9.3 Transpiration

Transpiration 

Loss of water vapour from leaves & stems of plants 

• Inevitable consequence of gaseous exchange 

Transpiration Stream

Movement of water from roots to leaves 

What happens when stomata are open?

• CO2 & O2 exchanged

• Water moves out

Transpiration Stream - How it Works

1. Water evaporates from surface of mesophyll cells into air spaces in leaf & move out of stomata into surrounding air by diffusion down conc. gradient

2. Loss of water by evaporation lowers water potential of cells, so water moves into cell from an adjacent cell by osmosis, along BOTH apoplast & symplast pathways

3. Is repeated across leaf to xylem

4. Water moves out of xylem by osmosis into cells of leaf

Adhesion and trasnpiration

Water molecules form hydrogen bonds w. carbohydrates in walls of narrow xylem vessels

Cohesion and transpiration

Water molecules form hydrogen bonds w. each other & so tend to stick together

What do the combined effects of adhesion & cohesion result in?

Water exhibiting capillary action

Capillary action of water

Process by which water can rise up a narrow tube against the force of gravity

Transpiration pull

Water is drawn up xylem in continuous stream to replace water lost by evaporation

• Results in a tension in xylem, which helps to move water across roots from soil

Transpiration Stream

• Water moves by osmosis across membranes and by diffusion in apoplast pathway from xylem thru cells of leaf

• Evaporates from freely permeable cellulose cell walls of mesophyll cells in leaves into air spaces

• Water vapor then moves into air thru stomata along diffusion gradient

Rate of water uptake

Distance moved by air bubble / time taken for air bubble to move

Factors affecting transpiration (5)

1. Light (increasing gives high no. of open stomata, increases rate)

2. Relative humidity (high = lowers rate due to reduced water vapor gradient)

3. Temperature (increases rate)

4. Air movement (Increases rate)

5. Soil-water availability (If soil dry, transpiration rate reduced)

Cohesion & Tension (Transpiration) 

Allow mass flow of water over long distances up the stem

1. Water evaporates from leaves (transpiration)

2. Creates tension, pulling more water into leaf

3. Water molecules are cohesive, so when are pulled into leaf, others follow

   1. Whole column of water in xylem moves upwards 

(Adhesion helps water rise up as water molecules attracted to walls of xylem vessels)

Evidence for cohesion-tension theory

• Changes in diameter of trees

  • During day, transpiration & tension in xylem vessels lowest, diameter shrinks

  • Night, transpiration & tension in xylem vessels lowest, diameter increases

• When xylem vessel broken, air drawn in, plant can’t move water up stem, cohesive forces broken 

How to measure water uptake of plant

Using potometer

Why do you cut the shoot underwater and slanted?

• To prevent air bubbles in xylem

• Slant to increase SA available for water uptake 

Why do you assemble the potometer underwater?

So no air enters stem

Using the potometer

Equation for rate of water uptake

Distance moved by air bubble / time for air bubble to move

• Units = cm s^-1