Centrifuge technique consistently overestimates vulnerability to water stress-induced cavitation in grapevines as confirmed with high-resolution computed tomography

Vulnerability to cavitation is a key variable defining the limits to drought resistance in woody plants (Kursar et al., 2009). This trait is typically assessed by a vulnerability curve, which can be generated by a range of methods, including dehydration (Sperry et al., 1988), air injection (Cochard et al., 1992), and centrifugation (Alder et al., 1997). Results from two recent papers suggest that one of the most widely used methods, the centrifuge technique, overestimates vulnerability to cavitation in species with very long vessels (Choat et al., 2010; Cochard et al., 2010). Typically, the centrifuge technique produces characteristic ‘R shaped’ curves for long-vesseled species, compared with ‘S shaped’ curves produced by the dehydration method (Cochard et al., 2010). Both research groups proposed that open vessels contained in the centrifuged samples were responsible for this artifact. Grapevine (Vitis vinifera L.), a liana species known to have unusually long and wide vessels, appears to be particularly susceptible to artifacts with the centrifuge method (Choat et al., 2010), but the conclusions of this paper have been challenged by Jacobsen & Pratt (2012). They contend the dehydration technique actually underestimates vulnerability to cavitation in grapevine because the production of gels and ⁄ or tyloses causes a decline in maximum specific hydraulic conductivity (Ks max) over time. On the basis of their results and previously published hydraulic data, they concluded that the centrifuge technique is the most appropriate technique for estimating vulnerability to embolism (see details in Jacobsen & Pratt, 2012). Here we demonstrate that declining Ks max did not influence the results of Choat et al. (2010) and present new evidence from high-resolution computed tomography (HRCT) to support our original conclusions and refute those of Jacobsen & Pratt (2012). We also contend that the analysis of previous literature presented in Jacobsen & Pratt (2012) was oversimplified and obscured the specific comparison of cavitation resistance in current year shoots of grapevine. Overall, the findings presented in Jacobsen & Pratt (2012) for V. vinifera cv Glenora are in direct contrast to published and unpublished results generated by our research groups for other V. vinifera varieties.