1. TRANSPIRATION

2. Definitions Transpiration is the loss of water vapour by evaporation from the aerial parts of a plant. The transpiration stream is the continuous flow of water from the roots to the leaves of a plant driven by evaporation.

3. Transpiration Transpiration involves evaporation followed by diffusion of water vapour down a water vapour potential gradient out of the leaves through the stomata.

4. Transpiration Stomata are open during daylight hours to allow carbon dioxide to enter the leaf. Water moves from the xylem across the mesophyll cells by the symplast route (by osmosis from cell to cell) and by the apoplast route(along the bundles of cellulose fibres forming the cell wall).

5. Cohesion -Tension The tension set up in the leaf draws water up the xylem vessels in the stem of the plant. Water molecules form hydrogen bonds causing cohesion. This produces an unbroken column up the xylem.

6. Cohesion-Tension A stronger rate of evaporation leads to more tension in the water column and a faster rate of water movement up the trunk. Capillary action plays a part in the movement of water up a stem. The water molecules adhere to the walls of the xylem vessels which are very narrow. Root pressure also contributes due to the build up of water in the xylem in the roots

7. Measuring the rate of water uptake by plants The potometer measures the rate of water uptake by a cut shoot. This can be used as an estimate of the rate of transpiration. Why is the uptake of water only an estimate of transpiration rate?

8. Setting up the potometer https://www.youtube.com/watch?v=oUr1P9RZnEU

9. The potometer- details An air bubble is introduced at the bottom of the capillary tubing. When it reaches the scale, the distance moved along the scale in one minute can be measured.

10. The potometer - details

11. The potometer- precautions The equipment must be water tight so that there are no leaks. The potometer is set up under water. The leaves must be allowed to dry before taking any readings. No air must be allowed to build up under the cut shoot and break the water column.