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Instructor: Chris Gillen Ryn Edwards Pat Heithaus Wade Powell Jennifer Smith

Phenotypic Plasticity in Oaks

The Beech family (Fagaceae) includes several species of red oak, which can be difficult to distinguish. The Northern red oak (Quercus rubra) is common in Ohio and throughout the East. It is a rapidly-growing, moderately shade-tolerant species that has commercial value.

Depending on light, temperature and moisture, oaks have several spurts of growth during a single growing season. Like all plants, oaks are built on a modular plan in which subunits such as leaves and branches are added as they grow. All of these modular units contain the same genetic information.

Begin by examining a group of red oak leaves. List several ways these leaves differ. Please do not change your responses as you work through this exercise.

Do you think the leaves are all from the same species of red oak? yes no unsure

If the leaves are all from the same species of oak, how might you explain the variability in shape?

genetic variation between individuals trees are growing in different habitats leaves are from different microhabitats all of the above

The leaves you examined are all from the same tree. From left to right, the leaves were exposed to increasing amounts of light as they developed. How does the size and amount of lobing change as the light level increases? Is there a pattern?

One of the most important resources for plants is sunlight. Plants convert the electromagnetic energy of the sunlight into a form of energy that can be used to convert carbon dioxide and water into glucose, the process called photosynthesis. Oaks leaves do not require full sunlight to reach full photosynthetic capacity. For oak seedlings, the US Forest Service estimates the amount of light required for full photosynthetic capacity is about 30% of the light intensity found in the open sun. This has implications for the adaptive geometry of oaks and their leaf shape.

Consider the fact that not all leaves grow at the same level in the canopy.

• Generate an experimental hypothesis that relates leaf shape and the level of sun exposure.
• Suggest a null hypothesis.
• What would you predict if your experimental hypothesis is true?

Since leaves are the focus of next week's lab we should briefly review leaf structure.

The three basic types of tissue seen in a cross-section of a lilac (Syringa spp.) leaf are similar to those found in oak leaves.

• Epidermis - consists of an upper and lower layer of cells that are in contact with the environment. Sometimes the epidermal layer is covered with a waxy layer known as a cuticle. The cuticle helps prevent water loss, but makes gas exchange difficult. Leaves with cuticle solve this problem by having small pores in the lower leaf surface. These pores, or stomates, are controlled by the guard cells on each side of the opening. The opening and closing of guard cells depends on the amount of water in their interior. When guard cells lose water, they become flaccid and the pore closes. This helps prevent water loss. When guard cells are turgid, the pore opens and water vapor, carbon dioxide and oxygen can be exchanged with the external environment. Trichomes also help prevent water loss by reducing air flow across the surface of the leaf.
• Mesophyll - forms the middle cell layer and is divided into palisade and spongy mesophyll layers. The mesophyll cells contain variable numbers of chloroplasts, which are the sites of photosynthesis. The chloroplasts have a light reddish stain and appear as granules. Examine the magnified image. Which of the two layers has the higher density of chloroplasts? spongy mesophyll palisade meosphyll they are the same
• Veins - are found within the mesophyll. Vascular bundles conduct water and nutrients as well as provide support.

Compare the structure of lilac leaves growing in full sunlight and shade. How do the mesophyll layers of sun and shade leaves differ?

In full sunlight, some of the energy reaching the leaf surface penetrates into the inner cell layers. Leaves growing in full sunlight take advantage of this by developing thick layers of photosynthetic palisade mesophyll cells. In contrast, leaves growing in the shade receive less intense light. In order to intercept sufficient light, they tend to expand their surface area at the expense of a thick mesophyll layer. Given the expanded mesophyll layer, it is possible that leaves found in full sunlight would have more chlorophyll per unit area than leaves from the shade..

Now compare a close up of lilac leaves grown in the sun vs. shade. In which type of leaf do you see the higher concentration of chloroplasts? shade sun no difference

Within an individual plant, all leaves contain the same genetic information, but they respond to this information differently depending on their loction. This plasticity of development increases the overall fitness of the plant if it maximizes total productivity. The ability of an organism to respond to different environmental signals by altering its morphology or physiology is known as phenotypic plasticity.

What are some of the adaptations one might predict?

• We have already seen that sun leaves are thicker than shade leaves. Based on this observation, you might also expect that they would weigh more per unit leaf area. This can be tested by determining the leaf area and weight and calculating the specific leaf weight (weight/area). Which leaves would you expect to have the higher specific leaf weight (SLW)? shade sun no difference
• Sun leaves have a thicker palisade mesophyll layer than shade leaves. Which leaves would you expect to have the higher chlorophyll content/area? shade sun no difference

Iin lab this week, you will test some of the predictions made during this tutorial and explore the ability of the Northern red oak (Quercus rubra) to modify leaf structure in response to their location in the canopy.

Statistics question of the week:

The leaves you will be working with this week will all come from the same tree to assure that they have the same genotype. Within the tree, the development of each leaf is independent of the development of any other leaf and depends on its position on a branch. Each leaf exists in a slightly different microenvironment. We will collect 16 sun leaves and 16 shade leaves from a single tree. What test would be appropriate to use if you want to determine if the mean surface area of sun leaves is different from the mean surface area of shade leaves? (Use your statistics guide to help you answer this question and defend your answer.)

Now take a look at the paper on leaf shape that can be accessed from the Biology 109 Moodle page. It presents an additional hypothesis about leaf shape.