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If you're able to isolate pieces of a leaf that are deep purple, and use, say, a hole punch to make circles in such a leaf, an experiment can be formulated.
Prepare a volume of water with 0.2% sodium bicarbonate (Baking soda, with 0.2% being .2g per 100 mL of water). Punch out 20-24 circles of leaf using a hole punch, and 20-24 circles of a green leaf to use as a control (fresh spinach works well).
Place the circles into a 10 mL syringe along with ~8 mL of the bicarbonate solution, plug the end with your finger, and *pull* on the plunger, creating a vacuum. You should see bubbles form on the outside of the leaf circles. This is oxygen leaving the leaf. All the leaf disks should sink.
Fill 4 cups or beakers about 2/3 full with sodium bicarbonate solution.
Carefully remove the leaf pieces and separate them into 4 cups or beakers: Two cups with 10 green each, two cups with 10 purple disks each. Place one of each cup in direct sunlight or another bright light, and the remaining two (one green one purple) in the dark.
If the experiment was set up correctly, all the leaf disks should start on the bottom of the beakers. If the leaf disks photosynthesize in the light, they should become more and more oxygen rich, therefore floating up to the surface. This process will take a number of minutes, and not all the disks may float. If you're looking for solid data, you may want to record the times at which the disks began floating, checked possible every 1-2 minutes.
The green leaves and the leaf disks in the dark will be regarded as controls. Basically, you compare the behavior of the experimental (the purple disks in the sunlight) to what normally happens with other "normal" leaves.
Hope this helps!
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