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Abstract Detail



Developmental and Structural Section

Schulte , Paul J [1].

Solid mechanics of the torus-margo in a conifer (Picea mariana) inter-tracheid bordered pit.

Water flow along the xylem for many tracheid-bearing plants depends on flow through bordered pits, which for many conifers include a torus within the pit membrane.  This torus is thought to act as a valve to seal the pit if one of the tracheids becomes air-filled as a result of cavitation.  The present study considered bordered pits in the conifer Picea mariana.  A computational solid mechanics model was constructed based upon electron micrographs of pit structure.  The cell wall material supporting the torus was constructed by subtracting drawings of margo pores (previously utilized in a model of fluid flow through the pit) from a solid disc representing the margo, leaving the margo strands.  A boundary load was applied to one side of the torus, representing a pressure difference across the torus. Model solutions gave physical displacements for all points in the model.  A model was developed for a pit from a sun-grown and a shade-grown tree having differences in pit size and the size and numbers of pores in the margo of the pit membrane.  In both cases, pressure differences arising from fluid flow through the pit lead to insignificant deflection of the torus.  However larger pressures, which could arise when one side of the pit becomes air-filled, lead to deflection of the torus.  For both pit models, torus deflection was linearly related to applied pressure and inversely related to the value specified for wall stiffness (Young's modulus).  Using literature values for Young's modulus, the pressure leading to sealing of the torus (deflection until the torus contacted the adjacent pit borders) was 0.18 MPa and 0.30 MPa for the "Sun" and "Shade" pit models, respectively.  The greater open area of the "Shade" pit membrane (margo solid fraction of 0.628) compared to the "Sun" pit membrane (margo solid fraction of 0.769), lead in a previous study to a lower membrane resistivity to flow for the "Shade" pit by a factor of 2.  Despite the apparent greater support of the torus in the "Sun" pit membrane, its greater pit diameter allowed for greater force deflecting the torus, thus requiring a lower pressure across the torus to seal the pit.  This study suggests that wider pits might compensate for their greater area over which pressure acts to deflect the torus by having more robust mechanical support in the margo.


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1 - University Of Nevada, Las Vegas, Life Sciences, 4505 S Maryland Pkwy, Las Vegas, NV, 89154-4004, USA

Keywords:
bordered pits
torus
solid mechanics
tracheid.

Presentation Type: Poster:Posters for Sections
Session: P
Location: Eyrie/Boise Centre
Date: Monday, July 28th, 2014
Time: 5:30 PM
Number: PDS003
Abstract ID:547
Candidate for Awards:None


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