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.