A comparison of two methods for measuring vessel length in woody plants

Abstract

Vessel lengths are important to plant hydraulic studies, but are not often reported because of the time required to obtain measurements. This paper compares the fast dynamic method (air injection method) with the slower but traditional static method (rubber injection method). Our hypothesis was that the dynamic method should yield a larger mean vessel length than the static method. Vessel length was measured by both methods in current year stems of Acer, Populus, Vitis and Quercus representing short- to long-vessel species. The hypothesis was verified. The reason for the consistently larger values of vessel length is because the dynamic method measures air flow rates in cut open vessels. The Hagen-Poiseuille law predicts that the air flow rate should depend on the product of number of cut open vessels times the fourth power of vessel diameter. An argument is advanced that the dynamic method is more appropriate because it measures the length of the vessels that contribute most to hydraulic flow. If all vessels had the same vessel length distribution regardless of diameter, then both methods should yield the same average length. This supports the hypothesis that large-diameter vessels might be longer than short-diameter vessels in most species. Vessels members have evolved from tracheids in ways to enhance hydraulic efficiency; members have become shorter and larger in diameter, and vessels have evolved from linear files of vessel members connected by end-walls with comparatively little obstruction to flow (scalariform perforation plates) or no obstructions (simple perforation plates), which again enhance hydraulic efficiency. Vessels have to be finite length (0.01 to 1m) for safety against cavitation in order to confine embolisms that result from cavitation to a very small percentage of the xylem system. Their length has become important with regard to recent controversies over artifacts in cavitron measurements of vulnerability to cavitation, which has made vessel length distribution measurement important and relevant for the PCE community.