Transpiration rates: environmental factors and data

Increasing temperature generally increases transpiration rate. Water molecules have more kinetic energy, so evaporation from the internal leaf surfaces is faster and water vapour diffuses more quickly.

Increasing humidity generally decreases transpiration rate. Humid air already contains more water vapour, so the concentration difference between the leaf's air spaces and the surrounding air is smaller. Less water vapour diffuses out per unit time. In drier air, the concentration gradient is steeper and transpiration is faster.

These predictions assume that other conditions remain the same. Comparing a warm, humid environment with a cool, dry one is less straightforward because temperature and humidity act in opposite directions.

Air movement and light intensity

In still air, water vapour leaving a leaf can build up around its surface. Increasing air movement carries this water vapour away and replaces it with less humid air. This maintains a steeper concentration gradient, increasing the rate of diffusion and therefore transpiration.

Increasing light intensity generally increases transpiration because stomata tend to open in the light. Open stomata allow carbon dioxide to enter for photosynthesis, but also allow more water vapour to escape. In darkness, stomata tend to close, reducing water loss.

These are general relationships, not promises that the rate will increase without limit. In dry conditions, plants may close their stomata to conserve water. Stomatal closure also restricts gas exchange.

Measuring a rate with a bubble potometer

A bubble potometer measures water uptake by a leafy shoot. As the shoot takes up water, an air bubble moves along a narrow capillary tube. A ruler measures its movement and a timer measures the elapsed time. Faster bubble movement indicates faster water uptake and is used to estimate transpiration rate.

The apparatus is filled with water, and the shoot is cut and fitted underwater to prevent air entering the xylem. Connections must be sealed so that they are airtight and watertight. The leaves should be dry. A single air bubble is introduced into the capillary tube, and the shoot is allowed to adjust to the conditions before measurements begin.

Record the bubble's starting position, wait for a measured time, then record its final position. A reservoir and tap allow the bubble to be reset for another measurement.

To investigate one environmental factor, change that factor while keeping the others constant. For example, move a lamp to change light intensity while keeping temperature, humidity and air movement constant. A lamp may also warm the leaves, so temperature needs checking. Use the same shoot to keep leaf area consistent, and repeat measurements at each setting to calculate a mean.

The measurement is an estimate, not a direct measurement of transpiration: the apparatus measures water taken up rather than water vapour lost, and some water is used by the plant rather than transpired.

Calculating and interpreting rates

A rate tells us how much change occurs per unit time. For water loss itself:

$$\text{transpiration rate} = \frac{\text{amount of water lost}}{\text{time}}$$

Depending on the measurement, units might be g/min for mass lost or cm³/min for volume lost. For a bubble potometer, a useful indicator is:

$$\text{bubble movement rate} = \frac{\text{distance travelled}}{\text{time}}$$

For example, suppose a bubble starts at the 10 mm mark and finishes at the 22 mm mark after 5 minutes. It has travelled $22 - 10 = 12$ mm, so its movement rate is $12 \div 5 = 2.4$ mm/min. This means that it moved an average of 2.4 mm each minute; it does not mean that the plant lost 2.4 mm³ of water each minute.

Dividing by time makes measurements taken over different durations comparable. Bubble movement rates should be compared using the same capillary tube dimensions and comparable shoots.

Turning a table into a graph

The following constructed practice data illustrate a possible humidity trend. They are not experimental observations. Each measurement uses the same shoot and capillary tube, with other environmental conditions held constant.

Calculate each rate by dividing its distance by 5 minutes. Put humidity, the independent variable, on the horizontal axis and bubble movement rate, the dependent variable, on the vertical axis. Both variables are continuous, so plot a graph of points with an appropriate line or curve rather than separate categorical bars.

Use regular numerical intervals and scales that spread the points across much of the available space. For these values, horizontal intervals of 20 percentage points and vertical intervals of 0.6 mm/min work well. Label both axes with their quantities and units, plot each pair accurately, and draw a suitable best-fit line or curve.

Data for Illustrative effect of humidity on water uptake

Reading biological information from data

To recover numerical information from the graph, start at a humidity value on the horizontal axis, move up to the line, then across to the vertical axis. At 60% humidity, the rate is 1.2 mm/min. This point can be transferred back into a table as the pair 60% and 1.2 mm/min.

The graph shows a negative relationship: as humidity increases, the rate decreases. Between 20% and 80% humidity, the rate falls from 2.4 to 0.6 mm/min. The biological explanation is that more humid surrounding air reduces the water-vapour concentration gradient out of the leaf.

Reading between plotted values is interpolation. This straight line gives an estimated rate of 1.5 mm/min at 50% humidity. Real results need not follow this exact line, and extending a trend beyond the measured range is less secure.

For any graph, chart or table, first check what is measured and its units. Then identify the pattern, support it with values, and connect it to the relevant biological mechanism.

Stomatal closure in dry conditions can reduce water loss.

Rates and measurements

$$\text{rate} = \frac{\text{amount of change}}{\text{time}}$$

For a bubble potometer:

$$\text{bubble movement rate} = \frac{\text{final position} - \text{starting position}}{\text{elapsed time}}$$

Distance in mm divided by time in minutes gives mm/min. A potometer measures water uptake, providing an estimate of transpiration.

Change one environmental factor, control the others and leaf area, and repeat measurements to obtain a mean.

Graphs and interpretation

• Independent variable: horizontal axis. Dependent variable: vertical axis.
• Use labelled axes with units and regular, suitable numerical scales.
• Plot continuous data as points with an appropriate line or curve.
• Read coordinates to transfer graph values into a table.
• Describe trends using values and units, then explain the biology.
• Interpolation estimates values within the measured range; trends beyond that range are less certain.