[NatureNS] xylem damage

From: David & Alison Webster <dwebster@glinx.com>
To: NatureNS@chebucto.ns.ca
Date: Tue, 08 Jul 2014 10:58:01 -0300
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Dear All,                                July 8, 2014
    I have not been into woods yet but I have noticed in the yard that many 
leaves of Ash, Sweet Cherry Seedling, Sugar Maple and Norway Maple are 
somewhat wilted, especially near branch tips, even early in the morning. 
This suggests structural damage to xylem elements such that gas (water vapor 
or air) blocks water movement.

    One great mystery in Plant Physiology is how occlusion of xylem elements 
by vapor doesn't happen routinely which would limit plant height to 30' and 
plant growth to very humid environments. The vapor pressure of water at 10o 
C is about 9 mm Hg, 4 mm at 0o C (standard atmosphere being 760 mm) so at 
tensions greater than 751 mm Hg (at 10o C) one would expect the water in 
xylem elements to boil. And water tensions can exceed 80 atm (drawing on 
memory) with often no Ill effects.

    As noted in a private Dec 2013 e-mail (below) I think the explanation 
may lie in the state of water in xylem elements; not truly free water but 
bound in a gel. Analogous to the movement of electrons in a conductor, only 
a small fraction of the water in a xylem element is free to move at any 
instant. Consequent the activity of water is very low and the vapor 
pressures determined on free water do not apply. This is speculation but it 
is the only mechanism, known to me, that makes any sense.

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    Interesting that Driwater is 2% Cellulose, 98% water and is solid. It 
would not be possible to directly demonstrate the presence of a weak gel in 
Xylem elements but it would be possible to compare maximum tensions attained 
in fine glass capillaries containing various dilutions of cellulose gel or 
silica gel or various hydrogels. The presence of cellulose chains that are 
attached to the wall only at one end in xylem elements is not so far 
fetched. Assuming secondary thickening from the primary wall inward with 
cellulose units sewed into the ends of various strands by metabolic action, 
how would the end of a spiral chain ever manage to become attached to the 
wall ? When the cell contents die and become digested by autolysis  (sp ?) 
are there not always going to be a swirl of loose cellulose ends ?
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Yt, DW

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