Document: UWE-3-32-13

Wood density and drought resistance of plant xylem.

HACKE, U.G.* ¹, J.S.SPERRY ¹, S.D.DAVIS ² and W.T.POCKMAN ³

University of Utah, Salt Lake City, UT, USA ¹
Pepperdine University, Malibu, CA, USA ²
University of New Mexico, Albuquerque, NM, USA ³

Abstract:
An important aspect of drought adaptation in plants is the ability to avoid cavitation, i.e. to keep xylem conduits water-filled under negative xylem pressures. Based on the correspondence between air permeability and cavitation pressure, cavitation is thought to occur by air seeding into xylem conduits. We tested the assumption that denser secondary xylem would be more resistant to air seeding and cavitation by a survey of over 70 diverse woody species. Wood types included conifers, diffuse-porous trees, ring-porous trees, Chaparral shrubs, and shrubs of the Great Basin and Sonoran deserts. In most wood types, there was a highly significant relationship between increasing wood density and increasingly negative xylem pressure causing 50% embolism (P₅₀). To investigate whether there was a direct mechanistic link between density and P₅₀ we developed a model based on a theoretical relationship between air seeding pressure, wall porosity, wall density, and wood density. The model predicted a curvilinear relationship between P₅₀ and wood density that fit measured P₅₀ values with an r² of 0.70. According to the analysis, the differences between wood types in the density vs. P₅₀ relationship were the result of different ratios of wall volume to total wood volume (wall fraction), which in turn were related to xylem conduit anatomy. For example, conifers achieved more negative P₅₀ at a given wood density than angiosperms because of the lower wall fraction associated with a xylem composed overwhelmingly of tracheids. The analysis suggests that an important cost of building low-pressure resistant xylem is the construction of dense wood. In a given wood type, this cost is steepest in the less negative (more vulnerable) P₅₀ range, and is less important as more negative P₅₀'s are achieved. Thus, fast growing woody plants, and plants adapted to favorable water supply, would be expected to show the smallest margins of safety from cavitation.

Keywords: Wood density, xylem cavitation, drought resistance

This abstract is being presented at: 9:00 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.