Meeting Abstract
The fluid-structure behavior of the torus-margo pit structure in bordered pits has important implications in understanding sap flow and cavitation and subsequent air-seeding phenomena in living gymnosperms. Pit aspiration (the state of the torus – a pectic disc supported by a porous cellulosic membrane [margo] – sealing the pit aperture) is assumed to localize the embolism. Until now, the force-displacement relations of the pit membrane have been hypothesized but never experimentally quantified. We present the first force-displacement curves for pit membranes of circular bordered pits. The test system consists of a quartz microprobe attached to a microforce sensor which is positioned and advanced with a micromanipulator mounted to an inverted microscope. Pit membrane displacement is measured from digital image analysis using Matlab algorithms. Unaspirated pits from never-dried wood of Larix and Pinus and aspirated pits from dried wood of Larix were tested. The force-displacement relations to translate and deform pit membranes from unaspirated to aspirated positions and then beyond until the seal of the pit aperture failed (for the never-dried wood) and the force-displacement relations needed to unaspirate pits (for the dried wood) were recorded. We observed two modes that would negate the imputed physiological advantages of pit aspiration; rupture or tearing of the pit membrane by the microprobe tip, and the stretching of pit membrane until the torus was forced out of the pit chamber through the pit aperture – torus prolapse. These results raise interesting questions about the hypothesized in situ force-displacement behavior of these structures during normal sap flow and as they relate to embolism containment following a cavitation event.
