Hacke, Uwe G. , Sperry, John S. .
Are xylem conduits long enough to minimize pit resistivity?.
Xylem conduits have presumably evolved to minimize their hydraulic resistance while maintaining safety from cavitation by air-seeding through pits. The hydraulic resistivity of xylem conduits can be approximated by the sum of the lumen and wall resistivities in series. Lumen resistivity can be minimized by increases in conduit diameter. However, if conduit length does not increase concomitantly with diameter, the wall resistivity – influenced by interconduit pitting – will become limiting. Short conduits have high resistivity because water must pass pits more often. A recently published model of ours predicted a minimum “saturating conduit length” at which the wall resistivity becomes negligible and conduit resistivity is minimized to the lumen value. Due to efficient torus-margo pits, the saturating length for an average conifer tracheid was only between 2 and 3 mm. In contrast, the saturating length for an average angiosperm vessel was much longer: between 2 and 3 cm, mostly because of less efficient pits. We tested whether vessels achieved their saturating length or whether wall resistivity remained significant. The influence of end wall resistance was assessed by measuring the resistivity of stem segments that were cut successively shorter, thereby increasing the number of open conduits. Measurement conditions eliminated potentially confounding end effects. Vessel length was measured with the paint infusion method. Xylem resistivity declined by 1.5 to 3 times depending on species as stems were shortened from 10 to 0.3 cm and end walls were removed. The results indicated that walls accounted for 43-80% of the total conduit resistivity. The magnitude of the pit limitation increased with vessel diameter. Vessels, despite being potentially unlimited in length, were 4 to 70 times shorter than their saturating lengths.
1 - University of Utah, Department of Biology, 257 S. 1400 E., Salt Lake City, Utah, 84112, USA
Presentation Type: Paper
Location: White Pine (Cliff Lodge)
Date: Monday, August 2nd, 2004
Time: 10:30 AM