9.3 - Transpiration

Question 1

Define transpiration.

Answer 1

• Loss of water vapour from the upper segments of the plant (mainly leaves)
• Occurs through evaporation from leaf surface and through stomata

Question 2

What are the components of the transpiration stream?

Answer 2

Transpiration stream: movement of water from roots to leaves

• Capillary action
• Transpiration pull

Question 3

What are the stages of transpiration?

Answer 3

1. Water moves by osmosis from the xylem to mesophyll cell in the leaf
2. Water evaporates from the surfaces of the spongy mesophyll into air spaces
3. Water vapour diffuses out of the leaf from the air spaces through the stomata down the conc. gradient

• The stomata remain open during the day to allow for gas exchange enabling photosynthesis
• Since the stomata are open, water vapour is lost so transpiration is a consequence of gaseous exchange

Question 4

What is capillary action?

Answer 4

Water rising through narrow tubing against gravity

Needs cohesion + adhesion

Adhesion: Formation of H-bonds between water molecules and carbs in xylem walls

Cohesion: Formation of H-bonds between water molecules

Question 5

What is the transpiration pull?

Answer 5

• Water moving out of xylem creating a pull to replace loss of water vapour from the leaves
• Since water is cohesive it moves as on body under tension, which pulls the water up the stem
  
  • known as cohesion-tension theory

Question 6

Show evidence for cohesion tension theory.

Answer 6

Change in tree diameter - rate of transpiration is highest during the day, xylem vessel tension is highest - tree diameter shrinks

At night transpiration rate is low and the tension on xylem is low, increasing tree diameter

Xylem vessel damage - air is drawn into xylem, plant can’t transpire, continuous water stream is broken

Question 7

What happens when stomata are flaccid?

Answer 7

Turgor pressure is low (guard cells are flaccid)

Pore closes

Question 8

What happens when stomata are turgid?

Answer 8

• When conditions are favourable guard cells actively pump solutes in increasing turgor
• Cellulose hoop stop cells from swelling widthways and they extend lengthways
• Inner cell wall less flexible than outer making a bean shape
• If water becomes scare - hormonal signals from roots trigger guard cell water loss closing the aperture.

Question 9

What are factors affecting transpiration?

Answer 9

• Light intensity - light needed for photosynthesis - stoma open, at night, no photosynthesis, stoma close
• Relative humidity - amount of water vapour in air
  
  • High humidity = low rate of transpiration due to decreased water vapour gradient vice versa
• Temperature - increases evaporation so there will be a higher water potential in leaf
  
  • Temp rise - also increases amount of water external air can hold before saturation, decreasing humidity and increasing water vapour gradient
• Air movement - if there is more wind this blows water vapour (trapped in the hairs at the surface of the leaf) increasing the water vapour gradient
• Soil-water availability - if there is not enough water in soil, plant will be under water stress reducing rate of transpiration.