Abstract
According to the hydraulic vulnerability segmentation hypothesis, leaves are more vulnerable to decline of hydraulic conductivity than branches, but whether stem xylem is more embolism resistant than leaves remains unclear. Drought-induced embolism resistance of leaf xylem was investigated based on X-ray microcomputed tomography (microCT) for Betula pendula, Laurus nobilis, and Liriodendron tulipifera, excluding outside-xylem, and compared with hydraulic vulnerability curves for branch xylem. Moreover, bordered pit characters related to embolism resistance were investigated for both organs. Theoretical P50 values (i.e. the xylem pressure corresponding to 50% loss of hydraulic conductance) of leaves were generally within the same range as hydraulic P50 values of branches. P50 values of leaves were similar to branches for L. tulipifera (-2.01 versus -2.10 MPa, respectively), more negative for B. pendula (-2.87 versus -1.80 MPa), and less negative for L. nobilis (-6.4 versus -9.2 MPa). Despite more narrow conduits in leaves than branches, mean interconduit pit membrane thickness was similar in both organs, but significantly higher in leaves of B. pendula than in branches. This case study indicates that xylem shows a largely similar embolism resistance across leaves and branches, although differences both within and across organs may occur, suggesting interspecific variation with regard to the hydraulic vulnerability segmentation hypothesis.
Highlights
Ever since the pioneering experiments by 18th and 19th century botanists, it has been well known that leaves play a crucial role in long-distance water transport of plants (Hales, 1727; Böhme, 1893; Strasburger, 1893)
As water transitions from liquid to vapour in leaf tissues, it generates an important pull, transmitted to the xylem water column through the cohesive strength of the liquid water.This results in a negative pressure in the hydraulic pathway from roots to stems and leaves (Tyree and Ewers, 1991; Buckley et al, 2017). there is general agreement based on a large body of evidence for the cohesion–tension theory (Dixon, 1914; Scholander et al, 1965; Dixon and Tyree, 1984; Holbrook et al, 1995; Pockman et al, 1995), an important, controversial question concerns the temporal and spatial frequency of embolism in xylem conduits along the entire xylem pathway at the wholeplant level (e.g. Jansen and Schenk, 2015;Venturas et al, 2017)
High embolism resistance was especially found in leaf veins of L. nobilis, despite the fact that all microcomputed tomography (microCT) scans were performed on cut branches, which may underestimate embolism resistance due to artificial air entry.The relatively high resistance to xylem embolism observed agrees with recent studies based on direct, non-invasive methods, such as the optical method
Summary
Ever since the pioneering experiments by 18th and 19th century botanists, it has been well known that leaves play a crucial role in long-distance water transport of plants (Hales, 1727; Böhme, 1893; Strasburger, 1893). As water transitions from liquid to vapour in leaf tissues, it generates an important pull, transmitted to the xylem water column through the cohesive strength of the liquid water.This results in a negative pressure in the hydraulic pathway from roots to stems and leaves (Tyree and Ewers, 1991; Buckley et al, 2017). there is general agreement based on a large body of evidence for the cohesion–tension theory (Dixon, 1914; Scholander et al, 1965; Dixon and Tyree, 1984; Holbrook et al, 1995; Pockman et al, 1995), an important, controversial question concerns the temporal and spatial frequency of embolism in xylem conduits along the entire xylem pathway at the wholeplant level (e.g. Jansen and Schenk, 2015;Venturas et al, 2017). Given concerns about potential artefacts associated with manipulating a transport system under negative pressure (Jansen and Nardini, 2014; Torres-Ruiz et al, 2014), various novel methods have been developed over the past years to improve the accuracy of the results, such as direct observation of embolism in conduits (Windt et al, 2006; Brodribb et al, 2016b; Choat et al, 2016). The high resolution and optimal phase contrast obtained with X-ray computed tomography are especially useful to study the dynamic and temporal aspects of embolism occurrence and spread across xylem in vivo (Brodersen et al, 2013; Choat et al, 2015; Bouche et al, 2016b; Scoffoni et al, 2017b; Nolf et al, 2017)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
