|MadSci Network: Botany|
From your description, it seems like you propose to have a cut shoot in a flask of water to measure transpiration. This is reasonable, however, it would not necessarily be the same as measuring transpiration of a potted plant which has a root system. Cut stems in water can get clogged by microbes or xylem blocked by air when they are cut. If you use cut shoots or cut flowers, you might also wish to investigate the effects of recutting stems under water and flower preservatives, such as Floralife, on transpiration of cut shoots or cut flowers. Covering the shoot with a plastic bag may make it difficult to accurately measure transpiration because the high humidity in the plastic bag can reduce transpiration. Probably the best way to measure tranpiration is not to cover the plant with a plastic bag but to use a balance and measure weight loss. One milliliter of water weighs one gram. For a potted plant, enclose the pot in a plastic bag and tie it around the stem. This will prevent water loss from the soil surface. For a cut shoot in a flask, place plastic film over the flask opening to prevent evaporation from the water surface. Another common technique is the potometer (see websites under references). It uses a cut shoot but can be tricky to set up. The advantage of a potometer is that it can give rapid results. Your treatments of light and dark are reasonable but having cold, warm and hot treatments in the dark probably will not give much difference because stomata close in the dark. If possible, it would be better to have temperature treatments in the light. One common treatment is to place plants in front of an electric fan to determine wind speed effects on transpiration. It is also easy to coat leaves with petroleum jelly to determine the effect of blocking stomata on transpiration. You can coat top leaf surface, bottom surface or both surfaces. Another type of experiment is to compare transpiration for different kinds of plants, such as cactus and tropical houseplants. With potted plants, you can also water the pots with salt or fertilizer solutions and see how that affects transpiration. One of the nice things about short term transpiration experiments with potted plants is that you can often use the same plant over again. For example, one day the plant can be placed in light, next day in dark, next day in front of a fan, etc. Be sure to have replication, such as 3 to 5 plants per treatment. One thing that can add to the experiment is to measure the leaf area after the experiment is completed and calculate transpiration rates on a leaf area basis. Leaf area can be measured by tracing leaves on graph paper and counting the blocks. Another measurement often done is the stomatal density (stomata per square millimeter). The technique requires a microscope and nail polish. References Hershey, D.R. 1995. Plant Biology Science Projects. New York: Wiley. Transpiration Lab Potometer Experimenting with the Potometer Transpiration Experiment
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