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B2 U) Investigating the Rate of Transpiration
B2 U) Investigating the Rate of Transpiration
We can estimate the rate of transpiration by using a piece of equipment called a potometer. However, a potometer measures the water uptake by a plant rather than the transpiration rate. But, as plants have transpiration streams, we can assume that the water uptake by a plant is in direct proportion to the loss of water from a plant due to transpiration. Therefore, by measuring the water uptake by a plant, we can get a good idea of the transpiration rate.
We can use a potometer to investigate how different conditions affect the rate of transpiration. In the previous section, we learnt that the rate of transpiration is influenced by light intensity, temperature, air movement (wind) and humidity.
When we are investigating the effect of one of these variables (e.g. light intensity), we must make sure that we keep all of the other variables constant (temperature, air movement and humidity) and only change the variable that we are investigating (in this case light intensity). This is so that we can say for certain that the change in transpiration/ water uptake that we observe is solely down to the variable that we changed.
We can use a potometer to investigate how different conditions affect the rate of transpiration. In the previous section, we learnt that the rate of transpiration is influenced by light intensity, temperature, air movement (wind) and humidity.
When we are investigating the effect of one of these variables (e.g. light intensity), we must make sure that we keep all of the other variables constant (temperature, air movement and humidity) and only change the variable that we are investigating (in this case light intensity). This is so that we can say for certain that the change in transpiration/ water uptake that we observe is solely down to the variable that we changed.
Using a Potometer
There is a diagram of a potometer below.
There is a diagram of a potometer below.
The potometer is set up under water. The plant shoot is cut under water at an angle. The shoot is cut underwater to avoid any air bubbles entering the xylem tubes because air entering the xylem tubes will affect transpiration streams. The shoot is cut at an angle so that there is a greater surface area for water to be taken up the xylem tubes.
We must make sure that all of the apparatus is watertight and airtight. We then lift the bottom of the capillary tube out of the water. When an air bubble has formed, we place the capillary tube back into the beaker of water. We can move the location of the air bubble closer to zero on the ruler (closer to the beaker of water) by opening the tap on the reservoir. This will add more water into the system, which pushes the air bubble towards zero on the ruler. We must also dry the leaves as wet leaves affect the rate of transpiration.
We are going to be measuring how far the air bubble moves in a given period of time. If the air bubble moves a great distance, it means that the rate of water uptake and transpiration is high. If the air bubble moves a small distance, it means that the rate of water uptake and transpiration is low. There is a ruler above the capillary tubing so that we can measure how far the bubble has moved and therefore find the rate of water uptake/ transpiration. We then work out the rate of transpiration by dividing the distance that the bubble has moved by the time taken. The equation for this is shown below:
We must make sure that all of the apparatus is watertight and airtight. We then lift the bottom of the capillary tube out of the water. When an air bubble has formed, we place the capillary tube back into the beaker of water. We can move the location of the air bubble closer to zero on the ruler (closer to the beaker of water) by opening the tap on the reservoir. This will add more water into the system, which pushes the air bubble towards zero on the ruler. We must also dry the leaves as wet leaves affect the rate of transpiration.
We are going to be measuring how far the air bubble moves in a given period of time. If the air bubble moves a great distance, it means that the rate of water uptake and transpiration is high. If the air bubble moves a small distance, it means that the rate of water uptake and transpiration is low. There is a ruler above the capillary tubing so that we can measure how far the bubble has moved and therefore find the rate of water uptake/ transpiration. We then work out the rate of transpiration by dividing the distance that the bubble has moved by the time taken. The equation for this is shown below:
We can measure these values in a few different units. We can measure the distance moved in millimetres (mm) and the time in minutes; this will measure transpiration in millimetres per minute (mm/min). We can also measure the distance in centimetres (cm) and the time in hours; this measures transpiration in centimetres per hour (cm/hour). Let’s have an example.
Questions
The air bubble on a potometer moves 15 mm in 5 minutes. Work out the transpiration rate in millimetres per minute.
We work out the transpiration rate by using the formula below.
Questions
The air bubble on a potometer moves 15 mm in 5 minutes. Work out the transpiration rate in millimetres per minute.
