit. The plug for the border is called the torus. This feature is shown below.

Evaporation from the leaves pulls water up the xylem

Evaporation from the intercellular spaces in the leaf into the atmosphere is a strong pull that removes water from the top of the column of water in the xylem. This process generates sufficient force to lift the column of water up against the gravity vector in tall trees. This evaporative pull should not be underestimated.

To understand how evaporation into a gaseous phase would interact with a fluid phase, we need to rework our water potential concept for gases.

ψ = R T Vw-1 ln(RH)

where: R is the gas constant, T is the absolute temperature, Vw is the partial molar volume of water, and RH is relative humidity (on a scale from 0 to 1).

Some values for water potential at various relative humidities are:

RHψ
1.000.0
0.99-1.4
0.96-5.5
0.90-14.2
0.50-93.6
0-∞

The first evaporation occurs from the leaf mesophyll cells into the humid atmosphere inside the leaf:

Let's take a closer look to see the path of water through a leaf

The pull is assisted by some capillary action in microscopic pits in the surfaces of the cell wall:

Once the water has evaporated into the internal atmosphere in the leaf, it moves out through the stomata into the relatively-drier outside atmosphere as shown below:

You might notice that the arrows have a "wiggly" edge as they pass through the epidermis via the stomata. These are meant to show a resistance to water movement caused by the small stomatal aperture and the boundary layer of "dead air." These two factors determine how rapidly the water will move from the interior atmosphere to the external atmosphere.

The rate of transpiration obviously will be a function of the size of the stomatal pore and the thickness of the boundary layer. The relationship is shown below:

To show you the magnitude of this evaporative pull I provide you with this diagram based on water potential. Remember water potential is a measure of energy, the more negative this value is, the stronger is the pull of water from one area to another. Notice that the diagram shown here is upside down, but in western cultures we read from top down, so I am sure you can understand the flow...

Xylem sap is a dilute solution of minerals and its composition is not that different from root to leaf. For each 10 meters of height, the gravitational potential rises 0.1 MPa in xylem. The tallest tree ever known was an Australian Eucalyptus at 150 meters. We have to explain water flow to the top of this tree. The pressure potential will have to overcome the frictional resistance of perforation plates and so on, so a strong vacuum will be needed.

The bundle sheath is composed of turgid living cells which help load sugar into the phloem and unload water from the xylem.

The water is pulled from those bundle sheath cells into the apoplast fluids that bathe the mesophyll. The evaporation of water from this fluid and the diffusion of sugars from the photosynthetic mesophyll cells make this solution quite concentrated. This is a non-cellular area, but water is being pulled through the wall pits so the pressure potential is slightly negative.

The water evaporates from the apoplastic fluid into the atmosphere in the intercellular spaces of the leaf. The gas here has high humidity, so the water potential is lower than the apoplastic fluid, but higher than the atmosphere outside the leaf.. This gives this the energy differences needed to drive the evaporation process.

The atmosphere outside the leaf is relatively dry air, its solute potential is even lower than that of the atmosphere inside the leaf. Obviously the difference in water potential will provide a driving force for movement of the water vapor out of the stomata and into the air.