Addressing controversies in the xylem embolism resistance–vessel diameter relationship

Abstract

Summary Although xylem embolism is a key process during drought‐induced tree mortality, its relationship to wood anatomy remains debated. While the functional link between bordered pits and embolism resistance is known, there is no direct, mechanistic explanation for the traditional assumption that wider vessels are more vulnerable than narrow ones. We used data from 20 temperate broad‐leaved tree species to study the inter‐ and intraspecific relationship of water potential at 50% loss of conductivity (P50) with hydraulically weighted vessel diameter (Dh) and tested its link to pit membrane thickness (TPM) and specific conductivity (Ks) on species level. Embolism‐resistant species had thick pit membranes and narrow vessels. While Dh was weakly associated with TPM, the P50–Dh relationship remained highly significant after accounting for TPM. The interspecific pattern between P50 and Dh was mirrored by a link between P50 and Ks, but there was no evidence for an intraspecific relationship. Our results provide robust evidence for an interspecific P50–Dh relationship across our species. As a potential cause for the inconsistencies in published P50–Dh relationships, our analysis suggests differences in the range of trait values covered, and the level of data aggregation (species, tree or sample level) studied.

Gas diffusion kinetics drive embolism spread in angiosperm xylem: evidence from flow-centrifuge experiments and modelling

“…The difference in the speed of embolism propagation between species is related to the ability of the species to resist embolism formation, and is likely associated with the vessel anatomy, including vessel diameter and length, and intervessel connectivity. However, more experiments with a larger number of species would be needed to test this hypothesis (Lens et al, 2011(Lens et al, , 2022Isasa et al, 2023).…”

Angiosperms follow a convex trade‐off to optimize hydraulic safety and efficiency

“…It is well known that large vessels in xylem are more efficient for conducting xylem sap than narrow ones considering the Hagen–Poiseuille equation (Sperry et al ., 2005). Yet, there is no clear evidence whether large vessels consistently embolize before the narrow ones (Isasa et al ., 2023). Intervessel pits, which interconnect the walls of adjacent vessels (also called ‘end walls’), have been assumed to provide the anatomical basis for a safety‐efficiency trade‐off, functioning either as valves that promote safety by avoiding embolism formation, or as valves that optimize hydraulic conductance without providing major resistance to flow between adjacent vessels.…”

The root of the problem: diverse vulnerability to xylem cavitation found within the root system of wheat plants

“…Xylem vessels have been noted to widen along the length of wheat seminal roots, meaning that the largest vessels in the plant are located towards the growing tips of these large roots (Wu et al ., 2009, 2011; Ouyang et al ., 2020). If xylem lumen diameter and vulnerability to cavitation were correlated (Scoffoni et al ., 2017b; Isasa et al ., 2023), then large wheat roots with large xylem would be expected to be highly vulnerable to xylem cavitation, becoming increasingly vulnerable moving towards the distal tips, unlike in leaves where the most peripheral veins are the smallest and most resistant to cavitation. Additionally, were small roots with small vessels to follow the pattern of leaves, these would be the most resistant part of the system (Schuldt et al ., 2013; Kirfel et al ., 2017; Isasa et al ., 2023).…”

“…If xylem lumen diameter and vulnerability to cavitation were correlated (Scoffoni et al ., 2017b; Isasa et al ., 2023), then large wheat roots with large xylem would be expected to be highly vulnerable to xylem cavitation, becoming increasingly vulnerable moving towards the distal tips, unlike in leaves where the most peripheral veins are the smallest and most resistant to cavitation. Additionally, were small roots with small vessels to follow the pattern of leaves, these would be the most resistant part of the system (Schuldt et al ., 2013; Kirfel et al ., 2017; Isasa et al ., 2023). This pattern, however, seems counterintuitive given that fine roots have rapid turnover and appear more replaceable than larger seminal roots (Lübbe et al ., 2022).…”