Lab #3
Plasticity and stomatal densities in native and invasive species
Leaf stomata are the principal structures for gas exchange in plants. When open, stomata allow carbon dioxide to enter the leaf for the synthesis of glucose, but they also allow water to exit the leaf. Plants must be able to balance the benefits of carbon dioxide intake against the costs of water loss. Plants can regulate the relative rates of carbon dioxide intake and water loss by controlling when stomata are open or closed. Plants may also regulate the rates of carbon dioxide intake and water loss by varying the position of the stomata (top of leaf vs. bottom of leaf) or by varying the density of stomata (number of stomata per square mm on a shade leaf vs. a sun leaf). In other words, plants have a variety of strategies for balancing or optimizing carbon dioxide intake against water loss. There is no single “perfect” stomatal pattern that we observe in plants- more stomata are not always better, nor are fewer automatically better.
One characteristic that could be beneficial for a plant species is to have more rather than less plasticity when it comes to stomatal densities on different leaf types throughout the plant body. Within the entire body structure of a plant, there is a variety of microenvironments, and the optimal stomatal density should vary from one environment to another. However, not all species are equally plastic, so not all plant species may have the plasticity necessary to produce different stomatal densities on different leaves.
The goal of this lab is to test the specific hypothesis that Russian olive (Elaeagnus angustifolia) has a higher degree of plasticity than plains cottonwood (Populus deltoides). Russian olive is a highly invasive species in the western U.S., and it is rapidly replacing the plains cottonwood. Russian olive possesses physiological and morphological traits (shade tolerant seedlings, nitrogen fixation, spines,…) that give it a competitive advantage over the native plains cottonwood. In this lab, we will investigate whether a higher degree of plasticity may also contribute to Russian olive’s abilities as an invasive species. We will specifically determine whether Russian olive and plains cottonwood differ in their degree of plasticity and evaluate the hypothesis that Russian olive may benefit from a higher degree of plasticity and a greater ability to balance carbon dioxide intake against water loss.
Goals and Assignments:
In this lab, students will measure stomatal densities to determine whether Russian olive and plains cottonwood differ in their degree of plasticity. Each pair of students will turn in a formal data analysis (statement of basic question, performance curves, summary statistics, p values, and concluding statement) on Tuesday, October 3.
Experimental Protocol:
1. Organize your leaves by species (RO or CW) and location (interior or exterior).
2. On the underside of the leaves, paint a thick swath of clear nail polish.
3. After the polish has dried, take a piece of clear tape and firmly attach it to the area with the dried nail polish.
4. GENTLY peel the nail polish swath from the leaf- this will provide you with an impression of the leaf surface.
5. Tape your leaf impression to a clean slide.
6. Bring your leaf impression into focus at 400X magnification.
7. Locate an area on your leaf impression that is undamaged and count all the stomata.
8. Collect data from 6 pairs of leaves (3 RO ext and int, and 3 CW ext and int). Calculate a plasticity index.
9. Convert your data to stomata/mm2 (area of field of view = pi*radius2).
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