PARENT SESSION
Oral Session #74: Plant Water Relations: Hydraulics and use.
Presiding: B. Engelbrecht
Thursday, August 8. 9:00 AM to 11:30 AM. Greenlee Meeting Room, TCC.
Is there a tradeoff between safety from xylem embolism and conductive efficiency in Californian chaparral?
Davis, Stephen*,1, Ewers, Frank2, Bowen, T.J.1, Jacobsen, Anna1, 1 Pepperdine University, Malibu, California2 Michigan State University, East Lansing, Michigan
ABSTRACT- There are two environmental factors that cause the disruption of water transport to leaves of evergreen, chaparral shrubs of California: severe water stress and freezing air temperatures. However, the mechanism of xylem cavitation and embolism (disruption of water transport by gas bubbles) is very different between the two environmental stress factors. Water stress-caused embolism is a function of the pore size in the cell walls (pit membranes) of xylem conduits whereas freezing-caused embolism is a function of conduit diameter (surrogate for transport efficiency). We surveyed 22 species of chaparral shrubs in the Santa Monica Mountains of southern California and found that minimum seasonal water potential was a good predictor of susceptibility to cavitation induced by water stress (r2 = 0.78) whereas efficiency of water transport (xylem specific conductivity) and vessel diameter were poor predictors (r2 = 0.31 and 0.32 respectively). We interpret the lack of a strong tradeoff between safety from embolism caused by water stress and efficiency in xylem water transport as a result of the confounding influence of selection for xylem traits that resist freezing-caused embolism (narrow vessels with inherently low conductive efficiency). An alternate likelihood for tradeoffs was found between susceptibility to cavitation by water stress and carbon allocation for increased wood density, higher resistance to stem breakage and greater implosion resistance of individual xylem conduits.
KEY WORDS: xylem, cavitation, chaparral, freezing