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A potometer measures the rate of water loss from a plant’s leaves. It can be made by inserting a stem with leaves into a plastic tube connected to a pipette filled with water. The water level in the pipette is observed over time to assess the rate of water uptake. The bubble potometer is a common design used in classrooms. Transpiration rates are affected by environmental factors such as temperature, light, humidity, ventilation, and water availability.
A potometer, or transpirometer, is a device used to measure the rate of transpiration, or the rate of water loss, from the leaves of a plant. Potentiometer readings typically vary based on environmental factors, such as temperature, light, humidity, ventilation, and water availability for the plant. A very simple potentiometer can be made by snugly inserting a plant stem with leaves into a piece of plastic tubing and connecting the tubing to a pipette, or graduated glass tube, filled with water. It is important to prevent air from entering the apparatus, so potometers are often assembled underwater, with everything submerged apart from the leaves. Changes in the water level in the pipette are noted over time to assess the rate of water uptake by the plant, which corresponds to water loss from the leaves.
Plant roots absorb water and minerals from the soil and transport them from the stem to the leaves through a specialized tissue known as xylem. Xylem consists of numerous small channels that run vertically throughout the plant. When water reaches the leaves, it evaporates through openings called stomata. Because water molecules tend to stick together, this evaporation from the top of the plant exerts an upward push on the vertical columns of water in the xylem. By setting up a potentiometer experiment, transpiration rates can be measured as various environmental factors are changed.
One type of potentiometer design commonly used in the biology classroom is the bubble potometer, in which the rate of movement of an air bubble within a column of water is used to measure the transpiration rate of a plant. The bubble is introduced into the system by drawing a small amount of air into the end of the glass tube connected to the implant, before immersing the tube in a container of water. As the water is absorbed by the plant you will see the bubble move along the tube and the marks made along the side of the tube allow you to measure how fast the bubble is progressing.
Using the potentiometer, transpiration rates can be evaluated in different situations. Leaf stomata tend to open in response to light, so more light means more transpiration. Insufficient water supply causes the stomata to close, which reduces transpiration. It is generally found that higher temperatures and drier air around leaves increase water evaporation and cause a faster rate of transpiration. Steady conditions allow water vapor to build up around the stomata, discouraging evaporation, so windy conditions tend to accelerate transpiration rates.
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