Changing Planet: Withering Plants - Stressing Over Lost Water
Students learn about how plants exchange gases and materials with their environment, and how they respond to stress. Next, they apply their understanding of the role of leaf stomata in regulating leaf temperature by expelling water through their openings to hypothesize about the quantity and variability of leaf stomata in various environmental conditions. Students then extract leaf stomata and examine them under the microscope to describe and quantify them in response to their hypothesis. They discuss their results with other teams in order to develop consensus about the role of stomata to regulate water loss in a plant.
Developed by NESTA/Windows to the Universe team members Missy Holzer, Jennifer Bergman, and Roberta Johnson, using resources from Windows to the Universe
7-9, although it may be adapted to higher grade levels using the extensions below
Student Learning Outcomes:
Hands-on laboratory investigation
- For background information on the how climate change impacts crops watch the Changing Planet episode, Withering Crops. Also, explore these topics on the Windows to the Universe website.
- Gather enough lab supplies for student teams of 3 or 4 students. Print out the student worksheet if you are using it instead of student journals. If time is an issue, prepare slides yourself, or acquire prepared slides from a science supply vendor. This activity assumes students have had some experience with microscopes. If students have little or no experience using a microscope, spend time orientating them to the proper usage and handling of the instrument.
- Introduce the lesson by showing the students 2 plants - a lush plant and a wilted plant. Brainstorm the conditions that led to current state of each plant. Students should respond with a list of the necessary components of plant growth - sunlight, water and carbon dioxide. Next discuss the pathway of each of these components, leading students to discuss the role of leaves in regulating the exchange of gases and the loss of water. Show students images of various types of plant stomata and discuss the role of the guard cells in regulating the flow of water and gases in the leaf under various environmental conditions. A higher stomata density is common during the growing season when greater control is needed over water loss and carbon dioxide uptake. Photosynthesis is optimized under moderate temperatures, and if temperatures should rise, the plant will release water through the stomata in order to cool.
- Distribute student worksheet, or alternatively, ask students to open their journals. Begin with the question: Is there variability in the stomata founded in wilted plants and fresh plants? Ask students to hypothesize about this question using the background knowledge from the previous step and the video.
- Slide preparation - Materials should be at lab stations ready for students to start the laboratory. Model the slide preparation process for the students while walking students through the procedure. Remind students to be patient while the nail polish dries. Once it is dry, they use the clear tape (cut to a size that will fit on the microscope slide) to extract the nail polish with the stomata casts. Next, they tape the stomata casts in place on the microscope slide, and label the slide. They should trim any excess tape from the slide. Now they are ready to collect data and make observations. Students may need extra practice with preparing the slides. Therefore this step may take longer than anticipated, and in this case, slide preparation may take place on one day, and data collection on another.
- Assist students with their microscope observations, and be sure they know what to look for. They are to count the number of open stomata, and the number of closed stomata on each slide. Once the data has been collected, they can create a bar graph representing the data. If time is available, they can use a graphing program to create the graph, and then present the data to their classmates using electronic means.
- Ask students to share their data with their classmates. Lead a class discussion that reflects on the data collect, laboratory methods, error, and ideas for future research.
Student should be evaluated on their laboratory skills and analysis. Consider using the extensions above to assess student understanding of the lesson concepts and their laboratory abilities.
Nail polish is volatile. Prepare slides in a well-ventilated room. Monitor student use of sharp and glass objects at all times. Glassware may break and create extremely sharp shards. Have clean-up materials handy if breakage should occur.
You may either save the slides for another time if they have been properly labeled, or discard them in a trash receptacle. Discard leaves in the trash or compost pile. Return all other materials to their proper location for future use.
- Allow students to develop their own stomata study, having them form a new question and hypothesis based on their own background research. Students should be given time to gather information, and then present their question and hypothesis to you for approval before beginning their work. Venture outside in the growing season to select plants that are actively growing.
- Besides extreme temperatures, soil water availability is another reason why plants stress to the point of dehydration and death. Depending on the ratio of soil constituents (organic matter, sand, silt, clay) some soils may retain water longer than others. Students can explore water retention in various types of soils by performing a controlled experiment. They can measure the mass of two dry soil samples, add a known volume of water to the samples, and determine the rate of evaporation after determining the length of time it takes for the sample to dry. Adding organic matter and clay to soils tends to enhance soil water retention.
"Though we often take the plants and trees around us for granted, almost every aspect of our lives depends upon them. They feed us, cloth us, absorb carbon dioxide, provide us with oxygen, and give us building materials and medications. When drastic changes occur to the vegetation around us, our health, economy, and environment are all affected." (from a NASA Earth Observatory Article)
Our Earth's global temperature is drastically changing; it is warming due to the addition of heat-trapping greenhouse gases which are increasing dramatically in the atmosphere as a result of human activities. Global warming changes the amount of precipitation received in different regions of the world, and it also increases the intensity and frequency of extreme weather events such as droughts and heat waves. These drastic changes are affecting vegetation already, with more changes yet to come.
