LIGHT DEPENDENCE OF PHOTOSYNTHESIS IN SUN AND SHADE PLANTS
...This process is called photosynthesis and is carried out mostly in the leaves of a plant. Obviously, the more leaves receiving direct sunlight, the more energy will be captured for growth, reproduction, etc. A three dimensional structure such as a tree has some of its leaves potentially shaded by other leaves. It would be most efficient for a tree to have some mechanism for allowing some sunlight to get past its outermost leaves and stimulate photosynthesis in its interior leaves. Many species of trees are adapted so that sunlight can penetrate the interior of the canopy and subsequently be absorbed by shaded leaves to maintain a high photosynthetic rate. The relationship between light intensity and photosynthesis is curvilinear; thus, a leaf does not have to be in full sunlight to photosynthesize at a maximal rate. Any leaf receiving about 250 w/m2 of sunlight, even if partially shaded, will photosynthesize as well as leaves that are more exposed. A tree with a large canopy can have adaptations to avoid excessive shading of the interior leaves. One of the ways of helping the entire tree to do maximum photosynthesis would be to have the outer leaves shaped to allow more light into the interior. Leaves in the deeper layers are shaped to intercept most of the fight received (Figure 2). In addition, leaves on the canopy edge have thicker layers of photosynthesizing cells because intense light penetrates the leaf to a greater depth. In this laboratory exercise we will investigate whether or not there is a difference in leaf shape between exterior (sun) and interior (shade) leav...