Anatomy and Physiology of Sugar Maple in Relation to Xylem Sap Pressure
- Cirelli, Damian
University of Maine Graduate School
In early spring sugar maples (Acer saccharum Marsh.) in New England and Québec are tapped for the production of maple syrup. Maples develop large positive pressures in their stems when day/night temperatures fluctuate above and below freezing. Several theories have been proposed to explain this phenomenon but it still remains one of nature's great mysteries. Two contrasting theories currently provide explanations for such phenomenon. One proposes that pressure development is purely physical, involving ice formation and gas compression a mechanism that requires neither living cells nor sucrose. An opposing theory invokes the involvement of sucrose (molecular weight 342 g mol-1) and an osmotic "membrane". In this theory vessel solution contains on average 2% sucrose while fibers contain only water (with salts, but at a higher energy state). An osmotic barrier between vessels and fibers must exist but the nature and location of this barrier is not known. In this study we look at wood of sugar maple and other species that also develop stem pressure, and analyze two key factors of the osmotic model: pits and cell walls. We look for pits directly using light and scanning electron microscopy (SEM) to identify their type and location, and indirectly by means of different fluorescent dyes of similar molecular weight to that of sucrose. The latter approach allows us to use a molecule that we can track as a surrogate for sucrose, using epifluorescence to reveal what paths the sucrose-like molecule follows. We hypothesize that the secondary cell wall is an effective osmotic barrier for non-polar molecules larger than 300 g mol-r. SEM work shows lack of pitting be- tween fibers and vessels, but also identifies fiber tracheids with profuse bordered pitting. Fiber tracheids are not mentioned in the literature and are presumably mistaken for true fibers. Fluorescein experiments suggest that large molecules do in fact remain confined to vessels, parenchyma cells and possibly fiber tracheids. These results directly support the vitalistic model.
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