Leaf Floating Techniques

Smooth broad leaves like maple make good floating leaf assay subjects.

The floating leaf disk assay is a means of demonstrating photosynthesis. Small disks are cut out of smooth leaves, then treated so as to draw a carbon dioxide-rich solution into them. This causes the leaf disks to sink in water. When a bright light source is applied, the leaf disks begin to create oxygen. As the oxygen infuses the leaf interior before release, the disks begin to rise up and float in the solution.

Leaf Preparation

Use a leaf floating assay test to demonstrate how photosynthesis works in dramatic fashion to school students. To make the demonstration work best, use smooth, broad leaves. Stay away from varieties with fuzz on them. To make it easy to infuse the leaves with carbon dioxide, small sections of the leaves are taken for the demonstration. Usually a paper punch is used to create disks, but a firm paper straw can be used to cut disk sections from the leaves. Take leafy sections, avoiding areas with prominent veins.

Carbon Dioxide Infusion

Create a 2 percent carbonate solution by mixing 1/8 tsp sodium bicarbonate, or common baking soda, with 300 ml water. Leaves feed on carbon dioxide, producing oxygen as a waste byproduct as photosynthesis occurs. The carbonate solution is used to infuse the leaf segments with carbon dioxide. Too much baking soda in the solution will cause bubbles to form on the leaves, defeating the purpose of the assay. A tiny bit of liquid soap is added to the solution to help the leaf disks absorb the carbon dioxide. Disks are put, with solution, into a syringe without a needle. Holding a finger against the open end, the teacher will withdraw the plunger for 10 seconds, creating a vacuum to help facilitate absorption, then releasing his finger to empty the vacuum. He will repeat this 3 or 4 times.

Control

Create a control to demonstrate the difference between carbon-infused leaf segments and ordinary segments. Another set of leaf disks is prepared in the same way that the carbon dioxide-infused disks are, except that only water and a little liquid soap are used for the solution in the syringe. These disks are given the same vacuum treatment in the syringe as the other disks. In both cases it helps absorb any existing bubbles in the sections of the leaves so they will sink when first dropped into clear plastic cups with the same baking soda solution used to infuse the experimental leaf segments. The control disks and the infused disks are put into separate marked cups, each containing the identical solution.

Photosynthesis

Put the two cups under a bright light source. The disks infused with carbon dioxide will begin floating to the top after about a minute and steadily continue until all eventually rise to the top. This is caused by the leaves, in the presence of bright light, processing the carbon dioxide and creating oxygen. The oxygen bubbles that form give the leaf disks buoyancy. The leaf disks in the control group, though dropped in the same solution and exposed to the same light, will remain at the bottom of their cup. Eventually, they would undergo enough photosynthesis to float, but the experiment demonstrates that the more carbon dioxide in the leaf at the time it is exposed to light, the faster it processes the carbon dioxide and creates oxygen.