Slide 44
Ascent of xylem sap
Outside air ψ = −100.0 Mpa
Leaf ψ (air spaces) = −7.0 Mpa
Leaf ψ (cell walls) = −1.0 Mpa
Trunk xylem ψ = −0.8 Mpa
Trunk xylem ψ = −0.6 Mpa
Soil ψ = −0.3 Mpa
Xylem sap
Mesophyll cells
Stoma
Stoma
Water molecule
Transpiration
Atmosphere
Adhesion by hydrogen bonding
Cell wall
Xylem cells
Cohesion and adhesion in the xylem
Cohesion by hydrogen bonding
Water molecule
Root hair
Soil particle
Water
Water uptake from soil
Water potential gradient
Slide 45
Water molecule
Root hair
Soil particle
Water
Water uptake from soil
Slide 46
Adhesion by hydrogen bonding
Cell wall
Xylem cells
Cohesion by hydrogen bonding
Cohesion and adhesion in the xylem
Slide 47
Xylem sap
Mesophyll cells
Stoma
Water molecule
Atmosphere
Transpiration
Slide 48
The movement of xylem sap against gravity is maintained by the transpiration-cohesion-tension mechanism.
Transpiration lowers water potential in leaves, and this generates negative pressure (tension) that pulls water up through the xylem.
There is no energy cost to bulk flow of xylem sap.
Slide 49
Leaves generally have broad surface areas and high surface-to-volume ratios.
These characteristics increase photosynthesis and increase water loss through stomata.
About 95% of the water a plant loses escapes through stomata.
Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape.
Slide 50
An open stoma (left) and closed stoma (right)
Slide 51
Changes in turgor pressure open and close stomata.
These result primarily from the reversible uptake and loss of potassium ions by the guard cells.
Slide 52
Stomatal Openings
Radially oriented cellulose microfibrils
Cell wall
Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed