Intra-specific variation in xylem cavitation in interior live oak (Quercus wislizenii A. DC.)

Xylem cavitation induced by water stress reduces plant hydraulic conductance and can indicate the habitat a species evolved in and its phylogenetic background. Species differ widely in cavitation resistance, but less is known about intra-specific variation. Cavitation resistance was assessed for field-collected adult and sapling size classes from three populations of interior live oak (Quercus wislizenii A. DC.) in California, USA. Root and stem cavitation resistance of two-year old seedlings from a greenhouse experiment was also measured. Cavitation resistance curves were determined by injecting air into the vascular system to induce cavitation and measuring the subsequent decline in hydraulic conductance. Based on the air-seeding hypothesis, the absolute value of the air pressures should be equivalent to the tensions that cause cavitation under dehydrating conditions. Conductance declined exponentially with applied pressure for both roots and stems. Comparisons between populations did not reveal significant differences despite good statistical power. The 50% loss in conductance point occurred between 1.0-1.6 MPa; conductance declined more slowly thereafter. Conductance was 21-30% of maximum at 4.0 MPa and 7-14% at 8.0 MPa. Saplings exhibited a nearly identical pattern compared with adults except at 4.0 MPa, where saplings exhibited slightly less cavitation (7%). Greenhouse seedling stems were more resistant compared with both field-collected adults and with seedling roots. The 50% loss in conductance point occurred at 0.83 and 2.6 MPa for seedling roots and stems, respectively. Seedling stems maintained conductance of 20.9% at 8.0 MPa while most roots were fully cavitated between 5.0-8.0 MPa.