L4 - Rate of transpiration and stomata

What factors effect rate of transpiration?

• Higher Wind speed - higher transpiration as keeps water potential gradient high

• More leaves - more leaves more surface area more transpiration

• More/bigger stomata - higher rate of transpiration

• Presence of waxy cuticle - waterproof layer prevent water loss reduces rate of transpiration

• Higher relative humudity than in plant - reduces rate of transpiration

How does a potometer work!?

Uses a trapped air bubble/dye in a piece of capillary tubing to measure water uptake of a plant.

Water uptake measured by recording time taken for bubble in tube to move set distance.

How does the stomata open?

• Light triggers proton pumps to pull hydrogen ions out of the guard cells.

• To balance charge, potassium H+ ions move in and lower water potential.

• Water enters the guard cells down water potential gradient from surrounding epidermal cells.

• Guard feels become turgid

• Due to inner cell walls being thicker than the outside walls, guard cells bend into crescent shape, opening pore.

How does the stomata close?

• When the plant becomes water stressed, protein hormone ABA (abscisic acid) bonds to receptors of guard cell membrane.

• ABA triggers opening of channel proteins allowing K+ and other ions to leave

• Water potential of guard cell increases and water leaves via osmosis.

• Guard cells become flaccid and close.

Two ways to measure rate of transpiration

Measuring change of mass of plant (g/min)

Measuring water uptake of plant (cm3/min)