Plant Water Relations

• List the components of water potential.
• Understand how water moves through plants.
• Explain how water potential and its components are measured.

This teaching strategy for plant water relations provides a one-hour presentation, with an animated tutorial, to show the components of water potential, how water moves through plants, and how water potential is measured.

1. Explain how water movement generally follows the laws of thermodynamics (generally from high to low potential).
2. Provide examples of how water tends to follow a path towards disorder (high activity to low activity, high temperature to low temperature, high pressure to low pressure, etc.) See http://www.merlot.org/merlot/viewMaterial.htm?id=83579
3. Ask participants to state major factors affecting water movement (pressure, osmotic, and gravitational) and write the water potential equation, Ψ = Ψ_p + Ψ_S + Ψ_g. See http://www.merlot.org/merlot/viewMaterial.htm?id=83579
4. Ask participants if water generally moves up the plant or down the plant to provide water for different plant organs. Explain that water generally moves up the plant (although it can move other directions depending on water potential).
5. Ask participants, based on their knowledge of the water potential equation, if they can identify one of the main mechanisms that causes water to move up the plant. Use transpiration as an example of the main driving force since the water potential of the atmosphere is usually less than that of plant tissues. See http://www.merlot.org/merlot/viewMaterial.htm?id=83580
6. Discuss plant adaptations and modifications that enable them to survive in dry climates (water stress). See http://www.merlot.org/merlot/viewMaterial.htm?id=83581
7. Introduce the concept of measuring water potential and components of water potential. Briefly explain methods such as thermocouple psychrometry, pressure bomb, and the Chardakov methods for measuring water potential. For an example using psychrometry, see http://www.merlot.org/merlot/viewMaterial.htm?id=420973

Supporting Files:

• Outline - Plant Water Relations (Microsoft Word 35kB Aug12 10)
• Questions - Plant Water Relations (Microsoft Word 35kB Aug15 10)

This teaching strategy provides discussion, as well as an animated tutorial to learn about water potential, water movement through plants, and how water potential is measured. Examples of how water moves from high potential to low potential helps students use logic to determine the direction of water flow. This concept can then be used to rationalize how water moves up the plant. Incorporation of animations keeps students interested in the topic. Applications of how plants have evolved to adapt to water stress, as well as instrumentation used to measure water potential, provide students with applications this important concept.

Participants may be tested on their comprehension of this learning material through multiple choice, short-answer, or essay exams. A few example questions are embedded in this Activity Sheet, entitled "Questions – Plant Water Relations."

MERLOT description and link to "Connecting Concepts: Plant Biology/Water Relations 1: Water Potential Equation," which provides examples and explanation of the water potential equation. See http://www.merlot.org/merlot/viewMaterial.htm?id=83579

MERLOT description and link to "Connecting Concepts: Plant Biology/Water Relations 2: Water Flow Through Whole Plants," which provides an animation of how water moves through plants. See http://www.merlot.org/merlot/viewMaterial.htm?id=83580

MERLOT description and link to "Connecting Concepts: Plant Biology/Water Relations 3: Plant Adaptations for Water Stress," which shows how plants have adapted for water stress. See http://www.merlot.org/merlot/viewMaterial.htm?id=83581

MERLOT description and link to "Psychrometer," which provides an example of instrumentation used to measure water potential. See http://www.merlot.org/merlot/viewMaterial.htm?id=420973