© 2006 Heron Publishing—Victoria, Canada
Scaling of angiosperm xylem structure with safety and efficiency
Uwe G. Hacke (1, 3), John S. Sperry (1), James K. Wheeler (1) and Laura Castro (2)
1. Department of Biology, University of Utah, Salt Lake City, UT 84112, USA / 2. Unidad de Anatomía, Fisiología y Genética Forestal, Escuela Técnica Superior de Ingenieros de Montes, Universidad Politécnica
de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain / 3. Corresponding author (hacke@biology.utah.edu) / Received June 9, 2005; accepted September 28, 2005; published online March 1, 2006
Summary
We tested the hypothesis that greater cavitation resistance correlates with less total inter-vessel pit area per vessel (the
pit area hypothesis) and evaluated a trade-off between cavitation safety and transport efficiency. Fourteen species of diverse
growth form (vine, ring- and diffuse-porous tree, shrub) and family affinity were added to published data predominately from
the Rosaceae (29 species total). Two types of vulnerability-to-cavitation curves were found. Ring-porous trees and vines showed
an abrupt drop in hydraulic conductivity with increasing negative pressure, whereas hydraulic conductivity in diffuse-porous
species generally decreased gradually. The ring-porous type curve was not an artifact of the centrifuge method because it
was obtained also with the air-injection technique. A safety versus efficiency trade-off was evident when curves were compared
across species: for a given pressure, there was a limited range of optimal vulnerability curves. The pit area hypothesis was
supported by a strong relationship (r2 = 0.77) between increasing cavitation resistance and diminishing pit membrane area per vessel (AP). Small AP was associated with small vessel surface area and hence narrow vessel diameter (D) and short vessel length (L)—consistent with an increase in vessel flow resistance with cavitation resistance. This trade-off was amplified at the tissue
level by an increase in xylem/vessel area ratio with cavitation resistance. Ring-porous species were more efficient than diffuse-porous
species on a vessel basis but not on a xylem basis owing to higher xylem/vessel area ratios in ring-porous anatomy. Across
four orders of magnitude, lumen and end-wall resistivities maintained a relatively tight proportionality with a near-optimal
mean of 56% of the total vessel resistivity residing in the end-wall. This was consistent with an underlying scaling of L to D3/2 across species. Pit flow resistance did not increase with cavitation safety, suggesting that cavitation pressure was not
related to mean pit membrane porosity.
Keywords:
hydraulic conductivity, trade-offs, vulnerability curves, water transport, wood structure, xylem anatomy, xylem cavitation.
