Does anybody know any science experiments based on photosynthesis?

you could plant a seed in three pots. put one in the dark constantly, another in light constantly and one outside for a control. water them all at the same time with the same amount of water and record which plant has the most progress in which it grows.

You don't say for what grade, but some of these sites list projects for all levels, so you're sure to find one you like. Good luck with it!

http://photoscience.la.asu.edu/photosyn/education/...

www.juliantrubin.com/fairprojects/.../photosynthesis.html

http://www.sciencefair-projects.org/biology-projec...

http://www.ipl.org/div/projectguide/choosingatopic...

http://www.biotopics.co.uk/plants/psfac2.html