The rate of water uptake by a leafy shoot can be measured with a potometer. Almost all the water taken up is lost by transpiration, so this gives an estimate of the rate of transpiration.
2. Assemble the potometer under water so that it is full of water with no air bubbles.
3. Fit the shoot into the potometer under water and seal the joints with Vaseline so the apparatus is airtight and watertight.
4. Dry the leaves, then introduce a single air bubble into the capillary tube.
5. Record the distance the bubble moves in a set time. Use the reservoir to push the bubble back to the start and repeat to calculate a mean.Calculating the ratevolume of water taken up = πr2 × distance moved by the bubble Worked example: a bubble moves 30 mm in 5 minutes in a capillary tube of radius 0.5 mm.
Volume = π × 0.52 × 30 = 23.6 mm3
Rate = 23.6 ÷ 5 = 4.7 mm3 min-1A potometer measures water uptake, not transpiration directly, because a small amount of water is used in photosynthesis and to keep cells turgid.
2. Assemble the potometer under water so that it is full of water with no air bubbles.
3. Fit the shoot into the potometer under water and seal the joints with Vaseline so the apparatus is airtight and watertight.
4. Dry the leaves, then introduce a single air bubble into the capillary tube.
5. Record the distance the bubble moves in a set time. Use the reservoir to push the bubble back to the start and repeat to calculate a mean.Calculating the rate
Volume = π × 0.52 × 30 = 23.6 mm3
Rate = 23.6 ÷ 5 = 4.7 mm3 min-1A potometer measures water uptake, not transpiration directly, because a small amount of water is used in photosynthesis and to keep cells turgid.