Investigating the existence of an osmotic barrier between xylem fibers and vessels in sugar maple (Acer saccharum) using microCT.

Abstract

Sugar maples (Acer saccharum Marshall) develop elevated stem pressures in springtime through the compression and expansion of gas bubbles present within xylem fibers. The stability of this gas within the fibers is hypothesized to be due to the elevated sugar concentration of maple sap and the presence of an osmotic barrier between fibers and vessels. Without this osmotic barrier, gas bubbles are predicted to dissolve rapidly. In this work, we investigated the existence of this osmotic barrier. We quantified the fraction of the xylem occupied by gas-filled fibers using synchrotron-based microCT. After imaging fresh stem segments, we perfused them with either a 2% sucrose solution or water, imaging again following perfusion. In this way we directly observed how total gas present in the fibers changed when an osmotic pressure difference should be present, with the 2% sucrose solution, and when it is absent, with the water. Following a first round of perfusion, we perfused stem segments with the other perfusate, repeating this multiple times to observe how switching perfusates affected gas-filled fibers. We found that perfusing stem segments with water resulted in a significant reduction in the xylem fiber gas, but perfusing stem segments with a sucrose solution did not significantly reduce the gas in the fibers. These results support the hypothesis that an osmotic barrier exists between fibers and vessels.