visualizing potato plasmolysis

Jon Monroe MONROEJD at jmu.edu
Wed Jan 31 09:58:41 EST 1996


John Hewitson wrote: 

> We have done a similar practical for many years, but our solution has been to
> use Rhubarb petiole epidermis (some of the cells contain pigment), or more
> recently, Onion bulb scales' epidermis (which being 1 cell thick, is easy tissue
> in which to observe plasmolysis).

I received several other similar replies like one from Carl Pike:

> ... I use leaves with a purple pigment in the
> vacuole of the epithelial cells, such as Rhoeo discolor. True, the students
> can't measure osmotic potential on the SAME plant that they used for water
> potential, but the results with these purple-pigmented are so dramatic and
> easy to see!

And one from at Frank Hoffman, Slippery Rock University:

> I use Elodea for this.  Half the groups used a graded sucrose series, and half
> used a graded NaCl series. When we converted concentration to bars, the values
> came out the same-- somwhere in the range of -9 - -11 bars.

The problem with neutral red staining was pointed out by Diane C. Robertson,
Grinnell College:

> Neutral red staining of vacuoles is highly pH dependent.  I have used it to
> stain onion epidermal cells; the pH in which vacuolar staining occurs is
> around 7.5 or above.  It takes that high a pH to assure a high
> concentration of the molecular form of the dye which is the only form that
> can diffuse into the cells.  At lower pH's the ionic form of the dye
> predominates and because it is charged (+) it will not pass the plasma
> membrane but only stains the walls.  The color of the staining solution
> changes with pH reflecting the relative abundance of molecular (yellow)
> form and the ionic form (red).  You can determine the relative abundance of
> each in your solution quite easily by extracting a few mls with chloroform.
> The molecular species will dissolve in the chloroform and the ionic
> species will stay in the aqueous layer.  The intensity of color in each
> phase shows how much of each you have.  Hope this helps.

Back to John:

> What puzzled my students and me was that the Pressure Potential which we
> calculated from the above gave us a figure of about 10 atmos - which seemed
> incredibly high - about the pressure of a lorry tyre!! Can this be true?

That value is similar to what my students got yesterday... 

Beets turned out to be an excellent tissue for measuring both water potential
(cylinders in sucrose) and osmotic potential (incipient plasmolysis), the later
aided by the pigmented vacuoles.  I have not yet learned of a way to stain
potatoes but I recall now that there are varieties of potato with purple flesh! 
I don't have access to any locally to try...  One problem we saw yesterday was
that the plasma membrane of potato cells is quite fragile.  Once the cells
plasmolyzed (rapidly) the plasma membrane broke releasing the amyloplasts so
they no longer appeared bunched in the center of the cells.

Thanks to all for your input!

Jon

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