Water transport

Matthew J. Linton linton at biology.ucla.edu
Thu Apr 8 15:43:08 EST 1999


I would also suggest that one read Sperry's reply in American Scientist.  I
actually find the "temperature" of this debate to be about right.  It has
paved the way for a "rebirth" of plant water relations and has led to many
new experiments and techniques as scientists have reconsidered water
transport in xylem, including papers in Science, Nature, new grant money,
etc...  Controversy is great for any scientific topic!

I worked with John Sperry as an undergraduate for 2 years, so I admit that I
am a bit partial to the traditional view.

The only reason that new theories have been proposed is because some find it
difficult to believe that xylem water can be under such large tensions (very
negative pressures) and remain in the liquid state.  The pressures are,
indeed, predicted to be incredibly low (as low as -10 MPa).  If one can make
the admission that large tensions can exist in xylem sap then there is no
need for competing hypotheses (including Canny's theory), since the
cohesion-tension is by far the simplest (and supported by huge amounts of
data).

There have been two groups of relatively recent experiments that have
supported either:

1)  Large negative pressure are not possible or 2) Large negative pressures
are possible

I'll say a bit about each:


1)The pressure probe experiments (Zimmermann, U and others, see:
Zimmermann, et al, Annals of Botany 76:545-551  and Plant, Cell and
Environment 17: 1169-1181) are elegant studies that attempted to measure
xylem pressures (and the point of xylem cavitation i.e., the pressure at
which the xylem sap vaporizes) directly.  These experiments suggested that
pressures in xylem conduits aren't nearly as negative as the C-T theory
predicts and that cavitation of xylem conduits occurs at pressure from 0
to -1 MPa.  The major question with the pressure probe technique is whether
the point of cavitation is the same for intact conduits as those that have
been punctured with a xylem probe (John Sperry says this all more elegantly
in the American Scientist article).  The answer is no.  In fact, the C-T
theory predicts that a pressure probe will be unable to measure xylem
pressure because it will cause xylem cavitation.

2) In support of the C-T theory and large negative pressures, there are the
centriguation experiments by both Sperry (see Pockman, et al Nature
378:715-716) and Holbrook (see Science 270: 1193-1194).  Here, centrifugal
forces were used to generate large negative pressure in xylem and measure
the effect on hydraulic transport.  Both these studies (and others
subsequently) show quite clearly that the pressures necessary for the C-T
theory can occur in xylem conduits and only cause limited cavitation
(depending on the species).

In the final analysis, the centrifuge experiments are much more convincing
that those of the pressure probe.

Very recently, Canny (and his wife, McCully-see Plant Physiology,
119:1001-1008) have come out with additional studies that look at cavitation
in corn roots that show cavitation and refilling at relatively high
pressures (from 0 to -1 MPa).  I admit that these data appear to support
quite strongly Canny's theories.  I will only say that there are reasonable
interpretations of their data within the context of the C-T theory.

My experience in talking with other plant physiologists--some of them quite
prominent (including Park Nobel, my graduate advisor) is that there are few,
if any. water-relations specialists that are ready to accept Canny's theory.
He brings up some very good arguements that should cause us to re-examine
our traditional understanding but they fall quite short of calling for an
entire rejection of the cohesion-tension theory.

I hope there is more comment on this topic.
--
Matthew J. Linton
Department of Organismic Biology, Ecology, and Evolution
UCLA
Los Angeles, CA 90095-1606
Phone:  (310) 206-8382
linton at biology.ucla.edu





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