Centrifuge measures of xylem resistance to cavitation: the current debate and best practices for the future

Background/Question/Methods

The impact of open vessels on centrifuge-based measures of xylem cavitation resistance has become a hot topic of debate in the field of plant hydraulics.  It has been suggested that species with long vessels may be susceptible to hydraulic measurement artifact and this has cast doubt on the reliability of centrifuge-based techniques in the generation of xylem vulnerability curves.  This is a potentially crucial issue that could drastically alter the state of our understanding of plant hydraulics and plant responses to water stress.  We review the origins of the open vessel debate and discuss the current state of knowledge on the potential impact of open vessels on centrifuge-based vulnerability curves.  Additionally, previously published data, generated using different centrifuge-based techniques, were compared to non-centrifuge-based methods to determine if vessel length was related to measurement reliability.

 Results/Conclusions

Because of the nature of the distribution of vessel lengths, even short vesselled species are likely to have a few vessels open in the types of samples used for determination of vulnerability to cavitation curves; however, we found no evidence for a vessel length artifact that is common to all centrifuge techniques.  When the standard centrifuge technique was used, water potentials at 50% loss in hydraulic conductivity (P₅₀) were highly predictive of P₅₀ from dehydration curves for both long and short vesselled species. Additionally, the slope of the regression between centrifuge P₅₀ and dehydration P₅₀ was not different from 1 (P > 0.05).  In contrast, other centrifuge-based techniques may be susceptible to long vessel artifact and long segment length relative to vessel length can also lead to measurement artifacts in some cases. We suggest a way forward in the debate that focuses on the core issue of the potential causes of the methodological discrepancies and identification of the specific techniques that appear to be reliable.