Transpiration

Question 1

Transpiration

Answer 1

Transpiration: Loss of water vapour from the upper parts of the plant, mostly the stomata in the leaves. Most water vapour leaves through stomata, which open to allow gaseous exchange for photosynthesis. Since photosynthesis only occurs in day, most water vapour is lost in the day, when stomata are open. Some may evaporate from leaf surface but limited by waxy cuticle.

Question 2

Importance of transpiration:

Answer 2

Transpiration is a consequence of gas exchange, but it vital for survival – creates transpiration stream which:
- Transports useful mineral ions up the plant to leaves.
- Maintains cell turgidity.
- Supplies water for growth, cell elongation and photosynthesis.
- Supplies water that, as it evaporates, can keep the plant cool in hot conditions.

Question 3

Environmental factors that affect transpiration rate:

Answer 3

•Light intensity (increases transpiration)
In light, stomata open to allow gaseous exchange for photosynthesis, this
allows water to diffuse out of the plant.

•Temperature (increases transpiration)
Increases rate of evaporation from cell surtaces so water-vapour potential in
the leaf rises.
Increases rate of diffusion through stomata as water molecules have more KE

•Air movement/wind (increases transpiration)
Air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient.

•humidity(decreases transpiration)
Higher relative humidity in the air will decrease the rate of diffusion, as there will be a smaller water vapour potential gradient between air spaces in the leaf and the air outside.

•Water availability (increases transpiration)
If there is little water in the soil, the plant cannot replace water lost by transpiration. If there is insufficient water, the stomata close - water stress.

Question 4

Practical – Measuring rate of transpiration:

Answer 4

Potometer used to ESTIMATE the rate of transpiration. It measures the rate of water uptake by a leafy shoot – some reacts in photosynthesis (assumes rate of water uptake is directly proportional to transpiration rate). Assuming the cells are turgid most water taken up is lost by transpiration so is a reasonable estimate. Water vapour is lost by the leaves and replaced by water in the capillary tube.
The effect of certain environmental factors can be investigated (e.g. fan – wind, black plastic bag (with holes so doesn’t affect humidity, not lamp as that would change temp) – light intensity, clear plastic bag with no holes – humidity, heater – heat.

Potometer setup – valid results:
- Shoot cut underwater, ensure apparatus is airtight,
apparatus set up underwater, insert shoot into apparatus underwater.
- so no air bubbles in xylem/apparatus
- Allow time for shoot to acclimatise
- Dry leaf (wet leaves reduce pot grad)
- Healthy shoot used
- Cut stem at angle so large SA in contact with water
- Ruler in fixed position, conditions constant & monitored
(only change 1 variable at a time).
Rate of transpiration = volume of eater taken up (πr2l)/time taken (m3s-1)