3.4 Transpiration Stream

Question 1

Transpiration stream

Answer 1

‘Engine’ that pulls water up from roots to:

Supply photosynthesis (1% - 2% of total)
Bring minerals from the roots for biosynthesis within leaf.
Cool leaf

Question 2

Transpiration

Answer 2

Process of water loss from plants, specifically leaf’s internal spaces.
Water is lost in the form of water vapour.
This occurs by diffusion’s through stomata.
Water is lost when leaves breath in C02 fo photosynthesis.
This water must be replaced by the leaves through the xylem of roots and stem.

Question 3

Limiting water loss.

Answer 3

Possible adaptations:
Small narrow leaves.
Thick cuticles on leaves.
Fine, wooly hairs on leaves.
Waxy ‘bloom’ on surface.
In rolled leaves - helps to stop the water loss of water via transpiration eg. Festuca rubrics.
Stomata in deep pits.

Question 4

Evaporation

Answer 4

Process where liquid water is transformed into gaseous state.
Can only occur when water is available.
Requires large amounts of energy.
Humidity of atmosphere must be less than that of evaporating surface.

Question 5

Absorption of water and mineral salts

Answer 5

Mainly through root, in root hair zone.

Water in the soil is held in a film around the soil particles, by capillary and surface tension forces.

Question 6

Factors controlling water uptake.

Answer 6

Available soil water.
Extent of root system.
Soil aeration.

Soil temperature

Osmotic pressure differences between root hair cell sap and soil water (diffusion pressure gradient).

Question 7

Definition of evapotranspiration.

Answer 7

Evaporation - liquid water transformed into a gaseous state.
Transpiration- process of water loss from plants, specifically leaf’s internal spaces - water lost in the form of water vapour.
Evapotranspiration - combined defects of evaporation and transpiration.

Question 8

Factors that control evapotranspiration.

Answer 8

Energy availability- more available energy the greater the rate of evapotranspiration.

Humidity gradient away from the surface - rate and quantity water vapour entering atmosphere - both become higher in drier air.

Wind speed immediately above surface - wind removes shallow layer of atmosphere just above surface which becomes saturated with drier air.

Water availability - evapotranspiration cannot occur if water is not available.