We work out the transpiration rate by using the formula below.
The question asks us to work out the transpiration rate in millimetres per minute. In order to do this, the distance moved needs to be in millimetres and the time needs to be in minutes. The question tells us that the distance moved is 15 mm and the time is 5 minutes. Both of these are in the correct units, so we sub them into the above equation.
This tells us that the rate of transpiration is 3 mm per minute (mm/min).
Factors Affecting the Rate of Transpiration
In the previous section, we learnt that the rate of transpiration is affected by light intensity, temperature, air movement (wind) and humidity. We can use a potometer to investigate how these factors affect the rate of transpiration.
Light Intensity
The best way to investigate light intensity is to see how the rate of transpiration varies over a whole day starting from it being dark to sunlight and then dark again. We work out the rate of transpiration for each hour and draw a graph like what is shown below.
In the previous section, we learnt that the rate of transpiration is affected by light intensity, temperature, air movement (wind) and humidity. We can use a potometer to investigate how these factors affect the rate of transpiration.
Light Intensity
The best way to investigate light intensity is to see how the rate of transpiration varies over a whole day starting from it being dark to sunlight and then dark again. We work out the rate of transpiration for each hour and draw a graph like what is shown below.
From the above graph, we can see that the rate of transpiration is at its highest during the middle of the day and this is because light intensity is the greatest during the middle of the day. The rate of transpiration is the lowest during the start and the end of the day because the light intensity is the lowest during these times.
We can also investigate the effect that light intensity has on the rate of transpiration by placing a lamp at different distances away from the plant. When we do this, we will observe that there will be a greater rate of transpiration when the light is closer to the plant. When we use a lamp to investigate the effect that light intensity has on the rate of transpiration, we need to ensure that the lamp does not produce heat as well as light. This is because transpiration is affected by temperature as well as light intensity. Therefore, if the lamp produced heat as well, we would be unable to determine whether the change in transpiration was down to light intensity or temperature.
Temperature
We can investigate the effect that temperature has on the rate of transpiration by changing the temperature of the room that the plant is in. We then leave our plant in different temperatures for a certain period of time (1 or 2 hours) and measure the water uptake/ rate of transpiration. When we do this, we will observe that a greater temperature results in a greater rate of transpiration.
Air Movement
We can investigate the effect that air movement has on the rate of transpiration by placing a fan a certain distance away from the plant. We then turn the fan on at different speed settings and calculate the water uptake/ rate of transpiration. We will observe that a higher speed setting results in a greater rate of transpiration.
Humidity
The final factor that effects the rate of transpiration is humidity. We can investigate the effect that humidity has on the rate of transpiration by placing the plant in different humidities. We then leave the plant for a certain period of time in each of the different humidies and measure the water uptake/ rate of transpiration. We will observe that a greater humidity results in a lower rate of transpiration.
We can also investigate the effect that light intensity has on the rate of transpiration by placing a lamp at different distances away from the plant. When we do this, we will observe that there will be a greater rate of transpiration when the light is closer to the plant. When we use a lamp to investigate the effect that light intensity has on the rate of transpiration, we need to ensure that the lamp does not produce heat as well as light. This is because transpiration is affected by temperature as well as light intensity. Therefore, if the lamp produced heat as well, we would be unable to determine whether the change in transpiration was down to light intensity or temperature.
Temperature
We can investigate the effect that temperature has on the rate of transpiration by changing the temperature of the room that the plant is in. We then leave our plant in different temperatures for a certain period of time (1 or 2 hours) and measure the water uptake/ rate of transpiration. When we do this, we will observe that a greater temperature results in a greater rate of transpiration.
Air Movement
We can investigate the effect that air movement has on the rate of transpiration by placing a fan a certain distance away from the plant. We then turn the fan on at different speed settings and calculate the water uptake/ rate of transpiration. We will observe that a higher speed setting results in a greater rate of transpiration.
Humidity
The final factor that effects the rate of transpiration is humidity. We can investigate the effect that humidity has on the rate of transpiration by placing the plant in different humidities. We then leave the plant for a certain period of time in each of the different humidies and measure the water uptake/ rate of transpiration. We will observe that a greater humidity results in a lower rate of transpiration.