Water potential by Pressure Chamber or Pressure bomb

Title: Procedure for the determination of water potential of a plant tissue/cell by Scholander type pressure chamber

History: It is a relatively quick method for measuring the water potential particularly of large pieces of tissues, such as leaves and small shoots. This method was first developed by Henry Dixon at Trinity College, Dublin, at the beginning of the 20th century. However, it did not come into widespread use until P. Scholander and coworkers at the Scripps Institution of Oceanography (San Diego, California USA) improved the instrument design and showed its practical use (Scholander et al. 1965). This type of pressure chamber is called “Scholander Type Pressure Chamber)

Reference: Scholander, P.F., Hammel, H.T., Bradstreet, E.D. and Hemmingsen, E.A., 1965. Sap pressure in vascular plants. Science, 148: 339-346.

Principal: The pressure chamber method for measuring plant water potential. The diagram at left shows a shoot sealed into a chamber, which may be pressurized with compressed gas. The diagrams at right show the state of the water columns within the xylem at three points in time:

  • (A) The xylem is uncut and under a negative pressure, or tension.
  • (B) The shoot is cut, causing the water to pull back into the tissue, away from the cut surface, in response to the tension in the xylem
  • (C) The chamber is pressurized, bringing the xylem sap back to the cut surface

Procedures:

  1. Excise (cut) the leaf or shoot just before dawn
  2. Place the excised material in the humid chamber to prevent water loss prior to measuring water potential
  3. Line the pressure chamberPlace the leaf in compression gland” in such a way that cut end of petiole or stem remains outside of the chamber
  4. Trim, if necessary, the cut end of petiole or stem/shoot to a smooth surface with a sharp razor
  5. Tighten the Allen screws with moderate pressure
  6. Place all this assembly in position on to the chamber and fasten down with retainer cap
  7. Close the bleed off valve
  8. Open the main control valve of nitrogen gas cylinder (do not use oxygen, to avoid explosion danger)
  9. Open the control valve very slowly and increase chamber pressure slowly, if leakage is apparent at the stem-gasket seal, retighten the Allen screws, taking care to use only the minimum pressure required to stop the leakage
  10. Use a magnifying glass to observe for the appearance of xylem sap at the cut end. As soon as sap is visible, close the control valve and note the gauge pressure

Calculations: The units of pressure most commonly used are the Bar (1 bar = 14.5 pounds per square inch) and the mega Pascal (1 MPa = 10 bars)

Precautions:

  1. Excise the leaf before dawn and determine water potential before sunrise. After sunrise, the leaves will experience increased transpirational stress thereby affecting results.
  2. Determine water potential soon after excising. Leaving the leaves unattended for longer periods will result in continued retreating the water column in leaf xylem and thus increased value of water potential.
  3. After excision, try to equally trim the petiole of all leaves. Unequal trimming will yield variable results.
  4. For larger experiments, try to determine water potential under almost similar soil moisture conditions. For example, the value of water potential is likely to increase soon after irrigation.

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