You measure xylem pressure in two plants using the pressure chamber. In one, you
ID: 34384 • Letter: Y
Question
You measure xylem pressure in two plants using the pressure chamber. In one, you apply 10 bars of pressure before you notice water coming from the xylem of your sample. In the other you apply 15 bars of pressure. Which plant is more water stressed? What is the pressure potential of the water stressed plant in MPa?
2. If (water potentional)P = 0.3 MPa and (water potentional)S = -0.45 MPa, the resulting (water potentional)W is ...
3. Briefly describe what happens in the xylem when cavitation due to water stress occurs. Mention one structural feature of xylem in conifers that protects against cavitation. (3 - 5 sentences).
4.Solute and pressure potential is measured in two different places in the same plant. Fill in the table below with plausible numbers (based on physiological principles). (Note: the plant is healthy and well-watered).
Solute (osmotic) Potential
(water potentional)s
(MPa)
Pressure Potential
(water potentional)p
(MPa)
Total water potential
(water potentional)w
(MPa)
Xylem in the stem
0.0
Root cell
- 1.4
Solute (osmotic) Potential
(water potentional)s
(MPa)
Pressure Potential
(water potentional)p
(MPa)
Total water potential
(water potentional)w
(MPa)
Xylem in the stem
0.0
Root cell
- 1.4
Explanation / Answer
First plant is more water stressed.
Water potential () = pressure potential (P) + solute potential (s)
Water potential () = 10 bars (note: 10 bars=1MPa (mega pascal))
Solute potential (s) = 0 (Note: Pure water at atmospheric pressure has a solute potential of zero.)
10 = pressure potential (P) + 0
Pressure potential (P) = 10-0
=10 bars or 1 MPa
2. Water potential () = pressure potential (P) + solute potential (s)
Water potential () = 0.3-0.45
=-0.15 MPa
3. Cavitation occurs in the xylem of vascular plants when the tension of water within the xylem becomes so great that liquid water (or sap) vaporizes locally and dissolved air within the water expands to fill either the vessel elements or tracheids. Due to cavitation, cells in the xylem expand due to osmosis. Presence of small diameter vessels in the xylem of conifers protects against cavitation.
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