This section explains how various environmental factors can change the rate of transpiration, and also examines how the structure of the leaves has adapted to minimise this water loss.
Learners will need to understand the factors that affect the transpiration rate such as temperature, light intensity, wind and humidity. Simple experiments can be conducted to demonstrate these factors.
Transpiration is a process that involves loss of water vapour through the stomata of plants. Transpiration is thought to be a 'necessary cost or evil' to allow the plant to absorb water from the soil. It is an inevitable process.
Turgidity, or turgor pressure, refers to the water content of cells and how this lends structural support to the plant. When cells absorb water, the vacuoles fill up and the cytoplasm increases, pushing against the cell membranes, which in turn push against the rigid cell walls. This makes the cells rigid, or turgid.
Transpiration is important in plants for three major reasons:
Transpirational pull: results from the evaporation of water from the surfaces of the mesophyll layer in the leaf to the atmosphere, through the stomata. Evaporation of water from the leaves surface causes a negative pressure (suction force) in the xylem that pulls water from the roots and soil. This results in water being drawn up the xylem vessel.
Transpirational pull draws water from the roots to the leaves because of the effects of capillary action. The primary forces that create the capillary action are adhesion and cohesion. Adhesion is the attraction that occurs between water and the surface of the xylem, and cohesion is the attraction between water molecules.
We will revisit transpirational pull and capillarity later in the chapter when we examine how water is transported in the plant.
Capillary action occurs when the adhesion of water molecules to the walls of the vessel is stronger than the cohesive forces between the water molecules. Have you ever seen fluid in a drinking straw move higher than the level of the fluid in the glass? This happens due to capillary action. The narrower the straw, the greater the capillary action, and therefore, the higher the fluid will rise in the straw.
Cohesion refers to the intermolecular, attractive forces that hold molecules in solids and liquids together. Imagine a drop of water on a waxy surface like wax paper. Even if the drop slides and rolls around, the water molecules will stay together due to the cohesive forces. Adhesion is the ability of a substance to stick to an unlike substance. If you were to take the same piece of wax paper and turn it upside down, some water droplets would still adhere to the paper. This indicates that there must be an attraction between the water and the wax paper. However, in this case the water-water cohesive force is stronger than the adhesive force between the molecules of the wax paper and the water.
This interactive website explains transpiration pull.
Of particular use to learners is an interactive animation that lets them determine the effect of different environmental factors on transpiration rate.
There is a close inter-relationship between transpiration and leaf structure. The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period. Plants regulate the rate of transpiration by opening and closing of stomata (Figure 5.14). There are, however, a number of external factors that affect the rate of transpiration, namely: temperature, light intensity, humidity, and wind.
Figure 5.14: The opening and closing of stomata. Different environmental conditions trigger both the opening and closing of stomata.
Temperature
Temperature affects the transpiration rate in two ways. Firstly, at warmer temperatures water molecules move faster, and the rate of evaporation from stomata is therefore much faster. Secondly, the water-holding capacity of warm air is greater than that of cold air. Assuming that cold air and warm air contain the same amount of water, the cold air may be saturated, and therefore have a shallow water concentration gradient, while the warm air may will be able to hold more water vapour, and will therefore have a steeper water concentration gradient.
Light intensity
At high light intensity, the rate of photosynthesis increases. As photosynthesis increases, the amount of stored glucose in the guard cells increases. This lowers the water potential of the leaf (i.e. the contents of the leaf are less dilute). As the water potential decreases, more water enters the guard cells making them more turgid. The turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration.
Relative humidity
The amount of water vapour in the air is referred to as the humidity. Water always moves down a concentration gradient. Therefore when the humidity is high (lots of water vapour in the air) the water potential gradient between the inside of the leaf stomata and the atmosphere is shallow and the rate of transpiration will be low. However, if the atmosphere is dry, there will be a steep water concentration gradient between the humid inside of the stomata and the outside air and the rate of transpiration will therefore be fast.
Wind
When water is lost from the leaf it forms a thin layer outside the leaf. This reduces the water potential between the leaf and the atmosphere outside. When there is wind, this layer is blown away, thus maintaining the water potential gradient across the leaf.
To measure the rate of transpiration we use a piece of equipment called a potometer. A potometer measures how factors such as light, temperature, humidity, light intensity and wind will affect the rate of transpiration. The main type of potometer is the 'bubble' potometer shown in Figure 5.19. The potometer measures the amount of water lost from a leafy shoot by monitoring the rate at which an air bubble moves along the narrow tube as the leafy shoot sucks up water to replace the water lost by the transpiration of the plant.
A potometer provides an indirect measurement of the transpiration rate – it measures how fast water is absorbed, which is related to how fast water vapour is being lost. It cannot measure how fast water vapour is being given off directly.
As the leafy twig transpires, the air bubble moves to towards the plant. The quicker the air bubble moves, the faster the leafy twig is transpiring.
Figure 5.19: Potometer measures the rate of transpiration.
Investigation: To determine the effect of environmental conditions on transpiration rate (using a simple photometer.
