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(American Journal of Botany. 2006;93:1265-1273.)
© 2006 Botanical Society of America, Inc.

Physiology and Biochemistry

Inter-tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection1

Jarmila Pittermann5, John S. Sperry, Uwe G. Hacke, James K. Wheeler and Elzard H. Sikkema

2Department of Integrative Biology, University of California, Berkeley, California 94720 USA; 3Department of Biology, University of Utah, Salt Lake City, Utah 84112 USA; 4Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138 USA

ABSTRACT

Plant xylem must balance efficient delivery of water to the canopy against protection from air entry into the conduits via air-seeding. We investigated the relationship between tracheid allometry, end wall pitting, safety from air-seeding, and the hydraulic efficiency of conifer wood in order to better understand the trade-offs between effective transport and protection against air entry. Root and stem wood were sampled from conifers belonging to the Pinaceae, Cupressaceae, Podocarpaceae, and Araucariaceae. Hydraulic resistivity of tracheids decreased with increasing tracheid diameter and width, with 64 ± 4% residing in the end wall pitting regardless of tracheid size or phylogenetic affinity. This end-wall percentage was consistent with a near-optimal scaling between tracheid diameter and length that minimized flow resistance for a given tracheid length. There was no evidence that tracheid size and hydraulic efficiency were constrained by the role of the pits in protecting against cavitation by air-seeding. An increase in pit area resistance with safety from cavitation was observed only for species of the northern hemisphere (Pinaceae and Cupressaceae), but this variable was independent of tracheid size, and the increase in pit resistance did not significantly influence tracheid resistance. In contrast to recent work on angiosperm vessels, protection against air-seeding in conifer tracheids appears to be uncoupled from conduit size and conducting efficiency.

Key Words: allometry • Araucariaceae • cavitation • conifer • Cupressaceae • Pinaceae • pits • Podocarpacae • resistivity • tracheid






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