Factors and Calculations of Transpiration Rate

Why conditions around a leaf matter

Water evaporates from moist surfaces inside a leaf, then water vapour diffuses out through its stomata. Water is drawn up through the xylem to replace it. Transpiration rate describes how quickly the plant loses water. The conditions around the leaf can change either how readily water evaporates or how quickly its vapour diffuses away. When comparing conditions, assume the other factors and the plant's water supply stay the same.

Temperature, humidity and moving air

At a higher temperature, water evaporates more quickly from the leaf's moist surfaces, so transpiration usually increases. In cooler conditions, evaporation is slower.

Humidity is the amount of water vapour in the surrounding air. In dry air, there is a large difference between the water vapour concentration inside the leaf and that outside it. Vapour therefore diffuses out more quickly. If the air is already humid, that difference is smaller and transpiration slows.

Air movement also changes the air immediately next to a leaf. In still air, water vapour leaving the stomata builds up nearby. Wind carries this humid air away and replaces it with less humid air, maintaining a steeper concentration gradient. Increasing air movement therefore usually increases transpiration.

Light intensity and stomata

In brighter light, photosynthesis can occur more readily. Stomata generally open to let carbon dioxide into the leaf, but water vapour can then escape too. Increasing light intensity therefore usually increases transpiration. In darkness, stomata tend to close, reducing water loss. Light affects the rate through stomatal opening, rather than simply by making water evaporate faster.

Estimating the rate with a potometer

A potometer follows water taken up by a leafy shoot. Cut the shoot under water to avoid drawing air into its xylem, then fit it into water-filled apparatus with a narrow capillary tube. Introduce an air bubble into the tube. As the shoot takes up water, the bubble moves towards it; measure how far the bubble moves in a measured time. The apparatus must be watertight so that movement reflects water uptake rather than a leak.

Water uptake is a useful estimate of transpiration because water lost from the leaves is replaced by water drawn into the shoot. It is not a direct measurement of the vapour escaping. To investigate one environmental factor, change that factor while keeping the others, the shoot's leaf area and the measurement time as consistent as possible. For example, a fan can change air movement while light and temperature are kept constant.

Calculating and reading rates

Rate is a compound measure: an amount divided by the time taken. If bubble movement is measured as a distance, use

$\text{rate of water uptake} = \frac{\text{distance moved by bubble}}{\text{time taken}}.$

For example, if the bubble moves 24 mm in 6 minutes, the rate is $24 \div 6 = 4$ mm/min. This means it moves an average of 4 mm each minute. If an investigation measures a volume of water taken up instead, divide that volume by time and give a volume-per-time unit.

A graph of water uptake against time puts time on the horizontal axis and water uptake on the vertical axis, with suitable numbered scales and units. Its gradient gives the uptake rate: a steeper line means a higher rate. The graph shows a bubble moving 24 mm in 6 minutes under warmer conditions, but only 12 mm in 6 minutes under cooler conditions. Their gradients are 4 mm/min and 2 mm/min respectively. The plotted values are constructed to show how to read rates, not experimental observations.

Data for Bubble movement over time at two temperatures

If a graph instead plots rate against an environmental factor, read the rate from the vertical axis at the chosen factor value and describe the trend. Check the axis labels and units before deciding whether to read a value or calculate a gradient.

Environmental factors

Higher temperature → faster evaporation → usually higher transpiration. Lower humidity or greater air movement → a steeper water vapour concentration gradient → faster diffusion out of the leaf. Greater light intensity → stomata generally open for carbon dioxide uptake → more water loss. Compare one factor at a time.

Measurement and data

A potometer estimates transpiration from water uptake by a leafy shoot. For bubble movement, $\text{rate}=\text{distance}\div\text{time}$, such as mm/min; use volume/time if volume is measured. On a water-uptake–time graph, gradient is rate. On a rate–factor graph, read the rate directly from its axis.