The relationship between xylem conduit diameter and cavitation caused by freezing

The centrifuge method for measuring the resistance of xylem to cavitation b y water stress was modified to also account for any additional cavitation t hat might occur from a freeze-thaw cycle. A strong correlation was found be tween cavitation by freezing and mean conduit diameter. On the one extreme, a tracheid-bearing conifer and diffuse-porous angiosperms with small-diame ter vessels (mean diameter <30 mu m) showed no freezing-induced cavitation under modest water stress (xylem pressure = -0.5 MPa), whereas species with larger diameter vessels (mean >40 mu m) were nearly completely cavitated u nder the same conditions. Species with intermediate mean diameters (30-40 m u m) showed partial cavitation by freezing. These results are consistent wi th a critical diameter of 44 mu m at or above which cavitation would occur by a freeze-thaw cycle at -0.5 MPa. As expected, vulnerability to cavitatio n by freezing was correlated with the hydraulic conductivity per stem trans verse area. The results confirm and extend previous reports that small-diam eter conduits are relatively resistant to cavitation by freezing. It appear s that the centrifuge method, modified to include freeze-thaw cycles, may b e useful in separating the interactive effects of xylem pressure and freezi ng on cavitation.