Research Aim 1: Sap Flow Mechanisms

May 2, 2022

We aim to provide a comprehensive understanding of sap exudation phenomena for both juvenile and mature trees. Using advanced, high-resolution, 3D, in-vivo imaging techniques we will non-destructively image the microstructural changes that occur during freeze-thaw induced sap flow.

Current Understanding

Tree syrup is produced by concentrating the sugars found in sustainably harvested tree sap. Yet, the mechanisms responsible for sap flow are not known, despite years of research and are likely different for different trees.

The leading explanation for sap exudation in sugar maple, walnut and related species is that positive pressure results from their unique xylem structure, osmotic forces generated by sucrose released by living cells, and pressure generated by the action of spring freeze-thaw cycles on gas bubbles within the sapwood. The importance of sucrose and xylem anatomy have been supported by recent experiments, but confirmation of the sap exudation mechanism requires observation of the behaviour of microscopic gas bubbles within xylem vessels and fibres, and the flow of liquid water within intact stems while they are subjected to temperature fluctuations.

Left: A transverse cross-section of a branch from a sugar maple sapling observed by x-ray micro-CT. Air-filled vessels appear as black space (labelled by arrows) while water-filled vessels appear light grey (labelled as arrowheads). Right: A 3D reconstruction of a maple sapling with a slice exposed to show the stem layers. Scale bar is 1 mm.

In birch, sap exudation does not require freeze-thaw cycles, but is similarly thought to result from osmotic movement of water from living cells into xylem vessels, with replenishment by water flow from the roots during the night. In other species (e.g. kiwifruit and grapes) spring exudation is generated by a poorly understood mechanism, operating in the roots, termed ‘root pressure’.

All of these systems share unresolved questions around the significance of variation in xylem anatomy; the behaviour of gas bubbles within the xylem; the movement of water and solutes between cells within the stem and roots; and the role of fluctuations in temperature.

Non-Destructive Analytical Methods

Existing analytical methods rely on measurement of temperature, pressure, and sap production, which can be correlated, but are not sophisticated enough to distinguish between the proposed theories to determine the sap flow mechanisms inside the tree.

Microstructural imaging (this Research Aim 1) will study small trees and saplings to examine the interactions between xylem anatomy, the pit membranes that interconnect xylem cells, bubble formation and movement, and the role of pit membranes in generating stem pressure and sap flow.

Light optical micrograph of maple sampling cross-section.

We are using a range of advanced tomographic techniques to non-destructively image the microstructural changes in juvenile trees that occur during freeze-thaw induced sap flow, and determining the influence of dissolved gases, liquid root water, and solutes.

The microstructure (see image above) of small trees and saplings will be imaged by high spatial resolution (~1μm) techniques: SEM and XMT. MRI will provide quantitative information on flow in vessels (resolution ~20μm). We will develop techniques for measurement of sugar content and pore sizes by MRI.

Initially measurements will be taken by transferring saplings from environmentally-controlled chambers to the imaging systems. We will then use the variable temperature capability of MRI and cryo-EM to study changes in flow and sugar concentration to understand seasonal variability (-10°C to +25°C).

We will springboard off our preliminary imaging experiments, carried out on live maples at the Australian Synchrotron, and supplement it with other imaging techniques. Research will be accelerated by working with our international collaborators in the northern hemisphere, to obtain two season’s data in a single year.

Our goal is to focus on maple, birch and a selected native tree.

The results from this research will inform and validate the predictive sap flow model (Research Aim 3).

Plant Physiology