Transpiration Rate and Stomatal Regulation

Transpiration is the loss of water vapour from aerial plant parts — mainly through stomata in leaves (90%) and a small amount through the cuticle.

It is not a waste — transpiration is the engine that drives water movement from roots to leaves.

VPD (Vapour Pressure Deficit) = difference between water vapour pressure inside leaf and outside. Higher VPD = faster transpiration.

1. Water evaporates from mesophyll cells into substomatal air spaces
2. This lowers water potential (ψ) in the leaf cells
3. Water is pulled from xylem to replace it
4. The entire water column in xylem is under tension (negative pressure)
5. Cohesion between water molecules maintains an unbroken column
6. Water is pulled up from roots passively — no pumping needed

This is the Cohesion-Tension mechanism — water is pulled up, not pushed.

Adhesion to xylem cell walls prevents the column from breaking under tension.

Stomata open and close via guard cells — kidney-shaped cells surrounding each pore.

Opening (daytime):

• Light → K⁺ ions pumped into guard cells (active transport)
• Water enters by osmosis → guard cells swell → pore opens
• CO₂ depletion (from photosynthesis) also signals opening

Closing (drought, night):

• ABA (Abscisic Acid) released during water stress
• ABA triggers K⁺ to leave guard cells
• Guard cells lose water → shrink → pore closes
• Ca²⁺ and other second messengers involved

Plants balance this by opening stomata in bright light (when photosynthesis benefit is high) and closing in drought (when water loss would be fatal).

Desert plants have evolved to minimise water loss:

• Sunken stomata — in pits, surrounded by still humid air
• Thick waxy cuticle — reduces cuticular transpiration
• Rolled leaves (marram grass) — trap humid air around stomata
• Small leaf area — fewer stomata per plant
• CAM photosynthesis — open stomata at NIGHT only (cacti, succulents)

• Drives mineral ion transport from roots upward
• Cooling — evaporative cooling reduces leaf temperature in hot sun
• Maintains water potential gradient that powers the entire water transport system
• Wilting when transpiration rate > water absorption rate

• VPD drives transpiration — not absolute humidity
• Cohesion-tension: water is under negative pressure in xylem — can be measured with pressure bomb
• ABA → guard cell → stomata closes: know the signalling cascade (ABA → Ca²⁺ → K⁺ efflux)
• Xerophyte vs hydrophyte vs mesophyte adaptations
• Root pressure also contributes (guttation) but is much weaker than transpiration pull
• Potometer measures transpiration rate — common experimental question