Question
I am currently working on a science fair project involving the transpiration rate of plants. I was wondering if you could perhaps offer me some assistance or ideas to  make my project the best that it can possibly be. Any help that you could give would be greatly appreciated.
Some general questions that i had were
What is the most accurate way to measure leaf transpiration rate?
What is the most accurate way to measure the surface area of a leaf?
What factors affect the transpiration rate of a leaf?
What types of plants have the fastest transpiration rates?
How can I control factors such as sunlight when I am measuring transpiration rate?


In my project, the effect of leaf diameter on transpiration rate will be investigated. Three leaves for each diameter will each be placed in a separate graduated cylinder filled with water. There will also be a graduated cylinder with no

leaf placed in it, to demonstrate that the water will not evaporate from the graduated cylinder, as all they graduated cylinders will be plugged at the top with clay. The amount of water in each graduated cylinder will be measured. All the graduated cylinders will be placed in the sunlight and the amount of water in the graduated cylinder will be measured every half hour. The more water that is removed from the beaker, the higher the transpiration rate is.


If you have any suggestions to improve this project, I would greatly appreciate any of your input.


Thank you for your time.  

Answer
If I understand right, the leaf stem (petiole) will be placed in water and clay will be packed around it?  That sounds good, although I don't immediately see why the clay is needed; the evaporation rate should be the same for all cylinders, so even though some water is lost through evaporation, it would still be a valid comparison.  If you have a control cylinder with no leaf, you would be able to tell exactly how much water evaporates vs. is lost through transpiration, right?  I think that would be the best way to set it up.

There are scientific instruments that can accurately measure surface area and I have used them for the exact purpose you want to.  You may be able to track one down at a local college, but that may be difficult.  Other than that, assuming the leaves you use are similar in shape, you could simply multiply the narrowest dimension by the widest, and get a unitless measure for comparison (call it a "surface area index" or something).

The most important factors that affect transpiration are heat, light, humidity, and air movement.  You could also argue that some species transpire water more quickly than others.  As your project will indicate, leaf area is also obviously a factor.

My *guess* is that very fast growing monocots like bamboo, sugarcane, or corn have the highest transpiration rates per unit surface area of leaf, and that xeriphytes like cacti have the slowest.

I don't think you need to control factors for your experiment.  Just do everything you can to have identical conditions among the treatments.  As long as all cylinders have nearly identical conditions, you will have a valid comparison.  Your only variable would be leaf area.

Also, it seems to me that you would want to use different size leaves of the same species.  Otherwise, you'd introduce a confounding variable.  That may be what you had in mind.

Also, I wouldn't bother trying to find "3 leaves" of each diameter; just calculate an area index (as I described above) and make some kind of graph plotting the relationship between area and transpiration rate.

You may also find that "every 30 minutes" is too often to measure the water, so be flexible.  Also, if you find that there aren't measurable differences, you could wait a longer time, use a narrower cylinder, or put several leaves per cylinder and measure the total area index per cylinder.  Also, you'll need to be careful to not let the water level get below the end of the petiole.  You could hypothetically add more water if needed during the experiment if you keep up with how much you add.

Good luck with your project!  Let me know if you need more help.

Trey