2.2 Xylem hydraulic traits
Xylem cavitation resistance of roots and branches was constructed using
the air injection method (Sperry & Saliendra, 1994). Previous studies
have shown that this method can obtain reliable measurements of
hydraulic vulnerability even on long-vesseled species, especially when
using a small pressure sleeve with no open vessels (Ennajeh et
al ., 2011). The maximum hydraulic conductivity
(K h max) was calculated as
dividing the volume flow rate of water flowing through the stem by the
hydrostatic pressure gradient along the stem (Johnson et al .,
2018). Xylem specific conductivity (K S) was
calculated as dividing K h by the cross-sectional
area of the xylem section being measured. During the measurement period,
the laboratory temperature was maintained at 20 ℃, and the solutions (10
mM KCl and 1 mM CaCl2) used in the measurement were
fully vented.
After determining K h max, a stem
was placed in a double-ended pressure sleeve (8 cm in length) and
pressurized for 5 min. Then, the stem was removed from the pressure
sleeve, and K h was measured using the same method
used for K h max. This process was
repeated at 0.5 to 1 MPa increments (depending on species and organ) of
increasing pressure until K h had fallen to less
than 10% of its maximum value. The percentage loss in hydraulic
conductivity (PLC) was calculated as PLC = (1 -K h / K hmax) × 100%. The pressure causing 50% loss of
conductivity (P 50) was calculated following
Pammenter & Willigen (1998) as PLC = 100 / (1 + exp (s / 25 ×
(P i - P 50)), wheres (% MPa-1) is the negative slope of the curve
at the inflection point, and P i is the xylem
pressure.