recap

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34.2 recap

The transpiration–cohesion–tension mechanism explains the ascent of xylem sap. Transpiration draws water out of leaves, resulting in tension that pulls water from the xylem. Because of cohesion between water molecules, water is pulled passively through the xylem vessels in continuous columns, always toward a region with lower pressure potential.

learning outcomes

You should be able to:

  • Solve problems relating to the transport of water by the xylem when given information on water potential.

  • Describe how scientists ruled out past hypotheses for upward flow of water in plants.

  • Discuss the interaction of the processes of transpiration, cohesion, and tension in the movement of water upward through the xylem.

  • Provide evidence for the importance of each of the processes of transpiration, cohesion, and tension in the movement of water through the xylem.

Question 1

What experiment ruled out the role of pressure from the roots in the upward flow of water in the xylem?

A tree was cut near its base, and the upper part of the cut stem was placed in a poisonous solution. The poison rose in the tree, killing cells as it went. This indicated that the fluid could rise without the need for the root.

Question 2

The table below shows measurements of water potential (Ψ) in a 100-meter-tall tree and its surroundings:

Region Ψ (MPa)
Soil water –0.3
Xylem of root –0.6
Xylem of trunk –1.2
Inside of leaf –2.0
Outside air –58.5

Gravity exerts a force of –0.01 MPa per meter of height above ground.

  1. Is the water potential in the leaf sufficiently low to draw water to the top of the tree?

  2. Would transpiration continue if soil water potential decreased to –1.0 MPa?

  3. What would you expect to happen to the xylem water potential if all of the stomata closed?

  1. Yes. The difference in water potential between the soil and the leaf (1.7 MPa) is enough to overcome gravity and draw water to the top of the tree.
  2. No. If the soil water potential decreased to –1.0 MPa, it would be more negative than inside the root cells and water would leave the roots (and enter the soil).
  3. If all the stomata closed, the leaf water potential would not be as negative. This in turn would make the xylem water potential less negative, and so on down to the roots. This would make the difference between the leaf water potential and the root water potential insufficient for water to flow from the roots to the leaves (toward a more negative water potential).

Question 3

Explain why all three parts of the transpiration–cohesion–tension mechanism are necessary for water transfer through the xylem.

Transpiration evaporates water from cell walls in the leaves. This must occur because it begins the process. The increase in negative pressure potential resulting from transpiration draws more water into the cell walls and begins to exert tension (the second part of the process) on the entire water column within the xylem. Cohesion draws water molecules together and makes them “stick.” It prevents water in the column from breaking, thus losing tension and failing to rise.

Question 4

What types of evidence provide support for the transpiration–cohesion–tension mechanism of water transport through the xylem?

The great difference between water pressure potential in the soil and the air is sufficient to pull water upward through the xylem, as water evaporates into the air. This is evidence for transpiration. The continuous column of water moving upward through the xylem is evidence for cohesion. Cut stems show high negative pressure potentials, showing that the xylem is under considerable tension.

Although transpiration provides the driving force for the transport of water and minerals in the xylem, it also results in the loss of tremendous quantities of water from the plant. How plants control this loss will be the subject of the next section.