Photosynthesis and Leaf Adaptation: How Sun and Shade Plants Respond to Light

In this practical investigation, students measure the rate of photosynthesis in plants from different habitats (sun and shade), and then consider what this demonstrates about leaf adaptation.

When leaf discs are immersed in a sodium hydrogen carbonate solution (a source of carbon dioxide) and illuminated, the oxygen produced by photosynthesis causes the leaves to float. The time the leaves take to float can thus be used as an indirect measure of the rate of photosynthesis - that is, the more quickly flotation occurs, the faster the rate of photosynthesis.

Leaf discs from sun and shade plants are illuminated with white light and the times taken to float noted. The experiment is then repeated, this time illuminating the discs with green light.
The experimental results should mimic the conditions in the plant’s natural habitat. For example, the sun plant in the canopy will receive white light and absorb the blue and red light from it in order to photosynthesise. However, the shade plant will receive the light that has already passed through the canopy. In order to photosynthesise, it may therefore have to absorb many other wavelengths of light, such as green.

This practical was developed by Science & Plants for Schools (SAPS) for the Scottish Highers, but the protocol can equally well be used for other UK specifications.