Is there a bright side to this story? The Earth is a place of extreme bounty - its crops and plants feed over 6.8 billion people worldwide, as well as forming the food web base for more than 1.25 million species of animals. One of the main greenhouse gases that humans contribute to the atmosphere that is a cause for increased global warming is CO2, carbon dioxide. More CO2 can actually be beneficial to plant growth. It is a necessary ingredient for photosynthesis, along with water from the soil and energy from the Sun. Increased CO2 concentrations can actually increase the rate of photosynthesis, though scientists agree that in real world situations, other factors that accompany higher CO2 concentrations combine to make photosynthesis decline.
Another consideration to take into account when talking about crop productivity and global warming is the world's population. Researchers project that the Earth's population could surpass 9 billion people by 2050. That's over two billion additional people that need to be fed every day. Combine this increase in global population with the fact that many plants are susceptible to stress due to warmer temperatures and drier conditions that are brought on by climate change, and it is clear we are on troubling path.
How do plants deal with increases in temperature? During the process of transpiration, plants lose water through tiny holes in their leaves called stomata. Because the water evaporates from the surface of the leaf, transpiration helps plants stay cool, in the same way perspiration keeps humans and animals cool. So the process of plants losing water through their stomata prevents overheating of the plant. Global warming is also projected to bring less precipitation or increased drought conditions in many parts of the world. How do plants deal with a lack of water? When less water from the soil is available, the plant closes the stomata on its leaves so water doesn't escape from the plant. Unfortunately, closed stomata don't let in necessary CO2 for photosynthesis and plant growth becomes stunted -- the plant must now choose between keeping its own water (stomata closed) and gaining food (stomata open). At the point when the plant cannot survive any more without food, it will be forced to open its stomata, letting its own water escape. The plant, which is fighting to survive, becomes wilted or withered.
Increased temperatures and drier soil conditions bring about crop withering, and crop withering has huge social implications. Even in the last decade, watching the world news would have tipped you off to the fact that drought produces crop withering and crop withering means food shortages as could be seen in Ethiopia in 2000, Niger in 2005 or Somalia and Haiti later in 2008. And before then, many were touched by photos and news that came out of the Sahel region of Africa, where thousands of people starved when the area's vegetation dried up during an extended drought. Of course, in what are very complex situations, drought and overheating, crop failure and depleted livestock can lead to famine, political unrest and in extreme cases, military conflicts like civil wars. Lest you think that these happenings will only occur on a continent or island far away, the IPCC has projected that Africa, Asia, Australia and New Zealand, Europe, Latin America, and small islands around the world will be adversely affected by climate warming and that crop yields would decrease to a certain extent in those areas. For those areas not as adversely affected (like North America and the Arctic), peoples in those regions will be affected in that rising temperatures and the heat and drought it brings will in the long run force food prices to rise as crops do wither and fail around the world. And, of course, as a global community, we are all affected when environmental problems cause the suffering of people around the world, whether they are the root of the problem like famine or only part of the problem like in the Darfur Conflict. These events that cause casualties, mass migrations of people away from their homeland, and the suffering of those that can't defend themselves become humanitarian crises that affect us all.
Luckily, people around the world and scientists are helping with these crop withering problems that could cause global crises. One way scientists are helping is through the Famine Early Warning Systems Network. FEWS NET is a decision support system that helps target more than two billion dollars of food aid to more than 40 countries each year. Through this system, scientists are helping with early identification of agricultural drought that might trigger food insecurity. Also, agricultural research worldwide is working on how to increase the yield of crops without consuming more resources like water or fertilizer. The research of these complicated subjects will no doubt lead to great discoveries that will ultimately feed the growing population of the world.
Even though global warming is in part brought about by increased CO2 and increased CO2 can actually increase the rate of photosynthesis, researchers have shown that overall photosynthesis diminishes with rises in air temperature. Many plants, especially grain crops that feed the people of our world are susceptible to warmer temperatures and dryer conditions/drought brought on by climate change. Our hope is that science, research and technology will bring about feast and not famine as we pore over these issues and figure out new and innovative solutions to the challenges set before us.
RELATED SECTIONS OF THE WINDOWS TO THE UNIVERSE WEBSITE:
- Changing Planet: Grapevines and Drought Classroom Activity and Video
- Effects of Climate Change Today
- Carbon Dioxide - CO2
- Heat Waves, Storms and Drought - Extreme Events and Human Health
- Soil Science Society of America Education Resources
- Earth Observatory drought page
- Earth Observatory drought images
- National Agricultural Statistics Service