The four groups should have leafy twigs of the same type of plant and about the same size, so that results in different environmental conditions can be reliably compared at the end. They should choose twigs with stems likely to fit tightly into a drinking straw and the stems should be strong enough not to be crushed when forced into the straws.
ALTERNATIVE METHOD:
Alternatively, if available, learners can use narrow clear plastic tubing filled with water that can be cut open / slit at one end to insert the twig. Plastic tubing is easier to seal with tape than drinking straws and it can be taped in place horizontally along a desk or table once an air bubble has been introduced at the open end. One learner will have to hold the plant off the desk. The tube is then placed into water and a ruler is taped in place next to it to track the movement of the bubble over time. They won’t be able to reset it, but that’s fine – take 3 or 4 measurements of how far the bubble moves in 2 minutes and get an average for each environmental condition. This is faster than doing it for an hour in each condition.
Learners can create the various environmental conditions in the laboratory.
Precautionary Measures:
Observations and Results:
Teachers should be able to guide the class in drawing up tables to compare the transpiration rate in the four conditions, but the following table is suggested as a guideline:
Conditions | Cumulative distance water has moved (mm): | ||||||
0 | 1 | 2 | 3 | 4 | 5 | 6 | |
Sunny, no wind | |||||||
Sunny, with wind | |||||||
Sunny, plastic bag around plant | |||||||
Shade, no wind |
Learners should find the following transpiration rates:
Bar graphs should have headings similar to the table heading. Environmental conditions will go onto the horizontal axis and rate of transpiration on the vertical axis. Since measurements were taken every 10 minutes for an hour, the total cumulative transpiration at the end of the 60 minute period will be the transpiration rate per hour.
Conclusions:
Improving the accuracy of findings: Accept any 2 of the following:
Questions:
Answers:
To prevent air from getting into the xylem. Cut at an angle so there is a large surface area through which the water can enter the plant. It is important to remember that cutting straight across the stem may crush and block the xylem.
The xylem of the stem.
Humidity, wind, light and moisture.
Learners should find the sunny area with wind to have the highest rate of transpiration. However, learners are to assess their own experiments and base answer on results.
To determine the effect of environmental conditions on transpiration rate using a simple potometer.
A potometer measures the rate of transpiration by measuring the movement of water into a plant. The following experiment uses a simple hand made potometer.
Learners will be divided into four groups as each group will investigate a different factor and then all the results can be shared at the end of the investigation.
Perform the following steps under water:
Each of the four groups that investigated different environmental conditions should contribute their results for the final analysis.
Record your observation from the table, bar graph and line graphs.
More information about potometer experiments can be found on the following websites:
In addition, the following website has a 'virtual laboratory' that allows you to perform the above experiment online:
Perform the experiment and complete the laboratory exercise given on the website.
To determine the effect of light intensity on transpiration.
Record the amount of water lost during the day and during the night.
Using the three plants, figure out the average water loss for each time period.
Plot a bar graph comparing the average amount of water loss in the day and night.
Write down anything you observed about the plants, the plastic bags and the rate of water loss from the plant.
What can you conclude regarding the rate of transpiration at different light intensities? Was there higher or lower water loss when you left the plant overnight compared to when you monitored it throughout the day?
How can you improve this experiment to determine the effects of different light intensities on transpiration?
In this experiment what are the key variables we are controlling for? Have we properly controlled for these?
Investigation: Determining the effect of light intensity on transpiration
The following activity can be done as a DEMONSTRATION and is optional. It is not easy to collect water in bags and then remove the bag without losing the water.
Conclusions:
The plant loses more water during the day, because it’s hotter and the light intensity is higher than at night.
Advantages of transpiration | Disadvantages of transpiration |
Cools the plant down | Excessive water loss causes the plant to wilt |
Assists in the transport of water from the soil | |
Important for transport of water through the xylem | |
Regulates the concentration of cell sap | |
Distribution of salts and minerals in the plant |
When the rate of transpiration is too high, it can have detrimental effects on the plant, as you will see in the next section on wilting and guttation. For this reason, plants have developed structural adaptations to minimise the amount of water loss.
Position of stomata: Stomata are found on both surfaces of the leaf but there are usually more on the ventral (lower) surface of the leaf. This means that less water vapour is lost because the ventral side of the leaf is in the shade and therefore does not get as hot.
Sunken stomata: some plants such as xerophytes have sunken stomata as a way of preventing water loss. Xerophytes (pronounced "zero-phytes") are plants that are normally found in hot, dry areas such as deserts. The sunken stomata creates a small pocket of moist air. The high humidity in the air pocket reduces the water potential gradient between the leaf air spaces and the exterior, and therefore decreases the rate of transpiration.
Figure 5.20: Sunken stomata.
Thickened cuticle: Some plants that occur in dry places have a thick cuticle that reduces transpiration.
Figure 5.21: Desert plants like cactus have thick cuticles to avoid water loss.
Hairs on leaves: Hairs trap a small layer of water vapour that works in three ways to reduce transpiration:
Figure 5.22: Hairy leaves to trap water.
Leaf spines: Some plants have spines instead of leaves. Spines usually have thicker cuticles and a very small surface area, which decreases transpiration.
Figure 5.23: Spiny leaves have a small surface area to decrease transpiration.