Preparation of the Shoot: The plant shoot must be cut under water to prevent air from entering the xylem vessels. An air lock in the xylem would break the continuous water column and stop the transpiration pull.
Ensuring an Airtight Seal: All connections between the plant, the rubber bung, and the glass tubing must be sealed with petroleum jelly (Vaseline). Any air leak will cause the bubble to move inconsistently or not at all.
Introducing the Bubble: By lifting the end of the capillary tube out of the water beaker for a moment and then replacing it, a single air bubble is introduced to act as an indicator.
Resetting the System: A water reservoir with a stopcock is used to push the air bubble back to the starting position (the zero mark) for repeated trials.
| Feature | Water Uptake (Measured) | Transpiration (Estimated) |
|---|---|---|
| Definition | Total water entering the plant vascular system. | Water vapor lost through stomata and cuticle. |
| Usage | Used for cell turgidity and photosynthesis. | Primarily a cooling and transport mechanism. |
| Accuracy | Measured precisely by bubble movement. | Slightly lower than uptake due to metabolic use. |
Identify the Variable: When presented with potometer data, always identify if the question asks for the 'rate' (distance/time) or 'volume' (using ).
Check the Setup: In 'identify the error' questions, look for dry leaves (which slow transpiration) or the absence of an airtight seal. Leaves should be blotted dry before starting the experiment.
Control Variables: Ensure that only one environmental factor is changed at a time. For example, if testing light intensity, use a heat shield (like a glass tank of water) to ensure the lamp's heat doesn't change the temperature.
The 'Transpiration = Uptake' Fallacy: Students often forget that a small amount of water is retained by the plant for maintaining cell turgor and for the chemical process of photosynthesis. In a rapidly transpiring plant, this error is small, but in a wilted or growing plant, it can be significant.
Air Bubbles in Xylem: If the shoot is not cut under water, air enters the xylem (embolism). This prevents the 'suction' from reaching the capillary tube, leading to a reading of zero even if the leaves are losing water.
Temperature Fluctuations: Changes in ambient temperature can cause the air in the potometer or the water itself to expand or contract, leading to false bubble movements that are not caused by the plant.
