Our team have recently published a journal paper showing how microCT imaging was used to examine changes in fibre embolisms within maple xylem following perfusion with water or sucrose solution.
James A.Robinson, Matt Rennie, Mike J. Clearwater, Daniel J. Holland, Abby van denBerg, Matthew J. Watson. 2024. Investigating the existence of an osmotic barrier between xylem fibres and vessels in sugar maple (Acer saccharum) using microCT. Tree Physiology.
Sugar maples develop elevated stem pressures in springtime through the compression and expansion of gas bubbles present within xylem fibres (thin, vertically aligned cells which fill much of the space within the tree stem). The stability of the gas found within the fibres is hypothesised to be due to the elevated sugar concentration of maple sap (which is around 2% sucrose) and the presence of an osmotic barrier between fibres and vessels. This barrier prevents sucrose from penetrating the fibres resulting in an osmotic pressure difference, without which gas bubbles are predicted to dissolve rapidly.
In this work we have investigated the existence of this osmotic barrier. We quantified the fraction of the xylem occupied by gas-filled fibres using synchrotron based microCT.
After imaging fresh stem segments, we perfused them with either a 2% sucrose solution or water (forcing the solution through the stem segments under pressure). We imaged again following perfusion. In this way we directly observed how total gas present in the fibres changed when an osmotic pressure difference should be present, with the 2% sucrose solution, and when it is absent, with the water.
We found that perfusing stem segments with water resulted in a significant reduction in the xylem fibre gas, and perfusing stem segments with a sucrose solution did not significantly reduce the gas in the fibres. These results support the hypothesis that an osmotic barrier exists between fibres and vessels.
Through this work we demonstrated that microCT was a viable approach for examining maple anatomy and water content at high resolution without sectioning, and can be used to study the phenomena associated with stem pressure development, including fibre embolism evolution.
