Transpiration
UNI Plant Physiology 2008
2008
www.uni.edu/berg
Water moves through plants
Into the air
Through the plant
From soil to plant
2008
www.uni.edu/berg
Key questions
• What is the route? (cell types)
• Why does it move? (Energy source)
– Whole system (soil/plant/air)
– Locally (cell to cell)
• What are the forces involved?
• What special properties are involved?
– Special properties of water
– Special properties of the plant
• How can the plant control it?
2008
www.uni.edu/berg
The route
From leaf cells to air
From leaf xylem to leaf cells
From stem xylem to
leaf xylem
From root xylem to
stem xylem
From root cells to
root xylem
From soil to root cells
2008
www.uni.edu/berg
What drives this? (Energy)
• Differences make things move
• Also need a route
• For water in plants
– Water potential (pressure and solute effects)
differences move water across membranes
– Pressure differences cause bulk flow in xylem
• Evaporating water to air
– Water potential differences again
2008
www.uni.edu/berg
Overall water movement
• Water potential
– Air < plant <soil
• Water flows “downhill”
energetically
– From higher to lower water potential
– From soil to plant to air
• Difference between soil and air
drives transpiration
• Plant provides the conduit
2008
www.uni.edu/berg
Cell-to-cell movement
• Live cells (with central vacuole)
– Membranes present
– Water potential difference drives movement
• Live cell and xylem “cell”
– Membrane present
– Water potential difference drives movement
• Xylem “cells”
– NO Membrane present
– Pressure potential difference drives
movement
2008
www.uni.edu/berg
Air, wall & tracheid
Xylem cell wall
Air in space between
this cell and next cell
Interface between xylem sap and air
Xylem sap in pore in wall
Xylem sap inside xylem “cell”
2008
www.uni.edu/berg
Capillary action holds water in
Well watered plant—not
Plant much
starts to dry—more
tension on water intension
tracheidon water in tracheid
Drier still—more tension
Quite dry—more tension
2008
www.uni.edu/berg
Failure of capillary action
• Tension in sap (water) pulls air/sap
interface closer toward interior of “cell”
• If tension is too great, the air/sap interface
is pulled out the bottom of the capillary
pore
• The air quickly expands to fill the tracheid
2008
www.uni.edu/berg
Failure of capillary action
Very fast, and makes a popping sound (too high frequency to hear)
2008
www.uni.edu/berg
Air-filled xylem “cell”
Sounds of air entry
• Dr. Mel Tyree
• University of Toronto, then University of
Vermont, then US Forest Service
• Also discovered the maple sap story
2008
www.uni.edu/berg