Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white
birch
Damián Cirelli (1, 2), Richard Jagels (1) and Melvin T. Tyree (3, 4)
1. School of Forest Resources, University of Maine, Orono, ME 04469, USA / 2. Corresponding author () / 3. Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada / 4. USDA Forest Service, 705 Spear Street, S. Burlington, VT 05403, USA / Received February 20, 2007; accepted February 26, 2008; published online June 2, 2008
Summary
Two theories have been proposed to explain how high positive pressures are developed in sugar maple stems when temperatures
fluctuate around freezing. The Milburn–O’Malley theory proposes that pressure development is purely physical and does not
require living cells or sucrose. The osmotic theory invokes the involvement of living cells and sucrose to generate an osmotic
pressure difference between fibers and vessels, which are assumed to be separated by an osmotic barrier. We analyzed wood
of Acer saccharum Marsh., Juglans cinerea L. and Betula papyrifera Marsh. (all generate positive pressures) examining three critical components of the osmotic model: pits in cell walls, selectivity
of the osmotic barrier and stability of air bubbles under positive xylem pressure. We examined the distribution and type of
pits directly by light and scanning electron microscopy (SEM), and indirectly by perfusion of branch segments with fluorescent
dyes with molecular masses similar to sucrose. The latter approach allowed us to use osmotic surrogates for sucrose that could
be tracked by epifluorescence. Infusion experiments were used to assess the compartmentalization of sucrose and to determine
the behavior of gas bubbles as predicted by Fick’s and Henry’s laws. The SEM images of sugar maple revealed a lack of pitting
between fibers and vessels but connections between fiber-tracheids and vessels were present. Fluorescein-perfusion experiments
demonstrated that large molecules do not diffuse into libriform fibers but are confined within the domain of vessels, parenchyma
and fiber-tracheids. Results of the infusion experiments were in agreement with those of the fluorescein perfusions and further
indicated the necessity of a compartmentalized osmolyte to drive stem pressure, as well as the inability of air bubbles to
maintain such pressure because of instability. These results support the osmotic model and demonstrate that the secondary
cell wall is an effective osmotic barrier for molecules larger than 300 g mol–1.
Keywords:
bubbles, pressure, sap flow, sucrose, sugar maple.
