Transpiration

Transpiration is the loss of water from plant leaves through their stomata by evaporation and diffusion. Water evaporates from the plant cells into the air spaces in the leaf and, when the stomata are open, diffuses out into the atmosphere down a concentration gradient. Water molecules are pulled up from the roots through the xylem to replace the water lost.

The movement of water from the roots up the stems to the leaves through the xylem is called the transpiration stream. It works because water molecules stick together more tightly than they stick to anything else. This property is called cohesion.

The stomata of the plant open for gas exchange, so the carbon dioxide (CO₂) needed for photosynthesis in the chloroplasts moves into the leaves by diffusion down a concentration gradient, while the oxygen produced diffuses out. Water vapour diffuses out of the open stomata down a concentration gradient at the same time. Stomata close at night when there is no light for photosynthesis, and this minimises water loss by transpiration at night.

The flow of water in transpiration is used by plants to transport mineral from the roots to the leaves through the xylem, to cool the leaves and even for roots to send chemical signals about the water content of the soil.

Transpiration flow in the xylem is driven by the loss of water from the leaves. The loss of water from the leaves over a measured period of time is the rate of transpiration. The rate of transpiration, and so the xylem flow, are affected by factors including whether the stomata are open in the first place, the air temperature, humidity and the rate of air flow over the leaf.

Measuring Transpiration

Transpiration can be measured directly or indirectly in a number of ways

Potometer

In a potometer, a leaf or shoot is sealed into a capillary tube and the rate of at which water is taken up through the stem is followed as the bubble moves along the measuring scale (see the diagram below). The uptake of water through the cut stem is assumed to be the same as the rate at which water is lost from the leaves by transpiration. However, the actual transpiration rate may be lower as some of this water could be used for photosynthesis or by the cells to maintain turgidity instead.

A graph of the volume of water lost over time has a slope that gives an estimate of the rate of transpiration of the leaf. By exposing the shoot to different conditions, we investigate the factors that affect the rate of transpiration. To use a potometer you have to cut a leaf or shoot from the plant. This limits the amount of data you can collect as the cut shoot does not remain active for a long time.

Weight loss

Placing a transpiring plant on a balance and recording the weight loss gives data to plot a graph of weight loss over time. The slope of the line is an estimate of the transpiration rate. This method is only useful for potted plants that are small enough to sit on a balance. It is likely to over estimate the rate of transpiration as water may also evaporate from the soil surface.

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Sap flow meter

Transpiration - loss of water by evaporation and diffusion from the leaves of a plant - pulls water up from the roots in tubes called xylem vessels. The greater the rate of transpiration from the leaves the greater the flow of water up the trunk. The rate of flow can be measured by recording how fast a heat pulse rises up the trunk over a given distance, detected by a mini thermometer. The rate of transpiration is estimated by recording how long the heat pulse takes to move a defined distance. Once the sap flow meter is installed, a continuous record of transpiration rate can be gathered. This method is being used in the BIFoR FACE experiment to gather data on the effects of elevated CO₂ levels in the air around trees on transpiration and xylem flow.

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A real experiment

At the BIFoR FACE there are six rings of pylons in the forest each enclosing a number of fully grown oak trees. Pipes in the pylons inject either air containing ambient CO₂ levels (400 ppm, aCO₂) or elevated CO₂ (550 pmm eCO₂) into the rings. There the three control aCO₂ rings and three eCO₂ rings. CO₂, temperature, light levels and wind speed are continuously monitored and recorded by data loggers.

The green icons in the 360 image below have further information and a short animation about how the Sap Flow Meter works.

The information for the three experiments is below.

Cloudy day vs Sunny day

In this experiment you examine the change in the transpiration rate of the trees over a daily cycle. You then see how this pattern is altered by an increase in sunshine and temperature.

Open the data sheet and plot the data as time of day (dependant variable) vs rate of sap flow (independent variable).

Describe the pattern of sap flow over the day for the cloudy day,
How can you explain this?
How is the pattern of sap flow different on the sunny day?
Suggest reasons for this.

aCO₂ vs eCO₂

In this experiment you examine how elevated CO₂ (eCO₂) might change the pattern of transpiration

Open the data sheet and plot the data as time of day (dependant variable) vs rate of sap flow (independent variable).

How does eCO₂ affect sap flow?
What reasons can you suggest for the change?

Sunny day vs Sunny Windy day

In this experiment you examine how conditions which greatly increase transpiration increase sap flow, and what the upper limits or constraints to transpiration might be.

Open the data sheet and plot the data as time of day (dependant variable) vs rate of sap flow (independent variable).

How does the wind affect sap flow in the morning?
Can you speculate on reasons for the large decline in sap flow in the afternoon in the sunny/windy treatment?

Post your favourite graph and a line of commentary in the anonymous Padlet below.

Transpiration

Measuring Transpiration

A real experiment

Cloudy day vs Sunny day

aCO2 vs eCO2

Sunny day vs Sunny Windy day

aCO₂ vs eCO₂