Water uptake can occur through woody portions of roots and facilitates localized embolism repair in grapevine

Abstract

Water acquisition is thought to be limited to the unsuberized surface located close to root tips. However, there are recurring periods when the unsuberized surfaces are limited in woody root systems, and radial water uptake across the bark of woody roots might play an important physiological role in hydraulic functioning. Using X-ray microcomputed tomography (microCT) and hydraulic conductivity measurements (Lp ), we examined water uptake capacity of suberized woody roots in vivo and in excised samples. Bark hydration in grapevine woody roots occurred quickly upon exposure to water (c. 4 h). Lp measurements through the bark of woody roots showed that it is permeable to water and becomes more so upon wetting. After bark hydration, microCT analysis showed that absorbed water was utilized to remove embolism locally, where c. 20% of root xylem vessels refilled completely within 15 h. Embolism removal did not occur in control roots without water. Water uptake through the bark of woody roots probably plays an important role when unsuberized tissue is scarce/absent, and would be particularly relevant following large irrigation events or in late winter when soils are saturated, re-establishing hydraulic functionality before bud break.

Small-scale characterization of vine plant root water uptake via 3-D electrical resistivity tomography and mise-à-la-masse method

“…At least two other phenomena may contribute to current release that is higher than expected. Firstly, Cuneo et al (2018) show that, although woody portions of roots act as an electrical barrier (also to microbial degradation), exchanges may occur during water uptake (in order to facilitate localized embolism repair in grapevines). Secondly, as discussed also in the introduction, some roots show anisotropic electrical conductivity, allowing current to flow radially more easily than longitudinally (Anderson and Higinbotham, 1976).…”

“…Field measurements were conducted in March 2017, using a 10-channel resistivity meter (Syscal Pro Switch 72, IRIS Instruments). For the 3-D scheme of ERT, a complete skip-two dipole-dipole scheme was adopted and produced some 5000 measurements, including reciprocal measurements used to estimate and reject bad data quality (Binley et al, 1995;Daily et al, 2004). A pulse duration of 250 ms for each measurement cycle and a target of 50 mV for potential readings were set as criteria for the current injection.…”

Differences in grapevine rootstock sensitivity and recovery from drought are linked to fine root cortical lacunae and root tip function

“…typically fourth order or higher roots) is possible (MacFall et al, 1990(MacFall et al, , 1991Cuneo et al, 2018) and might be particularly relevant when the absorption pathways of fine roots are scarce or damaged (Green & Clothier, 1999;Dubrovsky & North, 2002;Cuneo et al, 2016). Recently, direct evidence using micro-CT showed that water can enter woody roots through the lignified and suberized bark layer pointing to the water uptake potential of these roots (Cuneo et al, 2018). Direct evidence using magnetic resonance imaging (MacFall et al, 1990(MacFall et al, , 1991 and traditional hydraulic experiments (Queen, 1967;Chung & Kramer, 1975;Cuneo et al, 2018) showed similar results, thus we feel this is a feasible explanation for the recovery in Ψ stem in the drought-susceptible rootstock after re-watering.…”

“…Previous studies have reported that water uptake through woody root portions (i.e. typically fourth order or higher roots) is possible (MacFall et al, 1990(MacFall et al, , 1991Cuneo et al, 2018) and might be particularly relevant when the absorption pathways of fine roots are scarce or damaged (Green & Clothier, 1999;Dubrovsky & North, 2002;Cuneo et al, 2016). Recently, direct evidence using micro-CT showed that water can enter woody roots through the lignified and suberized bark layer pointing to the water uptake potential of these roots (Cuneo et al, 2018).…”

Positive pressure in xylem and its role in hydraulic function

“…Could positive xylem pressure after rewatering repair embolism that has formed during a drought period? According to modelling studies, if an embolus is compressed to slightly above atmospheric pressure, it should be resorbed in seconds to minutes (Pickard, 1989; Yang & Tyree, 1992), and even atmospheric pressure can be sufficient for embolism repair, including via capillary forces (Rolland et al ., 2015; Knipfer et al ., 2016; Cuneo et al ., 2018). By contrast, stems with many emboli require many hours to refill (Tyree & Yang, 1992), and refilling may not be possible after severe drought stress that causes embolism in leaf veins as well as damage to the photosynthetic machinery, as observed in wheat (Johnson et al ., 2018).…”