Vacuole transport lab, and red beet tissue

Scott T. Meissner smeissne at PRAIRIENET.ORG
Sat Nov 9 13:45:31 EST 1996


In response to Russ' comments concerning the outflow of
betacyanins from cut beet tissue placed in water let me
try to clarify the point I was trying to make.

While studying the water potential of red beet storage
tissue I found that this tissue is isotonic with from
0.2 to 0.4 M sorbitol, not water.  Amazed the heck out
of me since I got the idea previously that since the 
plant takes up ground water this fixes the base water
potential of its organs.  On reading the literature
I found lots of examples where storage sinks have 
a more negative water potential than other parts of
the plant.  Beet storage tissue also seems to be one
of these.  So normally the apoplast of this tissue
is not filled with water, but with a solution that is
isotonic with 0.2-0.4 osmolar.  

This means that if the beet storage organ is cut open, 
say by an insect or a plant physiologist :) ,  and 
exposed to rain water the cells take up this water, 
swell, get very high turgor, and some will lyse.  Other
cells will be dumping out sugars and cleaving sucrose
to fructose and glucose to create more osmotica in part
perhaps to dump these out and try to regulate the 
apoplastic tonicity.  Patrick and others have suggested
that it may be easier for the plant to regulate a small
apoplastic space rather than a large symplastic space.  So
this is what the beet tissue is trying to do.

Now, if you take a one cell layer thick section of beet 
tissue, all the cells will be exposed to the solution they
are placed in.  Under these conditions you can see the 
swelling and lysis of many of the cells over time.  The 
vacuole stretches and bursts into the cytosol, I have seen
this happen on many of the times I have been trying to 
measure the incipient plasmolysis point of this storage 
tissue.  So with a one cell thick layer the cells will
all suffer high turgor.

But if you use a plug of tissue things are different.  ONLY
the cells towards the surface of the plug experience the
water's tonicity.  The cells further in the plug can still
manage to dump enough osmotica into the apoplast so that 
they avoid this high turgor and can avoid lysing.  This is 
a means for the storage organ to survive being eatten into
by insects in the field.  Eventually, of course, the 
surviving cells will dedifferentiate and start to form 
callus.

One consequence of this attempt to regulate the 
tonicity of its apoplast is that the parenchyma cells will
be cleaving sucrose into hexoses to make more osmotica, and
to get energy to respond to this wound.  This is the classic
climateric effect that many workers have used to examine
aged (ie washed for hours in water) beet discs.  The 
cells in the center of the plug can, to a limited extent, 
regulate their apoplastic environment and attempt to recover
from the wound.  So there will be cells in the beet disc that
will survive for hours and not leak any betacyanins out into 
the media.  

I have looked at the water potential, osmotic content, and 
sucrose content of red beet discs that were placed in aerated water
for a day versus discs freshly harvested.  Interestly, if the disc is
thick enough you can still find that their internal water potential
is still more negative than water.  But they do lose sugar and 
osmotica over time.  Perry did some excellant work that showed 
this effect, though I do not recall if he looked at water potential
of the tissue, I believe he was mainly interested in sucrose levels.

My main point was that the beet storage tissue is not isotonic with
water.  Taking this tissue and sticking it in water will result in
extremely high turgor pressures in the exposed cells, damaging most
of them to the point of lysis.  Over time this will cause a release
of betacyanins until the cells that are towards the interior of the 
extracted plug can start adjusting and protecting themselves.  When
that has happened then you have a plug of tissue that can sit in 
water for hours without releasing much betacyanins.  But the main
release of betacyanins was not across the membrane, it was out of 
cells that were lysed!  So following the movement of betacyanins 
out of a beet tissue plug that is placed in water may not reflect
membrane transport processes as much as cell death.  This experiment 
needs to be run under conditions that are isotonic to the storage
tissue, and when it is then the cells will not be lysing due to 
high turgor and the leakage of betacyanins may be due to their 
crossing the membranes of living cells.  

I have most likely muddied the waters even more, but I hope that
this makes my point a bit clearer.


Aure Entuluva!




Scott T. Meissner, Division of Science and Mathematics
	McKendree College, 701 College Rd, Lebanon, IL  62254
	Tel: (618) 537-6934 
	E-mail: smeissne at a1.mckendree.edu
	You can't walk in step if you ain't got the beet.




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