Using the single-vessel method to investigate vulnerability of oak vessels to cavitation by air-seeding

Background/Question/Methods   The strong relationship observed across many angiosperm species between number of pits per vessel and mean cavitation pressure predicts that very large-vesseled species, such as ring-porous trees, should have a significant portion of earlywood vessels that are extremely vulnerable to cavitation by water stress. Indeed, typical techniques of studying a species' vulnerability to cavitation, such as centrifugation, air-injection, and dehydration, generally indicate that ring-porous species are extremely susceptible to cavitation, losing a large fraction of their conductivity at modest water potentials. However, the very long length of the vessels in ring-porous species complicates the use of these traditional methods in creating vulnerability curves. The single vessel technique, which allows the direct investigation of individual xylem vessels, provides a more direct method by which to investigate cavitation pressures in large-vesseled species. Here, air-seeding pressures of individual files of vessels in stems of Gambel oak (Quercus gambelii) were directly measured using the single vessel technique, and the resulting distribution of air-seeding pressures compared with vulnerability curves generated by the other traditional methods.

Results/Conclusions   Single vessel air-injection indicated a large proportion of vessels with very low air-seeding thresholds; approximately 40% of earlywood vessels had air-seeding pressures less than 1.0 MPa. However, the remaining 60% of vessels had air-seeding pressures evenly distributed over a wide range, with 11.5% exceeding 5 MPa. The mean air-seeding pressure of the distribution was 0.8 MPa. This result agreed well with centrifugation, air-injection, and dehydration vulnerability curves, which all showed steep drops in stem hydraulic conductivity at pressures below 2 MPa, again indicating a population of very vulnerable vessels in the species. A portion of the highly vulnerable earlywood vessels exhibited daily embolism-and-refilling cycles which were inhibited by phloem girdling. The vulnerability curves also showed a long tail, with conductivity maintained to pressures below -4 MPa, consistent with the long tail on the air-seeding distribution. Thus, the single vessel method of air-injection is a potentially very useful method for estimating vulnerability curves, particularly for species whose anatomy presents difficulties for using traditional methods.