Root resistance to cavitation is accurately measured using a centrifuge technique

Abstract

Plants transport water under negative pressure and this makes their xylem vulnerable to cavitation. Among plant organs, root xylem is often highly vulnerable to cavitation due to water stress. The use of centrifuge methods to study organs, such as roots, that have long vessels are hypothesized to produce erroneous estimates of cavitation resistance due to the presence of open vessels through measured samples. The assumption that roots have long vessels may be premature since data for root vessel length are sparse; moreover, recent studies have not supported the existence of a long-vessel artifact for stems when a standard centrifuge technique was used. We examined resistance to cavitation estimated using a standard centrifuge technique and compared these values with native embolism measurements for roots of seven woody species grown in a common garden. For one species we also measured vulnerability using single-vessel air injection. We found excellent agreement between root native embolism and the levels of embolism measured using a centrifuge technique, and with air-seeding estimates from single-vessel injection. Estimates of cavitation resistance measured from centrifuge curves were biologically meaningful and were correlated with field minimum water potentials, vessel diameter (VD), maximum xylem-specific conductivity (Ksmax) and vessel length. Roots did not have unusually long vessels compared with stems; moreover, root vessel length was not correlated to VD or to the vessel length of stems. These results suggest that root cavitation resistance can be accurately and efficiently measured using a standard centrifuge method and that roots are highly vulnerable to cavitation. The role of root cavitation resistance in determining drought tolerance of woody species deserves further study, particularly in the context of climate change.

Mechanical Failure of Fine Root Cortical Cells Initiates Plant Hydraulic Decline during Drought

“…Previous work has attributed the loss of hydraulic capacity of drought-stressed root systems to xylem embolism formation (McElrone et al, 2004;Pratt et al, 2015) and/or root shrinkage (Nobel and Cui, 1992;North and Nobel, 1997). Here, both embolism and tissue shrinkage lagged well behind lacunae formation.…”

“…Dry and warm atmospheric conditions would push leaves beyond a water potential threshold and trip the hydraulic fuse in the leaf or petiole, thereby preventing both excessive water losses to the atmosphere and the buildup of hydrostatic tension in the xylem that would lead to systemic and catastrophic embolism spread (McCulloh and Woodruff, 2012;Tyree et al, 1993;Zimmermann, 1983). However, root xylem has been reported to be more vulnerable to embolism formation than shoot xylem in many woody plants (McElrone et al, 2004;Alder et al, 1996;Sperry et al, 1998;Kolb and Sperry, 1999;Ewers et al, 2000;Hacke et al, 2000;Hacke, 2000;Pratt et al, 2015). While we found that the xylem of fine roots was more susceptible to droughtinduced embolism compared to coarse roots, which agrees with previous findings (Sperry and Ikeda, 1997), our current work highlights that the weakest link in the soil-plant-atmosphere continuum is in an even more distal tissue, yet still within the plant body.…”

“…Axial water transport in the xylem is considered a weak link, as roots of numerous species have been shown to be more susceptible to drought-induced xylem embolism compared to other organs within the same plant (i.e. trunks, stems, tap roots; Alder et al, 1996;Hacke and Sauter, 1996;McElrone et al, 2004;Pratt et al, 2015;Johnson et al, 2016). Moreover, Sperry and Ikeda (1997) found that smaller roots were the most vulnerable plant organ to xylem embolism, which would localize failure to inexpensive, distal, and easily replaceable portions of a root system.…”

Structural determinants of increased susceptibility to dehydration‐induced cavitation in post‐fire resprouting chaparral shrubs

“…Thus, as has already been shown for chaparral roots, which are also typically more hydrated than stems (Pratt et al . ), r ‐shaped curves appear to describe a relatively common hydraulic strategy that can be found in some organs, species and, now, recovery stages of woody plants.…”

Evidence for Hydraulic Vulnerability Segmentation and Lack of Xylem Refilling under Tension

“…The ratio between vessel and sample length impairs hydraulic measurements in long-vesseled species (Ennajeh et al, 2011;MartinStPaul et al, 2014;Torres-Ruiz et al, 2014;Choat et al, 2016), although this is disputed by other studies (Sperry et al, 2012;Pratt et al, 2015). Furthermore, the exponential-shaped vulnerability curves imply that a grapevine stem would be 50% embolized before its leaf and stomatal conductance decrease, which seems unlikely (Nardini and Salleo, 2000).…”