Water potential measurements
Psychrometers (PSY1, ICT International, Armidale, NSW, Australia) were
applied to obtain water potential values from the leaves that were
simultaneously measured with the optical and pneumatic method. Sand
paper with a grit size of 400 was used to carefully remove a small area
(10 mm2) of leaf cuticle under a stereomicroscope,
while paying special attention not to abrade the vascular bundles and
introduce air-entry into the xylem. Vaseline was used to seal the
psychrometer chamber to the abraded surface of the leaf. Water potential
measurements were taken every 10 min, and the measurements were stopped
when leaves were completely desiccated, or the water potential showed no
further decrease over a long period. The duration of measurements was
between one and two days.
At the beginning of each dehydration experiment, the leaf water
potential dropped rapidly within one hour. After that, the water
potential decreased slowly and steadily. To test the accuracy of the
xylem water potential values taken with psychrometers, we applied two
different approaches. Firstly, for leaves that were attached to long
branches, we measured not only leaf xylem water potential, but also stem
xylem water potential with another stem psychrometer (Fig. S1). This
comparison provided a reasonably good match between both organs,
although differences became more pronounced at high levels of
dehydration for some species, with xylem water potential decreasing more
quickly for leaves than stems. An exception was F. sylvatica ,
which had more negative xylem water potentials for leaves than stems
when water potentials in leaves were less negative than -5 MPa.
Secondly, water potential values of detached leaves that were measured
with the Pneumatron and optical method were compared to a second set of
leaves. Therefore, two leaves with a similar size were excised from the
same branch at the same time. One leaf was attached to a psychrometer,
while the other one was left to dry under similar conditions (same light
intensity, temperature, and humidity). The water potential of the second
leaf was measured with a pressure chamber (PMS Instrument Company,
Albany, OR, USA) at an interval of 20 to 60 min to validate the accuracy
of the stem psychrometer measurements during the first hours of
dehydration. These tests revealed an overall good agreement between both
methods for three species tested (C. betulus , F.
sylvatica , and Q. petraea ; Fig. S2).