SVERIGES LANTBRUKSUNIVERSITET - Epsilon Open Archive - SLU

The pressure chamber technique is much used for measuring leaf water
potential in the field due to its rapidity and reliability. Also it does not
require accurate controi of temperature. The principle of the technique,
bas ed upon a rediscovery of Scholander et al. (1965), is that a leaf or
branch is placed in the pressure chamber with the cut end just protruding
from the chamber through a rubber bung which seals the chamber (Fig. 6).
During the measurement the leaf is sealed in a bag to minimize evaporation
and hereby ch ange of water potential during the measurement.
POlythene bag
Fig. 6 Pressure chamber with the leaf sealed in a polyethylene bag in order
to minimize dehydration through evaporation. (Af ter Jones, 1983).
The pressure of the chamber is slowly increased (10-30 kPa s-l) to a
pressure sufficient to balance the tension by which the water is held in the
xylem and force the meniscus back to the cut surface which easily can be
recorded visually in a microscope. As the osmotic potential of the xylem
sap is high (> - 0.05 MPa) the negative balance pressure therefore nearly
equals the leaf water potential and is most of ten used without correcting
for osmotic potential of the xylem sap. Further cautions and procedures are
given by Turner (1988) and Bannister (1986). Turner (1988) lists several
commercially available pressure chambers.
The thermocouple psychometer technique is based on eq. (2), Le. that
water potential of plant tissue can be obtained from measurement of
relative humidity obtained af ter equilibration of the tissue in a small
chamber.
A detailed description of the use of psychrometer technique for measuring
water potential and its components is given by Wiebe et al. (1971) and
Slavik (1974). Savage et al. (1981) have in detail investigated the
calibration of psychrometers. Temperature compensated thermocouple
psychrometers for conventionaI use on plant tissue and for in situ
measurement of leaf water potential (Campbell and Campbell, 1974) are
commercially available through Wescor Inc., 459 South Main Street, Logan,
Ut ah 84321, USA.
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