How do floating discs correspond to the rate of photosynthesis?

Floating leaf disk assay procedure is a method that is used in the biological laboratory to measure the rate of photosynthesis in plant tissues. In this procedure, a vacuum is used to remove trapped air and infiltrate the interior of the plant sample with a solution containing bicarbonate ions which serve as a carbons source for photosynthesis.

The rate of photosynthesis in the disk of the leave tissue is inversely proportional to the time required for submerged leaf disk to float.