The centrifuge has become a common tool to induce embolisms in xylem conduits and thereby assess how vulnerable plants are to water stress. However, the method has become controversial in recent years because of the assertion that the induced emboli are artifacts, and that this causes long-vesseled species to appear more vulnerable than in situ. Here, we propose that a separate artifact causes species with short conduits to appear less vulnerable than in situ.
The spinning motion of a centrifuge creates a negative pressure profile within xylem with pressures most negative at the axis of rotation. Typical methodology for creating a vulnerability curve is to: 1) spin plant material at the velocity that will produce a target pressure at the center of rotation; 2) measure hydraulic conductivity of the material and calculate the reduction due to spinning; 3) associate the minimum pressure with the total reduction. The potential problem with this last step is that the total reduction is a function of the distribution of emboli, which is a function of the distribution of pressures while spinning and the in situ vulnerability of the xylem.
We created a model to assess how substantial this potential artifact is by predicting what vulnerability would be observed based on in situ vulnerability, conduit length, and the pressure profile across the stem during centrifugation.
Results/Conclusions
The model suggested that xylem will generally appear less vulnerable in the centrifuge than in situ. The predicted artifact was greatest when vulnerability curves were shallow and conduits were short relative to segment lengths. For example, the predicted P50 (the pressure corresponding to 50% loss of conductivity) was 1.6 MPa more negative than the in situ P50 (-4.2 MPa) when the vulnerability curve was nearly linear and conduits averaged 200 times shorter than the segment length. The artifact became negligible as the conduit/segment length ratio increased and vulnerability curves became very steep. The model was also used to back-calculate in situ vulnerability from predicted vulnerability. Preliminary empirical tests support the model results and further tests are pending.
Our results suggest that vulnerability has generally been underestimated in previous studies using the centrifuge. This artifact should be most pronounced in resistant conifers. In a twist to the debate about the vulnerability of long-vesseled species, our model suggests that, given species with the same in situ vulnerability, long-vesseled species will appear more vulnerable because there is less of an artifact.