I plan to investigate the variation of solute potential in onion epidermal cells as distance from bulb surface varies. Do you have any tips?
This is an ideal topic for a practical investigation.
I am assuming that you know how to measure solute potential by finding the point of "incipient plasmolysis" when 50% of the cells are plasmolysed.
The problems you will encounter are many - and part of your investigation is to discover the difficulties and design ways of overcoming them.
The first tip is to make sure that you mount the epidermis "torn surface down" ie the surface that was in contact with the rest of the onion scale needs to be in contact with the microscope slide, so you view it through the smooth surface.
A second tip is that you will find that most of your slides are either 100% or 0% plasmolysed and you will become very skilled at making different concentrations of sucrose from a concentrated stock solution to try to find the 50% point. Strong sucrose is not as easy to mix with water as you might guess and a "whirlimixer" or vigorous shaking is needed to ensure that the solution is homogeneous.
A third tip is to use a scalpel to cut a piece of epidermis the right size BEFORE you strip it off the scale, with forceps - just cut through the epidermis and not through the scale. You will find that the cells at the edges of your piece plasmolyse easier than those in the middle, so you need to pop the epidermis in a dish of the right sucrose solution for several minutes and not just mount it on a slide with a cover slip on top. You will also find that the epidermis tends to roll up and you may find a small piece (or a large piece!) is easier to unroll - part of your investigation will be to see what works best for you.
Anyway, the thing to do is to "have a go" and to get started without worrying too much.
As to what results you should expect - that's up to you to make a best guess, based on what YOU know about the onion bulb and solute potential - so you know WHY you have guessed that particular pattern. Then you can see whether YOUR results fit YOUR prediction. I would be interested to know what pattern you find! Be prepared for your guess to be wrong, or even for there to be no pattern at all. The important thing is to get the experimental results and to let the onion tell you the way it is - your experiment is no less valid if it leads you to conclude that there is no reliable pattern, or that the pattern varies with some other factor which you hadn't thought of until you were part way through the experiments!
John Hewitson
