Document Detail

Toward an improved model of maple sap exudation: the location and role of osmotic barriers in sugar maple, butternut and white birch.
MedLine Citation:
PMID:  18519246     Owner:  NLM     Status:  MEDLINE    
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).
Damián Cirelli; Richard Jagels; Melvin T Tyree
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Publication Detail:
Type:  Journal Article; Research Support, Non-U.S. Gov't; Research Support, U.S. Gov't, Non-P.H.S.    
Journal Detail:
Title:  Tree physiology     Volume:  28     ISSN:  0829-318X     ISO Abbreviation:  Tree Physiol.     Publication Date:  2008 Aug 
Date Detail:
Created Date:  2008-06-03     Completed Date:  2008-09-18     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  100955338     Medline TA:  Tree Physiol     Country:  Canada    
Other Details:
Languages:  eng     Pagination:  1145-55     Citation Subset:  IM    
School of Forest Resources, University of Maine, Orono, ME 04469, USA.
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MeSH Terms
Acer / metabolism*,  ultrastructure
Betula / metabolism*,  ultrastructure
Biological Transport / physiology
Cell Membrane Permeability
Cell Wall / metabolism,  ultrastructure
Fluorescein / analysis
Juglans / metabolism*,  ultrastructure
Microscopy, Electron, Scanning
Microscopy, Fluorescence
Models, Biological*
Osmosis / physiology
Osmotic Pressure
Plant Exudates / metabolism*
Sodium Chloride / metabolism,  pharmacology
Sucrose / metabolism,  pharmacology
Wood / metabolism,  ultrastructure
Xylem / metabolism,  ultrastructure
Reg. No./Substance:
0/Plant Exudates; 2321-07-5/Fluorescein; 57-50-1/Sucrose; 7647-14-5/Sodium Chloride

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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