The standard centrifuge method accurately measures vulnerability curves in a long-vesselled tree

Background/Question/Methods

The standard centrifuge method has been used for nearly 20 years to advance our understanding of water transport in plant xylem, including tradeoffs between water transport efficiency and safety and the impacts of drought and freeze/thaw stress. However, this method has recently been questioned. It has been hypothesized that open vessels may lead to measurement artifact when samples are centrifuged.  This artifact has been predicted to lead to exponential-shaped vulnerability curves and incongruous vulnerability curves when centrifuged samples are compared to other reference methods.  In this case study, we provide specific tests of the open vessel artifact hypothesis, using current-year’s shoots of olive (Olea europea), a long-vesselled evergreen tree. Three specific predictions of the open-vessel artifact hypothesis were evaluated: 1) Shorter stems with more open vessels would show a greater loss of hydraulic conductivity in the standard centrifuge method than longer stems with fewer open vessels; 2) Standard centrifuge method curves would have higher percentage loss conductivity (PLC) than benchtop dehydration curves, especially at high water potentials; and 3) The presence of long vessels would cause an exponential shape of the vulnerability curve. Additionally, we tested if more accurate curves could be obtained by spinning stem samples only once.

Results/Conclusions

Experimental evidence did not support the open vessel artifact hypothesis. We found no difference between the vulnerability curves of stem segments that differed in their proportion of open vessels. Centrifuge curves agreed with measurements of native and benchtop dehydrated xylem pressure and corresponding Ks values. Vulnerability curves were nearly linear, despite a high proportion of vessels extending beyond the center of measured stem segments. Spinning stem samples repeatedly slightly over-estimated PLC at very negative pressures. Thus, if many pressure increments are required for constructing a vulnerability curve, spinning stem samples only once might be a more reliable alternative. This study shows that accurate vulnerability curves can be obtained for a long-vesselled tree with the standard centrifugation method, suggesting that the standard centrifuge method is not susceptible to an open vessel artifact.