Biology of Dryland Plants
Hacke, Uwe G. , Sperry, John S. .
Xylem ecophysiology in the drylands.
The xylem of most dryland plants is resistant to cavitation, but often has a low conducting efficiency. This and the fact that xylem is usually not more resistant than required in a specific habitat suggests that there are costs involved in developing cavitation-resistant xylem. More negative xylem pressure requires greater wall reinforcement to prevent conduit collapse. This is reflected on the tissue level, as xeric plants typically show greater wood densities than riparian and mesic plants. Besides this mechanical cost of developing safe xylem, there is a hydraulic cost. In angiosperms, we found that resistant vessels have less pit membrane area than vulnerable ones. This suggests that cavitation is caused by the rare largest membrane pore, whose average size increases with total pit area per vessel. Total pit area was related to vessel surface area. Consequently, resistant vessels had not only less pit area than vulnerable ones, but resistant conduits were also smaller. This resulted in a trade-off between vessel resistivity and cavitation resistance. The constraint on pit area and vessel surface area also explains why vessels were not longer, and why pits accounted for c. 55% of the total xylem resistivity. In conifers, we observed a trend for decreasing tracheid diameter with increasing cavitation resistance. Although there is no known causal link between these two parameters, there may be an indirect constraint on tracheid diameter. Wider conduits require thicker walls to prevent implosion at a given xylem pressure. There may be a developmental limit to wall thickness in earlywood tracheids, which would constrain tracheid diameters in xeric species. Notably, torus-margo pits of conifers showed much greater conductances than the simpler pits of angiosperms, thus compensating to a large degree for the smaller conduit size in conifer xylem.
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1 - University of Utah, Department of Biology, 257 S. 1400 E., Salt Lake City, Utah, 84112, USA
Presentation Type: Symposium or Colloquium Presentation
Location: Salon K - Austin Grand Ballroom/Hilton
Date: Wednesday, August 17th, 2005
Time: 11:30 AM