title: Mechanical reinforcement of tracheids compromises the hydraulic efficiency of conifer xylem.
abstract: Wood structure and function of juvenile wood from 18 conifer species from four conifer families (Araucariaceae, Cupressaceae, Pinaceae and Podocarpaceae) were examined for a trade-off between wood reinforcement and hydraulic efficiency. Wood density and tracheid 'thickness-to-span' ratio were used as anatomical proxies for mechanical properties. The thickness:span represented the ratio of tracheid double wall thickness to lumen diameter. Hydraulic resistivity (R) of tracheids on a cross-sectional area basis (RCA) increased over 50-fold with increasing density and thickness:span, implying a strength versus efficiency conflict. The conflict arose because density and thickness:span were increased by narrowing tracheid diameter rather than by thickening walls, which may be developmentally difficult. In the Pinaceae and Cupressaceae species, density and thickness:span correlated strongly with protection from drought-induced embolism, suggesting that mechanical strength was required in part to withstand tracheid collapse by negative sap pressure. These species showed a corresponding trade-off between increasing RCA and embolism protection. In contrast, species of Podocarpaceae and Araucariaceae were overbuilt for their embolism protection and were hydraulically inefficient, having greater density, thickness:span and RCA, none of which were correlated with vulnerability to embolism.
authors.affiliation: Department of Integrative Biology, University of California, Berkeley 94720, USA. pittermann@berkeley.edu
authors.forename: Jarmila
authors.initials: J
authors.lastname: Pittermann
authors.forename: John S
authors.initials: JS
authors.lastname: Sperry
authors.forename: James K
authors.initials: JK
authors.lastname: Wheeler
authors.forename: Uwe G
authors.initials: UG
authors.lastname: Hacke
authors.forename: Elzard H
authors.initials: EH
authors.lastname: Sikkema
mesh_terms: D001696:Biomechanical Phenomena; D018517:Plant Roots / Q000033:anatomy & histology / Q000378:metabolism / Q000502:physiology; D018547:Plant Stems / Q000033:anatomy & histology / Q000378:metabolism / Q000502:physiology; D064028:Tracheophyta / Q000033:anatomy & histology / Q000378:metabolism* / Q000502:physiology; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water