Water transport

Tom Buckley buckley at BIOSERVER.VSB.USU.EDU
Fri Apr 9 13:34:26 EST 1999


[This is a follow-up to Matthew Linton's discussion of Zimmerman et al's
xylem pressure probe experiments (Plant, Cell and Environment 17:
1169-1181).  Be aware that I'm also a friend of Sperry's - he was on my
Ph.D. committee and gave me this particular question on my comps - so I
may be biased as well.]

There are some other problems with using the probe for measuring
negative xylem tensions.

(1)  The hydraulic continuum within the probe itself can cavitate,
especially likely given its relatively large internal volume
dimensions.  The Zimmerman paper itself showed that cavitation occurred
with the probe connected to a gravity osmometer at -.4 to -1.0 MPa.  It
is impossible to know where this cavitation occurred, but it may have
been at the probe-osmometer interface, in the water within the
osmometer, or within the probe iself.  Their measurements of tensions in
wet tissue showed cavitation at -.32 MPa; this likely occurred in the
probe itself or at the interface, as the tissue is always open to
atmospheric air.  Since CT theory requires tensions on the order of -5
to -10 MPa, it is not clear that Zimmerman's probe can measure these
tensions.

(2)  The hydraulic volume within the probe apparatus may represent an
enormous volume with respect to that contained within the xylem under
study.  If so, the equilibrium pressure of the new system (probe plus
xylem) will be a volume-weighted average of the probe's initial
hydraulic status (say, X ml, at roughly +.1 MPa before cell insertion in
Zimmerman et al's expts) and the xylem's (say Y ml at -Z MPa).  The
Zimmerman paper didn't address this.

(3)  When you push a probe tip through tissues to puncture a cell, the
tip will NOT be particularly clean when it hits the target cell.  Any
particulate matter on the probe tip could help nucleate an embolism in
water under high tension.

(4)  The greater the pressure gradient across a probe-cell interface,
the greater the chance of a leak.  I've done many pressure probe
experiments on guard cells in epidermal peels, and it's very difficult
to get a non-leaky seal even under ideal, physically stable conditions
with the ability to see what you're puncturing and a self-healing fluid
cell membrane (of course absent in xylem vessels).  My own experience
makes me very hesitant to believe one could use the probe to measure
large tensions in cells lacking membranes and hidden from direct view.

(These last two points are really an explanation of how the probe could
cause cavitation, as Linton pointed out.)

I agree with Matt Linton that the tone of this debate has been
appropriate.  Any less candor, any more 'tact' for its own sake, would
be counterproductive.  It's better to get all the issues and questions
out in the open for rigorous debate than to complacently jump on either
bandwagon.  (Vive la [xylem] resistance!)

Tom Buckley

--
Thomas N. Buckley
Biology Department
Utah State University
Logan, UT 84322-5305
voice: 435-797-3567
fax: 435-797-1575
email: buckley at biology.usu.edu
------------------------------
"Science is the belief in the
   ignorance of experts."
 - Richard P. Feynman
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