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What Is Transpiration in Plants? - Definition, Rate & Process

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  • 0:00 Definition of Transpiration
  • 0:45 Transpiration Process
  • 1:25 Transpiration Rates
  • 4:20 Lesson Summary
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Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Did you know that plants 'sweat?' Transpiration is both an important and costly process for plants, and it requires that a delicate balance between it and other necessary cellular processes be maintained.

Definition of Transpiration

Imagine yourself on a hot summer day. When you get thirsty, you drink water to rehydrate. But where does this water go? Some of it goes to bodily processes, but, on a hot day, you are likely sweating. Sweating, or evaporative cooling, is how your body prevents overheating. Water comes out through sweat glands and evaporates as it hits the air, leaving your skin feeling cooler.

Plants also 'sweat,' but this process is called transpiration. Plants use their roots in the ground to draw up water and nutrients. Plants also use much of this water for cellular processes, but some of it leaves the plant and goes into the air.

Transpiration Process

Similar to the sweat glands on your skin, plants have openings on their leaves that allow water to escape, called stomata (singular: stoma). Stomata are usually found on the underside of a leaf to reduce excess water loss, and they're surrounded by guard cells that open and close the pores.

Though stomata release water, their main purpose is to exchange gases. Plants need to 'breathe' carbon dioxide from the atmosphere in order to photosynthesize, or change sunlight into usable chemical energy. They also need to release oxygen back into the atmosphere as a waste product of cellular processes. This gas exchange occurs through the stomata, and, while this happens, some water is lost from the plant.

Transpiration Rates

A number of factors affect transpiration rates for plants, and the tricky part is regulating the amount of water loss while still exchanging the necessary amount of gas through stomata. Plants naturally transpire at different rates, but there are some factors that can affect the rate of water loss for all plants.

Stomata can regulate the rate of transpiration by simply controlling how open or closed they are via the guard cells. Stomata that are open wider will facilitate a greater rate of transpiration, while stomata that are more narrowly opened will reduce the rate of transpiration. Having more stomata will also allow for more transpiration. Having more or larger leaves also increases the overall number of stomata and will, therefore, increase the rate of transpiration as well.

Environmental factors can have an effect on transpiration rates. Since stomata open during photosynthesis to take in carbon dioxide, more light will signal the stomata to open, which will in turn increase the rate of transpiration. Transpiration rates also increase as temperature increases. Warmer temperatures are usually associated with sunlight and the growing season, so higher temperatures cause guard cells to open the stomata, while cold weather signals the guard cells to close the stomata.

The amount of water available plays a large role in transpiration rates. Think back to our sweating analogy. If you sweat more than you drink, you get dehydrated. The same happens to a plant, so when there is less water available for the plant to take in, there will be less water for the plant to lose. This signals the plant to slow transpiration in order to avoid losing too much water.

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