Testing the ‘microbubble effect’ using the Cavitron technique to measure xylem water extraction curves

Abstract

Xylem vulnerability to cavitation is an important trait in characterizing woody species' drought tolerance. However, artifacts arise for long-vesselled species when using the in situ flow centrifuge method, also known as the Cavitron. We tested the role of microbubbles as a potential mechanism for this bias by constructing vulnerability and xylem water extraction curves for species with different maximum vessel lengths in different rotor sizes. Our results show a major difference in xylem vulnerability to cavitation for long-vesselled species between methods and support the microbubble effect hypothesis.

Abstract

Plant resistance to xylem cavitation is a major drought adaptation trait and is essential to characterizing vulnerability to climate change. Cavitation resistance can be determined with vulnerability curves. In the past decade, new techniques have increased the ease and speed at which vulnerability curves are produced. However, these new techniques are also subject to new artefacts, especially as related to long-vesselled species. We tested the reliability of the ‘flow rotor’ centrifuge technique, the so-called Cavitron, and investigated one potential mechanism behind the open vessel artefact in centrifuge-based vulnerability curves: the microbubble effect. The microbubble effect hypothesizes that microbubbles introduced to open vessels, either through sample flushing or injection of solution, travel by buoyancy or mass flow towards the axis of rotation where they artefactually nucleate cavitation. To test the microbubble effect, we constructed vulnerability curves using three different rotor sizes for five species with varying maximum vessel length, as well as water extraction curves that are constructed without injection of solution into the rotor. We found that the Cavitron technique is robust to measure resistance to cavitation in tracheid-bearing and short-vesselled species, but not for long-vesselled ones. Moreover, our results support the microbubble effect hypothesis as the major cause for the open vessel artefact in long-vesselled species.