Greedy Plants: Water use and nighttime transpiration in the tallgrass prairie

This post is a highlight of a research article recently published by K-State alum Dr. Kimberly O’Keefe and her PhD advisor, Dr. Jesse Nippert.

On the Biology Division office front desk sits a small bowl of candy. Students often grab one or two pieces on their way in and out of the office, as expected, but you would not necessarily expect a student to take the whole bowl. Likewise, you would not expect other organisms to use more resources than they need, not necessarily because they have manners, but because it is not efficient to gather more resources than necessary. Yet recent evidence suggests that in some grasslands, certain plants may be doing just that—absorbing more than they need to cripple their neighbors’ success. These greedy plants are not hoarding candy, but water, which is essential to life.

Understanding the way that water moves is critical to the development of water management and irrigation plans; scientists and farmers need to know when and where water will be available. At first glance, the journey of water may seem simple—rain falls from clouds, and is evaporated by heat to form new clouds. But the water cycle is much more complicated—water can be trapped underground, stored in ice, drunk by animals, and absorbed by plants. How does the rain absorbed by plants make its way back to the atmosphere?

Rainwater is important for photosynthesis, the process through which plants use water’s hydrogen and oxygen components to convert the sun’s energy into sugars—the stored energy we gain when we eat plants. In addition to water, photosynthesis also requires carbon dioxide from the atmosphere. Without a mouth to breathe in this gas, plants have evolved small openings in their leaves called stomata. By opening these stomata, plants can let carbon dioxide in, but this also allows water to evaporate out of the plant back into the air, a process known as transpiration (check out our previous post about ongoing research at K-State to find out more about how stomata and other structures affect water loss in plants).

For decades, researchers thought that water loss by transpiration only occurred in the daytime—that plants opened their stomata in the daytime to get carbon dioxide for photosynthesis and closed them at nighttime to conserve water when there is no light to perform photosynthesis. Because of this, scientists did not include nighttime transpiration in their water cycle models. In recent years, however, they have discovered that plants often keep their stomata open and lose water at night. This discovery affects the way we understand water movement, but we still do not understand what environmental factors lead plants to keep the stomata open in the first place. Without understanding the conditions that lead to nighttime transpiration, we cannot reasonably account for its effect on local and global water movement patterns. Enter Dr. Kimberly O’Keefe.

Grasses and other plants in the tallgrass prairies of Kansas open their stomata for nighttime transpiration. Dr. Kimberly O’Keefe measures environmental conditions around these plants to figure out what leads to nighttime transpiration. Photos courtesy of Dr. Kimberly O’Keefe.

As a doctoral student at Kansas State University, Dr. O’Keefe set out to investigate the nighttime transpiration of common tallgrass prairie plants. Dr. O’Keefe sampled common grasses, flowering plants, and shrubs. She used a sophisticated array of sensors to measure environmental factors around selected plants, including air temperature, humidity, and soil moisture. Over two summers, Dr. O’Keefe went out to her study plots in the daytime and nighttime to measure the gas exchange occurring on individual leaves. By relating this measure of transpiration to the measurements of the soil and air around the plants, Dr. O’Keefe identified the environmental conditions that lead to nighttime transpiration.

Left: Dr. Kimberly O’Keefe uses a hydraulic corer to take a deep soil sample. Right: A gas analyzer that allows scientists to measure transpiration. Photos courtesy of Dr. Kimberly O’Keefe.

Dr. O’Keefe not only found that all of the sampled plants transpired at nighttime, but also that this transpiration could contribute more to the movement of water than previously thought. The data also indicated that transpiration is related to air moisture and soil moisture. Perhaps most intriguing, though, is her discovery that transpiration rate varies by plant type; of the three plant groups studied, grasses had higher rates of nighttime transpiration, especially when the soil was very wet.

Why would grasses allow so much transpiration? In a recent paper in the scientific journal Functional Ecology, Dr. O’Keefe suggests that these grasses may be intentionally losing water when the soil is very wet, an idea at odds with scientists’ previous understanding of water use. Of all the plants studied, these grasses are the best at dealing with dry conditions like droughts—when the soil is driest, they are better at out-competing the other plant types. Using nighttime transpiration, these greedy grasses may be drying wet soil to intentionally harm their competing neighbors, like a student who takes the whole bowl of candy from the front desk and dumps it so the other students have none.

Dr. O’Keefe’s work on plant transpiration has contributed to our understanding of water cycling, prairie water use, and plant interactions. Since her graduation from Kansas State, she has continued her work on related questions as a postdoctoral researcher at the University of Wisconsin-Madison. Her current projects include using stable isotopes to understand competition for water among plants in grasslands, and studying nightly uptake of dew by trees in the Madison Arboretum. To learn more about her work visit her website or the website of her PhD advisor, Dr. Jesse Nippert. Read her Functional Ecology article here.

sarah_winnickiThis post was written by Sarah Winnicki. Sarah is a Master’s student studying how cowbirds (which lay their birds in other birds’ nests) affect the growth and development of threatened grassland songbird nestlings (for more information check out


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