Evidence for hydraulic vulnerability segmentation and lack of xylem refilling under tension

The vascular system of grapevine has been reported as being highly vulnerable, even though grapevine regularly experiences seasonal drought. Stomata would consequently remain open below water potentials that would generate a high loss of stem hydraulic conductivity via xylem embolism. This situation would necessitate daily cycles of embolism repair to restore hydraulic function.. However, a more parsimonious explanation is that some hydraulic techniques are prone to artifacts in species with long vessels, leading to overestimation of vulnerability. The aim of this study was to provide an unbiased assessment of (i) the vulnerability to drought-induced embolism in perennial and annual organs, and (ii) the ability to refill embolized vessels in two Vitis species. X-ray micro-CT observations on intact plants indicated that both V. vinifera and V. riparia were relatively vulnerable, with the pressure inducing 50% loss of stem hydraulic conductivity (Ψ50Stem) = -1.7 and -1.3MPa, respectively. In V. vinifera, both the stem and petiole had similar sigmoidal vulnerability curves, but differed in Ψ50 (-1.7 and -1.0MPa for stem and petiole, respectively). Refilling was not observed as long as bulk xylem pressure remained negative (e.g. at the apical part of the plants): P=-0.11{plus minus}0.02MPa; ∆PLC=0.02{plus minus}0.01%). However, positive xylem pressure was observed at the basal part of the plant (P=0.04{plus minus}0.01MPa), leading to recovered conductance (∆PLC=-0.24{plus minus}0.12%). Our findings provide evidence that grapevine is unable to repair embolized xylem vessels under negative pressure, but its hydraulic vulnerability segmentation provides a significant protection of the perennial stem