An investigation of the mechanisms responsible for sap exudation in sugar maple trees (Acer saccharum Marsh.)

Abstract

The sap from sugar maple (Acer saccharum Marsh.) trees contains high levels of sugar, which is collected for the production of maple syrup. Despite centuries of research on the topic, and a multi-million-dollar industry which relies on the process, the specific mechanisms that enable springtime sap exudation in maple trees remain unexplained. Theories on the topic can be categorized into two main mechanisms; the physical models, which explain pressure development within the tree stem as a purely physical mechanism in which gas is compressed within the xylem conduits as a result of freezing and thawing temperatures, and the osmotic theories which hypothesize that a semi-permeable membrane exists between the vessels, filled with sap containing high concentrations of sugars, and the rest of the xylem conduits which creates an osmotic pressure. The most accurate explanation of the springtime sap exudation observed in sugar maple trees is the combined effect of many factors.

The objective of this thesis was to establish methods in which to investigate the process of springtime maple sap exudation at a cellular level, and further to use the techniques developed to determine the exact mechanisms responsible. In the past few decades since the last major contribution to the theories of maple sap exudation, there have been vast advancements in imaging technology. With the combined results from a wide range of novel imaging techniques, I have worked to provide direct evidence of sugar maple sap exudation at a cellular level. I focus on the physical theories of sap exudation, aiming to document the effects of freezing and thawing at a cellular level within the tree stem. Experiments on young sugar maple saplings were conducted using light optical microscopy, scanning electron microscopy (SEM), cryo-SEM, micro-computed tomography (micro-CT) and synchrotron-based micro-CT (synchrotron).

The results obtained offer proof of concept that the techniques used are reliable and effective in observing the mechanisms involved in maple sap exudation at a cellular level. Light optical and scanning electron microscopy provided an extensive amount of detailed anatomical information on the xylem layer of a sugar maple stem. Experiments using cryo-SEM, micro-CT, and synchrotron facilities produced thousands of detailed images of sugar maple branches and stem sections in a hydrated state, providing a thorough understanding of the distribution of sap and gas within the xylem layer. Further images were obtained from saplings which had been frozen offering direct comparisons and a wealth of information on how an induced freeze will affect the xylem conduits. The resulting images offered visualizations of vessel embolism throughout induced freeze-thaw cycles. Images from Cryo-SEM experiments showed direct evidence of gas-filled fibres in the sugar maple xylem, and the build-up of ice within the fibre lumens when the stem is exposed to freezing temperatures. This was the very first direct evidence that a process, which for decades has only been speculated to occur, could explain the process of sugar maple sap exudation.

The work conducted throughout this thesis provided an in-depth understanding of maple sap exudation at a cellular level and the role that freeze-thaw cycles might play. Despite the wealth of information obtained, it is difficult to draw conclusive evidence on the findings presented due to the small number of individual trees that were imaged using each technique. The techniques presented in this thesis are extremely promising, repeating these experiments on a larger sample population will provide information to end the decades of controversy on the topic.