title: Recalcitrant vulnerability curves: methods of analysis and the concept of fibre bridges for enhanced cavitation resistance.
abstract: Vulnerability curves (VCs) generally can be fitted to the Weibull equation; however, a growing number of VCs appear to be recalcitrant, that is, deviate from a Weibull but seem to fit dual Weibull curves. We hypothesize that dual Weibull curves in Hippophae rhamnoides L. are due to different vessel diameter classes, inter-vessel hydraulic connections or vessels versus fibre tracheids. We used dye staining techniques, hydraulic measurements and quantitative anatomy measurements to test these hypotheses. The fibres contribute 1.3% of the total stem conductivity, which eliminates the hypothesis that fibre tracheids account for the second Weibull curve. Nevertheless, the staining pattern of vessels and fibre tracheids suggested that fibres might function as a hydraulic bridge between adjacent vessels. We also argue that fibre bridges are safer than vessel-to-vessel pits and put forward the concept as a new paradigm. Hence, we tentatively propose that the first Weibull curve may be accounted by vessels connected to each other directly by pit fields, while the second Weibull curve is associated with vessels that are connected almost exclusively by fibre bridges. Further research is needed to test the concept of fibre bridge safety in species that have recalcitrant or normal Weibull curves.
authors.affiliation: College of Forestry, Northwest A&F University, Yangling, Shaanxi, 712100, China; Qinling National Forest Ecosystem Research Station, Yangling, Shaanxi, 712100, China.
authors.forename: Jing
authors.initials: J
authors.lastname: Cai
authors.forename: Shan
authors.initials: S
authors.lastname: Li
authors.forename: Haixin
authors.initials: H
authors.lastname: Zhang
authors.forename: Shuoxin
authors.initials: S
authors.lastname: Zhang
authors.forename: Melvin T
authors.initials: MT
authors.lastname: Tyree
mesh_terms: D031281:Hippophae / Q000033:anatomy & histology / Q000502:physiology*; D008962:Models, Theoretical; D018520:Plant Shoots / Q000033:anatomy & histology / Q000502:physiology; D018547:Plant Stems / Q000033:anatomy & histology / Q000502:physiology; D018526:Plant Transpiration / Q000502:physiology*; D014197:Trees; D014867:Water / Q000502:physiology*; D014934:Wood / Q000033:anatomy & histology / Q000502:physiology; D052584:Xylem / Q000033:anatomy & histology / Q000502:physiology
chemical_list: D014867:Water
keywords: Hippophae rhamnoides L.; dual Weibull; hydraulic weight; vessel length