Inter‐tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection

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.

Maximum height in a conifer is associated with conflicting requirements for xylem design

“…Hydraulic conductivity of conifer wood is a function of both the tracheid lumen conductance and pit conductance (Fig. S2), with the latter factor as the main determinant of wholewood xylem conductivity in conifers (20)(21)(22)(23). Pit conductance is controlled more by aperture conductance than by membrane conductance (9,12,20).…”

A physiologically explicit morphospace for tracheid-based water transport in modern and extinct seed plants

“…Because they allow comparable rates of water transport with fewer pits, torusand-margo tracheids are mechanically stronger than those of pycnoxylic Paleozoic woods. The torus-margo structure also allows for a tighter seal against embolism, if such events are rare; when subject to repeated embolism events, the torus may become stuck against the aperture, permanently sealing the tracheid against fluid flow (Hacke et al 2001b, Pittermann et al 2005, Pittermann et al 2006a, Pittermann et al 2006b, Sperry et al 1994, Sperry and Tyree 1990, Zimmermann 1983. Perhaps the tracheids observed today in conifers and Ginkgo present the optimum balance among mechanical support, cavitation avoidance, and water conductivity in plants developmentally committed to pycnoxylic wood.…”

“…Torus-margo pits allow individual pores to be up to 1 μm in diameter (Choat et al 2008, Lancashire and Ennos 2002, Pittermann et al 2005, Pittermann et al 2006a. These large pores dramatically increase the porosity of an individual conifer or Ginkgo tracheid, making them competitive with angiosperms (Pittermann et al 2005, Pittermann et al 2006a, Zimmermann 1983.…”

“…Recent work has shown that there are significant differences between these two pit types when pit resistance is normalized to area; because of the large pores in the margo, conifers have pit area resistances up to two orders of magnitude lower than those of angiosperms (Pittermann et al 2005, Pittermann et al 2006a, Pittermann et al 2006b). Consequently, there are significant effects of pit membrane pore size on values of r p .…”

“…We chose pore size and number based on a pore diameter value that would satisfy this relationship. The fraction of cell walls occupied by pits was measured directly from specimens through serial longitudinal section or taken from anatomical descriptions, figures, or other reports in the literature (Greguss 1968, Greguss and Balkay 1972, Pittermann et al 2006a). …”