(ii) practical investigations to estimate transpiration rates

The idea

Transpiration is the loss of water vapour from aerial parts of a plant, mainly through stomata. Practical investigations estimate the rate indirectly by measuring water uptake, usually with a potometer. A shoot is cut under water and sealed into a capillary tube containing an air bubble; as the shoot transpires, water is drawn along the tube and the bubble moves. The distance moved in a set time, converted to volume using the tube's cross-sectional area, gives an estimate of rate. Learners must control variables: light intensity, temperature, humidity and air movement all affect stomatal behaviour, so only one should be varied while others are held constant. Repeats and a control shoot help reliability. The estimate assumes water uptake equals water lost, which is only approximately true because some water is used in photosynthesis and turgor.

What earns the marks

• Transpiration is water loss as vapour, mainly through stomata, and is estimated indirectly by measuring water uptake with a potometer.
• A shoot is cut under water and sealed into a capillary tube with an air bubble; the bubble moves as water is drawn in, and distance moved per unit time estimates rate.
• Volume transpired is calculated from bubble distance multiplied by the capillary tube's cross-sectional area, giving a rate per unit time.
• Environmental variables such as light intensity, temperature, humidity and air movement must be controlled so only the chosen variable changes.
• Repeats and a control improve reliability, and the method assumes water uptake approximately equals water lost, though some water is used in photosynthesis and turgor.

Common mistakes and how to put them right

• Thinking a potometer measures transpiration directly.

  Put it right: It measures water uptake, which only approximates transpiration because some water is retained for photosynthesis and turgor.

• Failing to cut the shoot under water or to seal joints, which lets air enter and breaks the water column so the bubble does not move reliably.

• Changing several environmental factors at once, so any change in bubble movement cannot be attributed to the variable being tested.

Exam tips

• State clearly that the potometer estimates rather than measures transpiration, and explain the assumption behind that estimate.
• Name the controlled variables explicitly (light intensity, temperature, humidity, air movement) and say why each must be held constant.
• Describe how bubble distance is converted to a rate using the tube's cross-sectional area, and mention repeats for reliability.