Print PagePlants transport water in a metastable state under negative pressure and cavitation is one of the chief threats to this vital pipeline. The study of xylem vulnerability to cavitation was greatly facilitated by the advent of centrifuge techniques to efficiently and precisely generate negative xylem pressures in excised organs (stems or roots). This approach is more efficient that previous techniques and has led to a rapid accumulation of vulnerability data. Researchers have long been concerned that long vesselled species might be prone to artifacts when analyzed using centrifugal force as open vessels that are spun may drain leading to artifact. A key early design bathed cut ends in water while stem segments were spinning to prevent the open vessels from draining. Debate has been sparked by two recent papers that have found disagreement between vulnerability curves (vcurves) generated using centrifugal force and vcurves using other methods for some species. The hypothesis has been forwarded that long vessels are the root cause of the discrepancy and that the centrifuge technique is not a reliable method for generating vcurves in species that have long vessels. The purpose of the present study is to test the effect of vessel length on vulnerability curves.
Results/Conclusions
We found no evidence of a long vessel artifact in vcurves generated using a long vesselled oak species. We found that vcurves generated using using short segments (14 cm) that have on average 10% of their vessels open yield similar vcurves to segments that are 27 cm and have 0.2% of their vessels open (most have 0% open). Centrifuge vcurves agreed well with vcurves generated via benchtop dry down and to native embolism measurements. We found some discrepancies between the drydown method and the centrifuge method that could be explained by disequilibrium between leaf and stem water potential in drydown branches. This was overcome by using stem psychrometers to directly measure xylem water potential. Reliance on xylem water potentials measured with a pressure chamber when using benchtop dehydration can lead to errors when leaves or small twigs have suffered runaway cavitation even when plants are triple bagged and equilibrated overnight. We suggest that not all centrifuge techniques are the same and this likely explains part of the disagreement found in previous studies. We conclude that long open vessels alone are not the sole cause of artifacts and that the centrifuge technique is robust for at least some long vesselled species.
