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Resource Acquisition and Transport in Vascular Plants
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Slide 44

Ascent of xylem sap

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

Water molecule

Root hair

Soil particle

Water

Water uptake from soil

Slide 46

Adhesion by hydrogen bonding

Adhesion by hydrogen bonding

Cell wall

Xylem cells

Cohesion by hydrogen bonding

Cohesion and adhesion in the xylem

Slide 47

Xylem sap

Xylem sap

Mesophyll cells

Stoma

Water molecule

Atmosphere

Transpiration

Slide 48

Xylem Sap Ascent by Bulk Flow: A Review

Xylem Sap Ascent by Bulk Flow: A Review

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

Stomata help regulate the rate of transpiration

Stomata help regulate the rate of transpiration

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)

An open stoma (left) and closed stoma (right)

Slide 51

Mechanisms of Stomatal Opening and Closing

Mechanisms of Stomatal Opening and Closing

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

Stomatal Openings

Radially oriented cellulose microfibrils

Cell wall

Guard cells turgid/Stoma open Guard cells flaccid/Stoma closed

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