Harvard Forest Symposium Abstract 2005

Title: The contribution of gas filled fibers to positive sap pressures in sugar maple
Primary Author: Fulton Rockwell (Harvard University)
Additional Authors: David Hollinger (USDA Forest Service)
Abstract:
The occurence of positive sap pressures in Acer saccharum during periods of alternating freezing and thawing temperatures in the spring is a well known phenomena, yet the underlying physiological mechanisms remain poorly understood. Previous work has shown that for positive pressures to arise in the xylem both a period of dessication under frozen conditions, as well as the presence of sucrose in the sap, are required. A physical model has been proposed whereby the compression of gas in xylem fibers by ice growth during a freeze provides a source of positive pressure by the expansion of the gases upon thawing. However, the actual disposition of sap and gas in maple xylem during freeze thaw events has not, to our knowledge, ever been directly investigated. We intend to examine, by cryo-scanning electron microscopy, the distribution of gas between vessels and fibers in sugar maple xylem during the sap flow season and individual freeze thaw events, as well as in response to treatments of stem segments in a freezing chamber. By directly observing the disposition of gas and sap in the frozen state we hope to establish whether in fact ice growth in gas filled fiber spaces is required for the development of positive pressure upon thawing.
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